From 9c62a5046b829a7d803226212f63a13b5968f53f Mon Sep 17 00:00:00 2001
From: Stephen Toub <stoub@microsoft.com>
Date: Fri, 9 Aug 2019 11:32:32 -0400
Subject: [PATCH 1/6] Contribute to IDE0044 (make field readonly)

I used the auto-fix and then undid changes in areas where the fields were potentially mutated in native, e.g. I undid many of the changes in reflection.  We can revisit those individually later.
---
 .../shared/Internal/Win32/RegistryKey.cs      |  2 +-
 .../shared/System/AggregateException.cs       |  2 +-
 .../shared/System/ArgumentException.cs        |  2 +-
 .../System/ArgumentOutOfRangeException.cs     |  2 +-
 .../shared/System/Array.Enumerators.cs        | 10 +++---
 .../shared/System/AttributeUsageAttribute.cs  |  2 +-
 .../shared/System/BadImageFormatException.cs  |  4 +--
 .../shared/System/CLSCompliantAttribute.cs    |  2 +-
 .../shared/System/Collections/ArrayList.cs    | 34 +++++++++----------
 .../shared/System/Collections/Comparer.cs     |  2 +-
 .../System/Collections/Generic/Dictionary.cs  |  4 +--
 .../shared/System/Collections/Hashtable.cs    | 12 +++----
 .../Collections/ListDictionaryInternal.cs     | 14 ++++----
 .../Collections/ObjectModel/Collection.cs     |  2 +-
 .../ObjectModel/ReadOnlyCollection.cs         |  2 +-
 .../shared/System/CurrentSystemTimeZone.cs    |  6 ++--
 .../Contracts/ContractFailedEventArgs.cs      |  8 ++---
 .../System/Diagnostics/Contracts/Contracts.cs | 16 ++++-----
 .../shared/System/Diagnostics/StackFrame.cs   |  2 +-
 .../Diagnostics/Tracing/ActivityTracker.cs    |  2 +-
 .../Diagnostics/Tracing/EventDescriptor.cs    | 16 ++++-----
 .../System/Diagnostics/Tracing/EventSource.cs | 29 ++++++++--------
 .../Tracing/IncrementingPollingCounter.cs     |  2 +-
 .../Diagnostics/Tracing/PollingCounter.cs     |  2 +-
 .../Diagnostics/Tracing/StubEnvironment.cs    |  2 +-
 .../Tracing/TraceLogging/EventPayload.cs      |  4 +--
 .../Tracing/TraceLogging/SimpleTypeInfos.cs   | 10 +++---
 .../Globalization/CultureNotFoundException.cs |  4 +--
 .../Globalization/DateTimeFormatInfo.cs       |  2 +-
 .../System/Globalization/DateTimeParse.cs     |  7 ++--
 .../System/Globalization/HebrewNumber.cs      |  2 +-
 .../shared/System/Globalization/IdnMapping.cs |  2 +-
 .../System/Globalization/SortVersion.cs       |  2 +-
 .../System/Globalization/TaiwanCalendar.cs    |  2 +-
 .../System/Globalization/TimeSpanParse.cs     |  2 +-
 .../shared/System/IO/BinaryReader.cs          |  8 ++---
 .../shared/System/IO/BinaryWriter.cs          |  2 +-
 .../shared/System/IO/MemoryStream.cs          |  4 +--
 .../System/IO/PinnedBufferMemoryStream.cs     |  2 +-
 .../shared/System/IO/Stream.cs                |  6 ++--
 .../shared/System/IO/StreamReader.cs          |  2 +-
 .../shared/System/IO/TextWriter.cs            |  2 +-
 .../System/IO/UnmanagedMemoryStreamWrapper.cs |  2 +-
 .../shared/System/Math.cs                     |  2 +-
 .../shared/System/MathF.cs                    |  2 +-
 .../shared/System/NotFiniteNumberException.cs |  2 +-
 .../Number.NumberToFloatingPointBits.cs       |  4 +--
 .../shared/System/ObjectDisposedException.cs  |  2 +-
 .../shared/System/ObsoleteAttribute.cs        |  4 +--
 .../shared/System/Random.cs                   |  2 +-
 .../Reflection/AssemblyFlagsAttribute.cs      |  2 +-
 .../Resources/FileBasedResourceGroveler.cs    |  2 +-
 .../ManifestBasedResourceGroveler.cs          |  2 +-
 .../MissingSatelliteAssemblyException.cs      |  2 +-
 .../Resources/ResourceFallbackManager.cs      |  6 ++--
 .../System/Resources/ResourceManager.cs       |  8 ++---
 .../shared/System/Resources/ResourceReader.cs |  2 +-
 .../DateTimeConstantAttribute.cs              |  2 +-
 .../DecimalConstantAttribute.cs               |  2 +-
 .../InteropServices/ArrayWithOffset.cs        |  6 ++--
 .../shared/System/Runtime/MemoryFailPoint.cs  | 22 ++++++------
 .../System/Runtime/Remoting/ObjectHandle.cs   |  2 +-
 .../Serialization/SerializationInfo.cs        |  4 +--
 .../Versioning/TargetFrameworkAttribute.cs    |  2 +-
 .../shared/System/StringComparer.cs           |  2 +-
 .../System/Text/DecoderBestFitFallback.cs     |  2 +-
 .../System/Text/DecoderExceptionFallback.cs   |  4 +--
 .../shared/System/Text/DecoderNLS.cs          |  2 +-
 .../System/Text/DecoderReplacementFallback.cs |  4 +--
 .../System/Text/EncoderBestFitFallback.cs     |  2 +-
 .../System/Text/EncoderExceptionFallback.cs   |  8 ++---
 .../shared/System/Text/EncoderNLS.cs          |  2 +-
 .../System/Text/EncoderReplacementFallback.cs |  4 +--
 .../shared/System/Text/Encoding.cs            | 30 ++++++++--------
 .../shared/System/Text/EncodingProvider.cs    |  2 +-
 .../shared/System/Text/UTF7Encoding.cs        |  2 +-
 .../System/Threading/ReaderWriterLockSlim.cs  |  6 ++--
 .../shared/System/Threading/Tasks/Future.cs   |  2 +-
 .../System/Threading/Tasks/FutureFactory.cs   |  8 ++---
 .../shared/System/Threading/Tasks/Task.cs     |  2 +-
 .../System/Threading/Tasks/TaskScheduler.cs   |  4 +--
 .../shared/System/Threading/ThreadLocal.cs    |  4 +--
 .../shared/System/Threading/ThreadPool.cs     | 10 +++---
 .../shared/System/Threading/Timer.cs          |  2 +-
 .../System/TypeInitializationException.cs     |  2 +-
 .../System/UnhandledExceptionEventArgs.cs     |  4 +--
 .../Runtime/InteropServices/ComActivator.cs   | 16 ++++-----
 .../ObjectModel/ReadOnlyDictionary.cs         |  2 +-
 .../System/Diagnostics/Eventing/EventPipe.cs  | 14 ++++----
 .../Eventing/EventPipeEventDispatcher.cs      | 10 +++---
 .../src/System/Exception.CoreCLR.cs           | 10 +++---
 .../TypeDependencyAttribute.cs                |  2 +-
 .../Runtime/InteropServices/ComEventsInfo.cs  |  2 +-
 .../InteropServices/ComEventsMethod.cs        |  2 +-
 .../EnumeratorViewOfEnumVariant.cs            |  2 +-
 .../WindowsRuntime/Attributes.cs              | 14 ++++----
 .../WindowsRuntime/CLRIPropertyValueImpl.cs   |  4 +--
 .../WindowsRuntime/CLRIReferenceImpl.cs       |  6 ++--
 .../WindowsRuntime/ConstantSplittableMap.cs   |  6 ++--
 .../WindowsRuntime/CustomPropertyImpl.cs      |  2 +-
 .../EnumeratorToIteratorAdapter.cs            |  4 +--
 .../EventRegistrationTokenTable.cs            |  2 +-
 .../WindowsRuntime/ICustomPropertyProvider.cs |  8 ++---
 .../IteratorToEnumeratorAdapter.cs            |  4 +--
 .../ManagedActivationFactory.cs               |  2 +-
 .../WindowsRuntime/WindowsRuntimeMarshal.cs   |  6 ++--
 .../WindowsRuntime/WindowsRuntimeMetadata.cs  |  6 ++--
 .../src/System/RuntimeType.CoreCLR.cs         | 10 +++---
 .../src/System/Threading/Overlapped.cs        |  6 ++--
 .../Threading/Tasks/AsyncCausalityTracer.cs   |  2 +-
 .../src/System/TypeNameParser.cs              |  2 +-
 111 files changed, 290 insertions(+), 290 deletions(-)

diff --git a/src/System.Private.CoreLib/shared/Internal/Win32/RegistryKey.cs b/src/System.Private.CoreLib/shared/Internal/Win32/RegistryKey.cs
index 013b442b6f4a..90ffabb0e7ac 100644
--- a/src/System.Private.CoreLib/shared/Internal/Win32/RegistryKey.cs
+++ b/src/System.Private.CoreLib/shared/Internal/Win32/RegistryKey.cs
@@ -29,7 +29,7 @@ internal sealed class RegistryKey : IDisposable
         private const int MaxKeyLength = 255;
         private const int MaxValueLength = 16383;
 
-        private SafeRegistryHandle _hkey;
+        private readonly SafeRegistryHandle _hkey;
 
         private RegistryKey(SafeRegistryHandle hkey)
         {
diff --git a/src/System.Private.CoreLib/shared/System/AggregateException.cs b/src/System.Private.CoreLib/shared/System/AggregateException.cs
index 49809b104814..e0f079fe2390 100644
--- a/src/System.Private.CoreLib/shared/System/AggregateException.cs
+++ b/src/System.Private.CoreLib/shared/System/AggregateException.cs
@@ -25,7 +25,7 @@ namespace System
     [System.Runtime.CompilerServices.TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
     public class AggregateException : Exception
     {
-        private ReadOnlyCollection<Exception> m_innerExceptions; // Complete set of exceptions. Do not rename (binary serialization)
+        private readonly ReadOnlyCollection<Exception> m_innerExceptions; // Complete set of exceptions. Do not rename (binary serialization)
 
         /// <summary>
         /// Initializes a new instance of the <see cref="AggregateException"/> class.
diff --git a/src/System.Private.CoreLib/shared/System/ArgumentException.cs b/src/System.Private.CoreLib/shared/System/ArgumentException.cs
index 2fd3c880ecee..41711870ade0 100644
--- a/src/System.Private.CoreLib/shared/System/ArgumentException.cs
+++ b/src/System.Private.CoreLib/shared/System/ArgumentException.cs
@@ -23,7 +23,7 @@ namespace System
     [System.Runtime.CompilerServices.TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
     public class ArgumentException : SystemException
     {
-        private string? _paramName;
+        private readonly string? _paramName;
 
         // Creates a new ArgumentException with its message
         // string set to the empty string.
diff --git a/src/System.Private.CoreLib/shared/System/ArgumentOutOfRangeException.cs b/src/System.Private.CoreLib/shared/System/ArgumentOutOfRangeException.cs
index 0ffc4e4aee71..3b9ae7e8e90d 100644
--- a/src/System.Private.CoreLib/shared/System/ArgumentOutOfRangeException.cs
+++ b/src/System.Private.CoreLib/shared/System/ArgumentOutOfRangeException.cs
@@ -22,7 +22,7 @@ namespace System
     [System.Runtime.CompilerServices.TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
     public class ArgumentOutOfRangeException : ArgumentException
     {
-        private object? _actualValue;
+        private readonly object? _actualValue;
 
         // Creates a new ArgumentOutOfRangeException with its message
         // string set to a default message explaining an argument was out of range.
diff --git a/src/System.Private.CoreLib/shared/System/Array.Enumerators.cs b/src/System.Private.CoreLib/shared/System/Array.Enumerators.cs
index 74349ab23809..8029ab218d4c 100644
--- a/src/System.Private.CoreLib/shared/System/Array.Enumerators.cs
+++ b/src/System.Private.CoreLib/shared/System/Array.Enumerators.cs
@@ -10,11 +10,11 @@ namespace System
 {
     internal sealed class ArrayEnumerator : IEnumerator, ICloneable
     {
-        private Array array;
+        private readonly Array array;
         private int index;
-        private int endIndex;
-        private int startIndex;    // Save for Reset.
-        private int[] _indices;    // The current position in a multidim array
+        private readonly int endIndex;
+        private readonly int startIndex;    // Save for Reset.
+        private readonly int[] _indices;    // The current position in a multidim array
         private bool _complete;
 
         internal ArrayEnumerator(Array array, int index, int count)
@@ -110,7 +110,7 @@ internal sealed class SZArrayEnumerator : IEnumerator, ICloneable
     {
         private readonly Array _array;
         private int _index;
-        private int _endIndex; // Cache Array.Length, since it's a little slow.
+        private readonly int _endIndex; // Cache Array.Length, since it's a little slow.
 
         internal SZArrayEnumerator(Array array)
         {
diff --git a/src/System.Private.CoreLib/shared/System/AttributeUsageAttribute.cs b/src/System.Private.CoreLib/shared/System/AttributeUsageAttribute.cs
index 06fb7f9a5060..978910b943d3 100644
--- a/src/System.Private.CoreLib/shared/System/AttributeUsageAttribute.cs
+++ b/src/System.Private.CoreLib/shared/System/AttributeUsageAttribute.cs
@@ -19,7 +19,7 @@ namespace System
     [AttributeUsage(AttributeTargets.Class, Inherited = true)]
     public sealed class AttributeUsageAttribute : Attribute
     {
-        private AttributeTargets _attributeTarget = AttributeTargets.All; // Defaults to all
+        private readonly AttributeTargets _attributeTarget = AttributeTargets.All; // Defaults to all
         private bool _allowMultiple = false; // Defaults to false
         private bool _inherited = true; // Defaults to true
 
diff --git a/src/System.Private.CoreLib/shared/System/BadImageFormatException.cs b/src/System.Private.CoreLib/shared/System/BadImageFormatException.cs
index 5d065e7bba27..aa39a9a95ad7 100644
--- a/src/System.Private.CoreLib/shared/System/BadImageFormatException.cs
+++ b/src/System.Private.CoreLib/shared/System/BadImageFormatException.cs
@@ -21,8 +21,8 @@ namespace System
     [System.Runtime.CompilerServices.TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
     public partial class BadImageFormatException : SystemException
     {
-        private string? _fileName;  // The name of the corrupt PE file.
-        private string? _fusionLog;  // fusion log (when applicable)
+        private readonly string? _fileName;  // The name of the corrupt PE file.
+        private readonly string? _fusionLog;  // fusion log (when applicable)
 
         public BadImageFormatException()
             : base(SR.Arg_BadImageFormatException)
diff --git a/src/System.Private.CoreLib/shared/System/CLSCompliantAttribute.cs b/src/System.Private.CoreLib/shared/System/CLSCompliantAttribute.cs
index d895b5ac7166..9de638ee8de2 100644
--- a/src/System.Private.CoreLib/shared/System/CLSCompliantAttribute.cs
+++ b/src/System.Private.CoreLib/shared/System/CLSCompliantAttribute.cs
@@ -16,7 +16,7 @@ namespace System
     [AttributeUsage(AttributeTargets.All, Inherited = true, AllowMultiple = false)]
     public sealed class CLSCompliantAttribute : Attribute
     {
-        private bool _compliant;
+        private readonly bool _compliant;
 
         public CLSCompliantAttribute(bool isCompliant)
         {
diff --git a/src/System.Private.CoreLib/shared/System/Collections/ArrayList.cs b/src/System.Private.CoreLib/shared/System/Collections/ArrayList.cs
index 3f1cba08514a..d4ae3da5350d 100644
--- a/src/System.Private.CoreLib/shared/System/Collections/ArrayList.cs
+++ b/src/System.Private.CoreLib/shared/System/Collections/ArrayList.cs
@@ -772,7 +772,7 @@ public virtual void TrimToSize()
         // Note this requires reimplementing half of ArrayList...
         private class IListWrapper : ArrayList
         {
-            private IList _list;
+            private readonly IList _list;
 
             internal IListWrapper(IList list)
             {
@@ -1235,8 +1235,8 @@ public void Reset()
 
         private class SyncArrayList : ArrayList
         {
-            private ArrayList _list;
-            private object _root;
+            private readonly ArrayList _list;
+            private readonly object _root;
 
             internal SyncArrayList(ArrayList list)
                 : base(false)
@@ -1590,8 +1590,8 @@ public override void TrimToSize()
 
         private class SyncIList : IList
         {
-            private IList _list;
-            private object _root;
+            private readonly IList _list;
+            private readonly object _root;
 
             internal SyncIList(IList list)
             {
@@ -1719,7 +1719,7 @@ public virtual void RemoveAt(int index)
 
         private class FixedSizeList : IList
         {
-            private IList _list;
+            private readonly IList _list;
 
             internal FixedSizeList(IList l)
             {
@@ -2031,7 +2031,7 @@ public override void TrimToSize()
 
         private class ReadOnlyList : IList
         {
-            private IList _list;
+            private readonly IList _list;
 
             internal ReadOnlyList(IList l)
             {
@@ -2343,12 +2343,12 @@ public override void TrimToSize()
         // made to the list while an enumeration is in progress.
         private sealed class ArrayListEnumerator : IEnumerator, ICloneable
         {
-            private ArrayList _list;
+            private readonly ArrayList _list;
             private int _index;
-            private int _endIndex;       // Where to stop.
-            private int _version;
+            private readonly int _endIndex;       // Where to stop.
+            private readonly int _version;
             private object? _currentElement;
-            private int _startIndex;     // Save this for Reset.
+            private readonly int _startIndex;     // Save this for Reset.
 
             internal ArrayListEnumerator(ArrayList list, int index, int count)
             {
@@ -2404,7 +2404,7 @@ public void Reset()
         private class Range : ArrayList
         {
             private ArrayList _baseList;
-            private int _baseIndex;
+            private readonly int _baseIndex;
             private int _baseSize;
             private int _baseVersion;
 
@@ -2819,13 +2819,13 @@ public override void TrimToSize()
 
         private sealed class ArrayListEnumeratorSimple : IEnumerator, ICloneable
         {
-            private ArrayList _list;
+            private readonly ArrayList _list;
             private int _index;
-            private int _version;
+            private readonly int _version;
             private object? _currentElement;
-            private bool _isArrayList;
+            private readonly bool _isArrayList;
             // this object is used to indicate enumeration has not started or has terminated
-            private static object s_dummyObject = new object();
+            private static readonly object s_dummyObject = new object();
 
             internal ArrayListEnumeratorSimple(ArrayList list)
             {
@@ -2910,7 +2910,7 @@ public void Reset()
 
         internal class ArrayListDebugView
         {
-            private ArrayList _arrayList;
+            private readonly ArrayList _arrayList;
 
             public ArrayListDebugView(ArrayList arrayList)
             {
diff --git a/src/System.Private.CoreLib/shared/System/Collections/Comparer.cs b/src/System.Private.CoreLib/shared/System/Collections/Comparer.cs
index 7775283367c8..e8f431be6db3 100644
--- a/src/System.Private.CoreLib/shared/System/Collections/Comparer.cs
+++ b/src/System.Private.CoreLib/shared/System/Collections/Comparer.cs
@@ -17,7 +17,7 @@ namespace System.Collections
     [System.Runtime.CompilerServices.TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
     public sealed class Comparer : IComparer, ISerializable
     {
-        private CompareInfo _compareInfo;
+        private readonly CompareInfo _compareInfo;
 
         public static readonly Comparer Default = new Comparer(CultureInfo.CurrentCulture);
         public static readonly Comparer DefaultInvariant = new Comparer(CultureInfo.InvariantCulture);
diff --git a/src/System.Private.CoreLib/shared/System/Collections/Generic/Dictionary.cs b/src/System.Private.CoreLib/shared/System/Collections/Generic/Dictionary.cs
index 7f14fd7cb90f..1bd9056265ce 100644
--- a/src/System.Private.CoreLib/shared/System/Collections/Generic/Dictionary.cs
+++ b/src/System.Private.CoreLib/shared/System/Collections/Generic/Dictionary.cs
@@ -1290,7 +1290,7 @@ object IDictionaryEnumerator.Key
         [DebuggerDisplay("Count = {Count}")]
         public sealed class KeyCollection : ICollection<TKey>, ICollection, IReadOnlyCollection<TKey>
         {
-            private Dictionary<TKey, TValue> _dictionary;
+            private readonly Dictionary<TKey, TValue> _dictionary;
 
             public KeyCollection(Dictionary<TKey, TValue> dictionary)
             {
@@ -1473,7 +1473,7 @@ void IEnumerator.Reset()
         [DebuggerDisplay("Count = {Count}")]
         public sealed class ValueCollection : ICollection<TValue>, ICollection, IReadOnlyCollection<TValue>
         {
-            private Dictionary<TKey, TValue> _dictionary;
+            private readonly Dictionary<TKey, TValue> _dictionary;
 
             public ValueCollection(Dictionary<TKey, TValue> dictionary)
             {
diff --git a/src/System.Private.CoreLib/shared/System/Collections/Hashtable.cs b/src/System.Private.CoreLib/shared/System/Collections/Hashtable.cs
index 974bdf9bef8a..2b3bb5774913 100644
--- a/src/System.Private.CoreLib/shared/System/Collections/Hashtable.cs
+++ b/src/System.Private.CoreLib/shared/System/Collections/Hashtable.cs
@@ -1251,7 +1251,7 @@ public virtual void OnDeserialization(object? sender)
         // class is created by the GetKeys method of a hashtable.
         private class KeyCollection : ICollection
         {
-            private Hashtable _hashtable;
+            private readonly Hashtable _hashtable;
 
             internal KeyCollection(Hashtable hashtable)
             {
@@ -1296,7 +1296,7 @@ public virtual int Count
         // this class is created by the GetValues method of a hashtable.
         private class ValueCollection : ICollection
         {
-            private Hashtable _hashtable;
+            private readonly Hashtable _hashtable;
 
             internal ValueCollection(Hashtable hashtable)
             {
@@ -1510,11 +1510,11 @@ internal override KeyValuePairs[] ToKeyValuePairsArray()
         // are made to the hashtable while an enumeration is in progress.
         private class HashtableEnumerator : IDictionaryEnumerator, ICloneable
         {
-            private Hashtable _hashtable;
+            private readonly Hashtable _hashtable;
             private int _bucket;
-            private int _version;
+            private readonly int _version;
             private bool _current;
-            private int _getObjectRetType;   // What should GetObject return?
+            private readonly int _getObjectRetType;   // What should GetObject return?
             private object? _currentKey;
             private object? _currentValue;
 
@@ -1614,7 +1614,7 @@ public virtual void Reset()
         // internal debug view class for hashtable
         internal class HashtableDebugView
         {
-            private Hashtable _hashtable;
+            private readonly Hashtable _hashtable;
 
             public HashtableDebugView(Hashtable hashtable)
             {
diff --git a/src/System.Private.CoreLib/shared/System/Collections/ListDictionaryInternal.cs b/src/System.Private.CoreLib/shared/System/Collections/ListDictionaryInternal.cs
index 2e23aa5bf73c..0585b08552be 100644
--- a/src/System.Private.CoreLib/shared/System/Collections/ListDictionaryInternal.cs
+++ b/src/System.Private.CoreLib/shared/System/Collections/ListDictionaryInternal.cs
@@ -269,9 +269,9 @@ public void Remove(object key)
 
         private class NodeEnumerator : IDictionaryEnumerator
         {
-            private ListDictionaryInternal list;
+            private readonly ListDictionaryInternal list;
             private DictionaryNode? current;
-            private int version;
+            private readonly int version;
             private bool start;
 
 
@@ -362,8 +362,8 @@ public void Reset()
 
         private class NodeKeyValueCollection : ICollection
         {
-            private ListDictionaryInternal list;
-            private bool isKeys;
+            private readonly ListDictionaryInternal list;
+            private readonly bool isKeys;
 
             public NodeKeyValueCollection(ListDictionaryInternal list, bool isKeys)
             {
@@ -425,10 +425,10 @@ IEnumerator IEnumerable.GetEnumerator()
 
             private class NodeKeyValueEnumerator : IEnumerator
             {
-                private ListDictionaryInternal list;
+                private readonly ListDictionaryInternal list;
                 private DictionaryNode? current;
-                private int version;
-                private bool isKeys;
+                private readonly int version;
+                private readonly bool isKeys;
                 private bool start;
 
                 public NodeKeyValueEnumerator(ListDictionaryInternal list, bool isKeys)
diff --git a/src/System.Private.CoreLib/shared/System/Collections/ObjectModel/Collection.cs b/src/System.Private.CoreLib/shared/System/Collections/ObjectModel/Collection.cs
index 6dec16fa5780..d87594e5e2a2 100644
--- a/src/System.Private.CoreLib/shared/System/Collections/ObjectModel/Collection.cs
+++ b/src/System.Private.CoreLib/shared/System/Collections/ObjectModel/Collection.cs
@@ -13,7 +13,7 @@ namespace System.Collections.ObjectModel
     [System.Runtime.CompilerServices.TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
     public class Collection<T> : IList<T>, IList, IReadOnlyList<T>
     {
-        private IList<T> items; // Do not rename (binary serialization)
+        private readonly IList<T> items; // Do not rename (binary serialization)
 
         public Collection()
         {
diff --git a/src/System.Private.CoreLib/shared/System/Collections/ObjectModel/ReadOnlyCollection.cs b/src/System.Private.CoreLib/shared/System/Collections/ObjectModel/ReadOnlyCollection.cs
index b68a03292adc..1f99b892cddd 100644
--- a/src/System.Private.CoreLib/shared/System/Collections/ObjectModel/ReadOnlyCollection.cs
+++ b/src/System.Private.CoreLib/shared/System/Collections/ObjectModel/ReadOnlyCollection.cs
@@ -13,7 +13,7 @@ namespace System.Collections.ObjectModel
     [System.Runtime.CompilerServices.TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
     public class ReadOnlyCollection<T> : IList<T>, IList, IReadOnlyList<T>
     {
-        private IList<T> list; // Do not rename (binary serialization)
+        private readonly IList<T> list; // Do not rename (binary serialization)
 
         public ReadOnlyCollection(IList<T> list)
         {
diff --git a/src/System.Private.CoreLib/shared/System/CurrentSystemTimeZone.cs b/src/System.Private.CoreLib/shared/System/CurrentSystemTimeZone.cs
index a3d00c102d04..285dcba29be3 100644
--- a/src/System.Private.CoreLib/shared/System/CurrentSystemTimeZone.cs
+++ b/src/System.Private.CoreLib/shared/System/CurrentSystemTimeZone.cs
@@ -29,9 +29,9 @@ internal class CurrentSystemTimeZone : TimeZone
         // no daylight saving is in used.
         // E.g. the offset for PST (Pacific Standard time) should be -8 * 60 * 60 * 1000 * 10000.
         // (1 millisecond = 10000 ticks)
-        private long m_ticksOffset;
-        private string m_standardName;
-        private string m_daylightName;
+        private readonly long m_ticksOffset;
+        private readonly string m_standardName;
+        private readonly string m_daylightName;
 
         internal CurrentSystemTimeZone()
         {
diff --git a/src/System.Private.CoreLib/shared/System/Diagnostics/Contracts/ContractFailedEventArgs.cs b/src/System.Private.CoreLib/shared/System/Diagnostics/Contracts/ContractFailedEventArgs.cs
index 3c58ca10da5b..b0cd50950ca1 100644
--- a/src/System.Private.CoreLib/shared/System/Diagnostics/Contracts/ContractFailedEventArgs.cs
+++ b/src/System.Private.CoreLib/shared/System/Diagnostics/Contracts/ContractFailedEventArgs.cs
@@ -12,10 +12,10 @@ namespace System.Diagnostics.Contracts
 {
     public sealed class ContractFailedEventArgs : EventArgs
     {
-        private ContractFailureKind _failureKind;
-        private string? _message;
-        private string? _condition;
-        private Exception? _originalException;
+        private readonly ContractFailureKind _failureKind;
+        private readonly string? _message;
+        private readonly string? _condition;
+        private readonly Exception? _originalException;
         private bool _handled;
         private bool _unwind;
 
diff --git a/src/System.Private.CoreLib/shared/System/Diagnostics/Contracts/Contracts.cs b/src/System.Private.CoreLib/shared/System/Diagnostics/Contracts/Contracts.cs
index a176746b4d4d..72762e8a71a7 100644
--- a/src/System.Private.CoreLib/shared/System/Diagnostics/Contracts/Contracts.cs
+++ b/src/System.Private.CoreLib/shared/System/Diagnostics/Contracts/Contracts.cs
@@ -42,7 +42,7 @@ public sealed class PureAttribute : Attribute
     [AttributeUsage(AttributeTargets.Class | AttributeTargets.Interface | AttributeTargets.Delegate, AllowMultiple = false, Inherited = false)]
     public sealed class ContractClassAttribute : Attribute
     {
-        private Type _typeWithContracts;
+        private readonly Type _typeWithContracts;
 
         public ContractClassAttribute(Type typeContainingContracts)
         {
@@ -62,7 +62,7 @@ public Type TypeContainingContracts
     [AttributeUsage(AttributeTargets.Class, AllowMultiple = false, Inherited = false)]
     public sealed class ContractClassForAttribute : Attribute
     {
-        private Type _typeIAmAContractFor;
+        private readonly Type _typeIAmAContractFor;
 
         public ContractClassForAttribute(Type typeContractsAreFor)
         {
@@ -121,7 +121,7 @@ public sealed class ContractRuntimeIgnoredAttribute : Attribute
     [AttributeUsage(AttributeTargets.Assembly | AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Method | AttributeTargets.Constructor | AttributeTargets.Property)]
     public sealed class ContractVerificationAttribute : Attribute
     {
-        private bool _value;
+        private readonly bool _value;
 
         public ContractVerificationAttribute(bool value) { _value = value; }
 
@@ -139,7 +139,7 @@ public bool Value
     [AttributeUsage(AttributeTargets.Field)]
     public sealed class ContractPublicPropertyNameAttribute : Attribute
     {
-        private string _publicName;
+        private readonly string _publicName;
 
         public ContractPublicPropertyNameAttribute(string name)
         {
@@ -178,10 +178,10 @@ public sealed class ContractAbbreviatorAttribute : Attribute
     [Conditional("CONTRACTS_FULL")]
     public sealed class ContractOptionAttribute : Attribute
     {
-        private string _category;
-        private string _setting;
-        private bool _enabled;
-        private string? _value;
+        private readonly string _category;
+        private readonly string _setting;
+        private readonly bool _enabled;
+        private readonly string? _value;
 
         public ContractOptionAttribute(string category, string setting, bool enabled)
         {
diff --git a/src/System.Private.CoreLib/shared/System/Diagnostics/StackFrame.cs b/src/System.Private.CoreLib/shared/System/Diagnostics/StackFrame.cs
index 1683206bbce0..47c82309756d 100644
--- a/src/System.Private.CoreLib/shared/System/Diagnostics/StackFrame.cs
+++ b/src/System.Private.CoreLib/shared/System/Diagnostics/StackFrame.cs
@@ -47,7 +47,7 @@ public partial class StackFrame
         /// <summary>
         /// This flag is set to true when the frame represents a rethrow marker.
         /// </summary>
-        private bool _isLastFrameFromForeignExceptionStackTrace;
+        private readonly bool _isLastFrameFromForeignExceptionStackTrace;
 
         private void InitMembers()
         {
diff --git a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/ActivityTracker.cs b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/ActivityTracker.cs
index 9647a7aaa3ba..3f2a12dbbfba 100644
--- a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/ActivityTracker.cs
+++ b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/ActivityTracker.cs
@@ -613,7 +613,7 @@ private void ActivityChanging(AsyncLocalValueChangedArgs<ActivityInfo?> args)
         private bool m_checkedForEnable;
 
         // Singleton
-        private static ActivityTracker s_activityTrackerInstance = new ActivityTracker();
+        private static readonly ActivityTracker s_activityTrackerInstance = new ActivityTracker();
 
         // Used to create unique IDs at the top level.  Not used for nested Ids (each activity has its own id generator)
         private static long m_nextId = 0;
diff --git a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/EventDescriptor.cs b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/EventDescriptor.cs
index 770ea4a2f5b9..1646a37fd372 100644
--- a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/EventDescriptor.cs
+++ b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/EventDescriptor.cs
@@ -43,21 +43,21 @@ struct EventDescriptor
     {
         #region private
         [FieldOffset(0)]
-        private int m_traceloggingId;
+        private readonly int m_traceloggingId;
         [FieldOffset(0)]
-        private ushort m_id;
+        private readonly ushort m_id;
         [FieldOffset(2)]
-        private byte m_version;
+        private readonly byte m_version;
         [FieldOffset(3)]
-        private byte m_channel;
+        private readonly byte m_channel;
         [FieldOffset(4)]
-        private byte m_level;
+        private readonly byte m_level;
         [FieldOffset(5)]
-        private byte m_opcode;
+        private readonly byte m_opcode;
         [FieldOffset(6)]
-        private ushort m_task;
+        private readonly ushort m_task;
         [FieldOffset(8)]
-        private long m_keywords;
+        private readonly long m_keywords;
         #endregion
 
         public EventDescriptor(
diff --git a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/EventSource.cs b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/EventSource.cs
index f7669b1a3b9d..771b5f1ff749 100644
--- a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/EventSource.cs
+++ b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/EventSource.cs
@@ -2431,8 +2431,8 @@ protected override void OnControllerCommand(ControllerCommand command, IDictiona
                 m_eventSource.SendCommand(listener, m_eventProviderType, perEventSourceSessionId, etwSessionId,
                                           (EventCommand)command, IsEnabled(), Level, MatchAnyKeyword, arguments);
             }
-            private EventSource m_eventSource;
-            private EventProviderType m_eventProviderType;
+            private readonly EventSource m_eventSource;
+            private readonly EventProviderType m_eventProviderType;
         }
 #endif
 
@@ -3913,7 +3913,7 @@ private bool SelfDescribingEvents
 
         private EventHandler<EventCommandEventArgs>? m_eventCommandExecuted;
 
-        private EventSourceSettings m_config;      // configuration information
+        private readonly EventSourceSettings m_config;      // configuration information
 
         private bool m_eventSourceDisposed;              // has Dispose been called.
 
@@ -4962,7 +4962,7 @@ internal EventWrittenEventArgs(EventSource eventSource)
         }
         private string? m_message;
         private string? m_eventName;
-        private EventSource m_eventSource;
+        private readonly EventSource m_eventSource;
         private ReadOnlyCollection<string>? m_payloadNames;
         private Guid m_activityId;
         private long? m_osThreadId;
@@ -6280,15 +6280,14 @@ private class ChannelInfo
         }
 #endif
 
-        private Dictionary<int, string> opcodeTab;
+        private readonly Dictionary<int, string> opcodeTab;
         private Dictionary<int, string>? taskTab;
 #if FEATURE_MANAGED_ETW_CHANNELS
         private Dictionary<int, ChannelInfo>? channelTab;
 #endif
         private Dictionary<ulong, string>? keywordTab;
         private Dictionary<string, Type>? mapsTab;
-
-        private Dictionary<string, string> stringTab;       // Maps unlocalized strings to localized ones
+        private readonly Dictionary<string, string> stringTab;       // Maps unlocalized strings to localized ones
 
 #if FEATURE_MANAGED_ETW_CHANNELS
         // WCF used EventSource to mimic a existing ETW manifest.   To support this
@@ -6301,17 +6300,17 @@ private class ChannelInfo
         private const int MaxCountChannels = 8; // a manifest can defined at most 8 ETW channels
 #endif
 
-        private StringBuilder sb;               // Holds the provider information.
-        private StringBuilder events;           // Holds the events.
-        private StringBuilder templates;
+        private readonly StringBuilder sb;               // Holds the provider information.
+        private readonly StringBuilder events;           // Holds the events.
+        private readonly StringBuilder templates;
 
 #if FEATURE_MANAGED_ETW_CHANNELS
-        private string providerName;
+        private readonly string providerName;
 #endif
-        private ResourceManager? resources;      // Look up localized strings here.
-        private EventManifestOptions flags;
-        private IList<string> errors;           // list of currently encountered errors
-        private Dictionary<string, List<int>> perEventByteArrayArgIndices;  // "event_name" -> List_of_Indices_of_Byte[]_Arg
+        private readonly ResourceManager? resources;      // Look up localized strings here.
+        private readonly EventManifestOptions flags;
+        private readonly IList<string> errors;           // list of currently encountered errors
+        private readonly Dictionary<string, List<int>> perEventByteArrayArgIndices;  // "event_name" -> List_of_Indices_of_Byte[]_Arg
 
         // State we track between StartEvent and EndEvent.
         private string? eventName;               // Name of the event currently being processed.
diff --git a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/IncrementingPollingCounter.cs b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/IncrementingPollingCounter.cs
index 364f4d5ec270..4050daf38cb0 100644
--- a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/IncrementingPollingCounter.cs
+++ b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/IncrementingPollingCounter.cs
@@ -47,7 +47,7 @@ public IncrementingPollingCounter(string name, EventSource eventSource, Func<dou
         public TimeSpan DisplayRateTimeScale { get; set; }
         private double _increment;
         private double _prevIncrement;
-        private Func<double> _totalValueProvider;
+        private readonly Func<double> _totalValueProvider;
 
         /// <summary>
         /// Calls "_totalValueProvider" to enqueue the counter value to the queue.
diff --git a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/PollingCounter.cs b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/PollingCounter.cs
index 2b94119fb5ee..3238fcf03579 100644
--- a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/PollingCounter.cs
+++ b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/PollingCounter.cs
@@ -42,7 +42,7 @@ public PollingCounter(string name, EventSource eventSource, Func<double> metricP
 
         public override string ToString() => $"PollingCounter '{Name}' Count {1} Mean {_lastVal.ToString("n3")}";
 
-        private Func<double> _metricProvider;
+        private readonly Func<double> _metricProvider;
         private double _lastVal;
 
         internal override void WritePayload(float intervalSec, int pollingIntervalMillisec)
diff --git a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/StubEnvironment.cs b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/StubEnvironment.cs
index 2bd5de50496c..7dcfbf1876f2 100644
--- a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/StubEnvironment.cs
+++ b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/StubEnvironment.cs
@@ -44,7 +44,7 @@ public static string GetRuntimeResourceString(string key, params object?[] args)
             return GetResourceString(key, args);
         }
 
-        private static System.Resources.ResourceManager rm = new System.Resources.ResourceManager("Microsoft.Diagnostics.Tracing.Messages", typeof(Environment).Assembly());
+        private static readonly System.Resources.ResourceManager rm = new System.Resources.ResourceManager("Microsoft.Diagnostics.Tracing.Messages", typeof(Environment).Assembly());
     }
 
 #if ES_BUILD_STANDALONE
diff --git a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/TraceLogging/EventPayload.cs b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/TraceLogging/EventPayload.cs
index ff5a048d94e5..a32770894e9a 100644
--- a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/TraceLogging/EventPayload.cs
+++ b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/TraceLogging/EventPayload.cs
@@ -150,8 +150,8 @@ public bool TryGetValue(string key, [MaybeNullWhen(false)] out object? value)
         }
 
         #region private
-        private List<string> m_names;
-        private List<object?> m_values;
+        private readonly List<string> m_names;
+        private readonly List<object?> m_values;
         #endregion
     }
 }
diff --git a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/TraceLogging/SimpleTypeInfos.cs b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/TraceLogging/SimpleTypeInfos.cs
index 466ad010021b..64579d119ad2 100644
--- a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/TraceLogging/SimpleTypeInfos.cs
+++ b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/TraceLogging/SimpleTypeInfos.cs
@@ -50,8 +50,8 @@ public override void WriteData(TraceLoggingDataCollector collector, PropertyValu
     /// </summary>
     internal sealed class ScalarTypeInfo : TraceLoggingTypeInfo
     {
-        private Func<EventFieldFormat, TraceLoggingDataType, TraceLoggingDataType> formatFunc;
-        private TraceLoggingDataType nativeFormat;
+        private readonly Func<EventFieldFormat, TraceLoggingDataType, TraceLoggingDataType> formatFunc;
+        private readonly TraceLoggingDataType nativeFormat;
 
         private ScalarTypeInfo(
             Type type,
@@ -96,9 +96,9 @@ public override void WriteData(TraceLoggingDataCollector collector, PropertyValu
     /// </summary>
     internal sealed class ScalarArrayTypeInfo : TraceLoggingTypeInfo
     {
-        private Func<EventFieldFormat, TraceLoggingDataType, TraceLoggingDataType> formatFunc;
-        private TraceLoggingDataType nativeFormat;
-        private int elementSize;
+        private readonly Func<EventFieldFormat, TraceLoggingDataType, TraceLoggingDataType> formatFunc;
+        private readonly TraceLoggingDataType nativeFormat;
+        private readonly int elementSize;
 
         private ScalarArrayTypeInfo(
             Type type,
diff --git a/src/System.Private.CoreLib/shared/System/Globalization/CultureNotFoundException.cs b/src/System.Private.CoreLib/shared/System/Globalization/CultureNotFoundException.cs
index e17385564b5c..c053ec44a540 100644
--- a/src/System.Private.CoreLib/shared/System/Globalization/CultureNotFoundException.cs
+++ b/src/System.Private.CoreLib/shared/System/Globalization/CultureNotFoundException.cs
@@ -10,8 +10,8 @@ namespace System.Globalization
     [System.Runtime.CompilerServices.TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
     public class CultureNotFoundException : ArgumentException
     {
-        private string? _invalidCultureName; // unrecognized culture name
-        private int? _invalidCultureId;     // unrecognized culture Lcid
+        private readonly string? _invalidCultureName; // unrecognized culture name
+        private readonly int? _invalidCultureId;     // unrecognized culture Lcid
 
         public CultureNotFoundException()
             : base(DefaultMessage)
diff --git a/src/System.Private.CoreLib/shared/System/Globalization/DateTimeFormatInfo.cs b/src/System.Private.CoreLib/shared/System/Globalization/DateTimeFormatInfo.cs
index a0afc8f76c0b..67a7b47c5e89 100644
--- a/src/System.Private.CoreLib/shared/System/Globalization/DateTimeFormatInfo.cs
+++ b/src/System.Private.CoreLib/shared/System/Globalization/DateTimeFormatInfo.cs
@@ -54,7 +54,7 @@ public sealed class DateTimeFormatInfo : IFormatProvider, ICloneable
         private static volatile DateTimeFormatInfo? s_invariantInfo;
 
         // an index which points to a record in Culture Data Table.
-        private CultureData _cultureData;
+        private readonly CultureData _cultureData;
 
         // The culture name used to create this DTFI.
         private string? _name = null;
diff --git a/src/System.Private.CoreLib/shared/System/Globalization/DateTimeParse.cs b/src/System.Private.CoreLib/shared/System/Globalization/DateTimeParse.cs
index 511a440588c0..8008f328f3c9 100644
--- a/src/System.Private.CoreLib/shared/System/Globalization/DateTimeParse.cs
+++ b/src/System.Private.CoreLib/shared/System/Globalization/DateTimeParse.cs
@@ -332,7 +332,7 @@ internal enum DS
         ////////////////////////////////////////////////////////////////////////////
 
         //          End       NumEnd      NumAmPm     NumSpace    NumDaySep   NumTimesep  MonthEnd    MonthSpace  MonthDSep   NumDateSuff NumTimeSuff     DayOfWeek     YearSpace   YearDateSep YearEnd     TimeZone   Era         UTCTimeMark
-        private static DS[][] dateParsingStates = {
+        private static readonly DS[][] dateParsingStates = {
 // DS.BEGIN                                                                             // DS.BEGIN
 new DS[] { DS.BEGIN, DS.ERROR,   DS.TX_N,    DS.N,       DS.D_Nd,    DS.T_Nt,    DS.ERROR,   DS.D_M,     DS.D_M,     DS.D_S,     DS.T_S,         DS.BEGIN,     DS.D_Y,     DS.D_Y,     DS.ERROR,   DS.BEGIN,  DS.BEGIN,    DS.ERROR},
 
@@ -5246,6 +5246,7 @@ private static void Trace(string s)
             // Internal.Console.WriteLine(s);
         }
 
+        // for testing; do not make this readonly
         private static bool _tracingEnabled = false;
 #endif // _LOGGING
     }
@@ -5274,10 +5275,10 @@ internal ref struct __DTString
         // The current character to be looked at.
         internal char m_current;
 
-        private CompareInfo m_info;
+        private readonly CompareInfo m_info;
         // Flag to indicate if we encouter an digit, we should check for token or not.
         // In some cultures, such as mn-MN, it uses "\x0031\x00a0\x0434\x04af\x0433\x044d\x044d\x0440\x00a0\x0441\x0430\x0440" in month names.
-        private bool m_checkDigitToken;
+        private readonly bool m_checkDigitToken;
 
         internal __DTString(ReadOnlySpan<char> str, DateTimeFormatInfo dtfi, bool checkDigitToken) : this(str, dtfi)
         {
diff --git a/src/System.Private.CoreLib/shared/System/Globalization/HebrewNumber.cs b/src/System.Private.CoreLib/shared/System/Globalization/HebrewNumber.cs
index a2054377d5dd..2eaf58ac2ab6 100644
--- a/src/System.Private.CoreLib/shared/System/Globalization/HebrewNumber.cs
+++ b/src/System.Private.CoreLib/shared/System/Globalization/HebrewNumber.cs
@@ -282,7 +282,7 @@ internal HebrewValue(HebrewToken token, short value)
         };
 
         private const int minHebrewNumberCh = 0x05d0;
-        private static char s_maxHebrewNumberCh = (char)(minHebrewNumberCh + s_hebrewValues.Length - 1);
+        private static readonly char s_maxHebrewNumberCh = (char)(minHebrewNumberCh + s_hebrewValues.Length - 1);
 
         ////////////////////////////////////////////////////////////////////////////
         //
diff --git a/src/System.Private.CoreLib/shared/System/Globalization/IdnMapping.cs b/src/System.Private.CoreLib/shared/System/Globalization/IdnMapping.cs
index 5e54571815fe..4925872348e6 100644
--- a/src/System.Private.CoreLib/shared/System/Globalization/IdnMapping.cs
+++ b/src/System.Private.CoreLib/shared/System/Globalization/IdnMapping.cs
@@ -183,7 +183,7 @@ private static unsafe string GetStringForOutput(string originalString, char* inp
 
 
         // Legal "dot" separators (i.e: . in www.microsoft.com)
-        private static char[] c_Dots = { '.', '\u3002', '\uFF0E', '\uFF61' };
+        private static readonly char[] c_Dots = { '.', '\u3002', '\uFF0E', '\uFF61' };
 
         private string GetAsciiInvariant(string unicode, int index, int count)
         {
diff --git a/src/System.Private.CoreLib/shared/System/Globalization/SortVersion.cs b/src/System.Private.CoreLib/shared/System/Globalization/SortVersion.cs
index d99a999cb50e..f2b8e56675e0 100644
--- a/src/System.Private.CoreLib/shared/System/Globalization/SortVersion.cs
+++ b/src/System.Private.CoreLib/shared/System/Globalization/SortVersion.cs
@@ -13,7 +13,7 @@ public sealed class SortVersion :
         IEquatable<SortVersion>
 #nullable restore
     {
-        private int m_NlsVersion; // Do not rename (binary serialization)
+        private readonly int m_NlsVersion; // Do not rename (binary serialization)
         private Guid m_SortId; // Do not rename (binary serialization)
 
         public int FullVersion => m_NlsVersion;
diff --git a/src/System.Private.CoreLib/shared/System/Globalization/TaiwanCalendar.cs b/src/System.Private.CoreLib/shared/System/Globalization/TaiwanCalendar.cs
index a0f64c9df25a..d9bd22153805 100644
--- a/src/System.Private.CoreLib/shared/System/Globalization/TaiwanCalendar.cs
+++ b/src/System.Private.CoreLib/shared/System/Globalization/TaiwanCalendar.cs
@@ -25,7 +25,7 @@ public class TaiwanCalendar : Calendar
         // When Gregorian Year 1912 is year 1, so that
         //    1912 = 1 + yearOffset
         //  So yearOffset = 1911
-        private static EraInfo[] s_taiwanEraInfo = new EraInfo[]
+        private static readonly EraInfo[] s_taiwanEraInfo = new EraInfo[]
         {
             new EraInfo( 1, 1912, 1, 1, 1911, 1, GregorianCalendar.MaxYear - 1911)    // era #, start year/month/day, yearOffset, minEraYear
         };
diff --git a/src/System.Private.CoreLib/shared/System/Globalization/TimeSpanParse.cs b/src/System.Private.CoreLib/shared/System/Globalization/TimeSpanParse.cs
index 20669bf13753..7dfac05aaddc 100644
--- a/src/System.Private.CoreLib/shared/System/Globalization/TimeSpanParse.cs
+++ b/src/System.Private.CoreLib/shared/System/Globalization/TimeSpanParse.cs
@@ -153,7 +153,7 @@ public bool NormalizeAndValidateFraction()
 
         private ref struct TimeSpanTokenizer
         {
-            private ReadOnlySpan<char> _value;
+            private readonly ReadOnlySpan<char> _value;
             private int _pos;
 
             internal TimeSpanTokenizer(ReadOnlySpan<char> input) : this(input, 0) { }
diff --git a/src/System.Private.CoreLib/shared/System/IO/BinaryReader.cs b/src/System.Private.CoreLib/shared/System/IO/BinaryReader.cs
index 3f8055a4ab78..e684940b93a7 100644
--- a/src/System.Private.CoreLib/shared/System/IO/BinaryReader.cs
+++ b/src/System.Private.CoreLib/shared/System/IO/BinaryReader.cs
@@ -31,12 +31,12 @@ public class BinaryReader : IDisposable
         private readonly Decoder _decoder;
         private byte[]? _charBytes;
         private char[]? _charBuffer;
-        private int _maxCharsSize;  // From MaxCharBytesSize & Encoding
+        private readonly int _maxCharsSize;  // From MaxCharBytesSize & Encoding
 
         // Performance optimization for Read() w/ Unicode.  Speeds us up by ~40%
-        private bool _2BytesPerChar;
-        private bool _isMemoryStream; // "do we sit on MemoryStream?" for Read/ReadInt32 perf
-        private bool _leaveOpen;
+        private readonly bool _2BytesPerChar;
+        private readonly bool _isMemoryStream; // "do we sit on MemoryStream?" for Read/ReadInt32 perf
+        private readonly bool _leaveOpen;
         private bool _disposed;
 
         public BinaryReader(Stream input) : this(input, Encoding.UTF8, false)
diff --git a/src/System.Private.CoreLib/shared/System/IO/BinaryWriter.cs b/src/System.Private.CoreLib/shared/System/IO/BinaryWriter.cs
index 1f20a5450f14..f0de8736cfa7 100644
--- a/src/System.Private.CoreLib/shared/System/IO/BinaryWriter.cs
+++ b/src/System.Private.CoreLib/shared/System/IO/BinaryWriter.cs
@@ -22,7 +22,7 @@ public class BinaryWriter : IDisposable, IAsyncDisposable
         private readonly Encoding _encoding;
         private readonly Encoder _encoder;
 
-        private bool _leaveOpen;
+        private readonly bool _leaveOpen;
 
         // Perf optimization stuff
         private byte[]? _largeByteBuffer;  // temp space for writing chars.
diff --git a/src/System.Private.CoreLib/shared/System/IO/MemoryStream.cs b/src/System.Private.CoreLib/shared/System/IO/MemoryStream.cs
index 038d96fc4f77..28ff0399b47b 100644
--- a/src/System.Private.CoreLib/shared/System/IO/MemoryStream.cs
+++ b/src/System.Private.CoreLib/shared/System/IO/MemoryStream.cs
@@ -21,7 +21,7 @@ namespace System.IO
     public class MemoryStream : Stream
     {
         private byte[] _buffer;    // Either allocated internally or externally.
-        private int _origin;       // For user-provided arrays, start at this origin
+        private readonly int _origin;       // For user-provided arrays, start at this origin
         private int _position;     // read/write head.
         private int _length;       // Number of bytes within the memory stream
         private int _capacity;     // length of usable portion of buffer for stream
@@ -29,7 +29,7 @@ public class MemoryStream : Stream
 
         private bool _expandable;  // User-provided buffers aren't expandable.
         private bool _writable;    // Can user write to this stream?
-        private bool _exposable;   // Whether the array can be returned to the user.
+        private readonly bool _exposable;   // Whether the array can be returned to the user.
         private bool _isOpen;      // Is this stream open or closed?
 
         private Task<int>? _lastReadTask; // The last successful task returned from ReadAsync
diff --git a/src/System.Private.CoreLib/shared/System/IO/PinnedBufferMemoryStream.cs b/src/System.Private.CoreLib/shared/System/IO/PinnedBufferMemoryStream.cs
index 5e663510cd71..6b60ce76a363 100644
--- a/src/System.Private.CoreLib/shared/System/IO/PinnedBufferMemoryStream.cs
+++ b/src/System.Private.CoreLib/shared/System/IO/PinnedBufferMemoryStream.cs
@@ -22,7 +22,7 @@ namespace System.IO
 {
     internal sealed unsafe class PinnedBufferMemoryStream : UnmanagedMemoryStream
     {
-        private byte[] _array;
+        private readonly byte[] _array;
         private GCHandle _pinningHandle;
 
         internal PinnedBufferMemoryStream(byte[] array)
diff --git a/src/System.Private.CoreLib/shared/System/IO/Stream.cs b/src/System.Private.CoreLib/shared/System/IO/Stream.cs
index 0db1fc161cf2..6910689ee31b 100644
--- a/src/System.Private.CoreLib/shared/System/IO/Stream.cs
+++ b/src/System.Private.CoreLib/shared/System/IO/Stream.cs
@@ -1026,10 +1026,10 @@ private sealed class SynchronousAsyncResult : IAsyncResult
             private readonly object? _stateObject;
             private readonly bool _isWrite;
             private ManualResetEvent? _waitHandle;
-            private ExceptionDispatchInfo? _exceptionInfo;
+            private readonly ExceptionDispatchInfo? _exceptionInfo;
 
             private bool _endXxxCalled;
-            private int _bytesRead;
+            private readonly int _bytesRead;
 
             internal SynchronousAsyncResult(int bytesRead, object? asyncStateObject)
             {
@@ -1114,7 +1114,7 @@ internal static void EndWrite(IAsyncResult asyncResult)
         // a lock for every operation making it thread safe.
         private sealed class SyncStream : Stream, IDisposable
         {
-            private Stream _stream;
+            private readonly Stream _stream;
 
             internal SyncStream(Stream stream)
             {
diff --git a/src/System.Private.CoreLib/shared/System/IO/StreamReader.cs b/src/System.Private.CoreLib/shared/System/IO/StreamReader.cs
index 6695684871ae..9fcfc80fab7b 100644
--- a/src/System.Private.CoreLib/shared/System/IO/StreamReader.cs
+++ b/src/System.Private.CoreLib/shared/System/IO/StreamReader.cs
@@ -67,7 +67,7 @@ public class StreamReader : TextReader
         // The intent of this field is to leave open the underlying stream when
         // disposing of this StreamReader.  A name like _leaveOpen is better,
         // but this type is serializable, and this field's name was _closable.
-        private bool _closable;  // Whether to close the underlying stream.
+        private readonly bool _closable;  // Whether to close the underlying stream.
 
         // We don't guarantee thread safety on StreamReader, but we should at
         // least prevent users from trying to read anything while an Async
diff --git a/src/System.Private.CoreLib/shared/System/IO/TextWriter.cs b/src/System.Private.CoreLib/shared/System/IO/TextWriter.cs
index 9dfb1405a943..7a1d707ffd70 100644
--- a/src/System.Private.CoreLib/shared/System/IO/TextWriter.cs
+++ b/src/System.Private.CoreLib/shared/System/IO/TextWriter.cs
@@ -37,7 +37,7 @@ public abstract partial class TextWriter : MarshalByRefObject, IDisposable, IAsy
         private string CoreNewLineStr = Environment.NewLine;
 
         // Can be null - if so, ask for the Thread's CurrentCulture every time.
-        private IFormatProvider? _internalFormatProvider;
+        private readonly IFormatProvider? _internalFormatProvider;
 
         protected TextWriter()
         {
diff --git a/src/System.Private.CoreLib/shared/System/IO/UnmanagedMemoryStreamWrapper.cs b/src/System.Private.CoreLib/shared/System/IO/UnmanagedMemoryStreamWrapper.cs
index 72dcd15b6fa2..9afb7d3c8779 100644
--- a/src/System.Private.CoreLib/shared/System/IO/UnmanagedMemoryStreamWrapper.cs
+++ b/src/System.Private.CoreLib/shared/System/IO/UnmanagedMemoryStreamWrapper.cs
@@ -21,7 +21,7 @@ namespace System.IO
     // UnmanagedMemoryStream from ResourceReader.
     internal sealed class UnmanagedMemoryStreamWrapper : MemoryStream
     {
-        private UnmanagedMemoryStream _unmanagedStream;
+        private readonly UnmanagedMemoryStream _unmanagedStream;
 
         internal UnmanagedMemoryStreamWrapper(UnmanagedMemoryStream stream)
         {
diff --git a/src/System.Private.CoreLib/shared/System/Math.cs b/src/System.Private.CoreLib/shared/System/Math.cs
index 18d15653f078..cdb11e6f0088 100644
--- a/src/System.Private.CoreLib/shared/System/Math.cs
+++ b/src/System.Private.CoreLib/shared/System/Math.cs
@@ -35,7 +35,7 @@ public static partial class Math
         private const double doubleRoundLimit = 1e16d;
 
         // This table is required for the Round function which can specify the number of digits to round to
-        private static double[] roundPower10Double = new double[] {
+        private static readonly double[] roundPower10Double = new double[] {
           1E0, 1E1, 1E2, 1E3, 1E4, 1E5, 1E6, 1E7, 1E8,
           1E9, 1E10, 1E11, 1E12, 1E13, 1E14, 1E15
         };
diff --git a/src/System.Private.CoreLib/shared/System/MathF.cs b/src/System.Private.CoreLib/shared/System/MathF.cs
index dfc5b59de41c..382af77a5aa1 100644
--- a/src/System.Private.CoreLib/shared/System/MathF.cs
+++ b/src/System.Private.CoreLib/shared/System/MathF.cs
@@ -28,7 +28,7 @@ public static partial class MathF
         private const int maxRoundingDigits = 6;
 
         // This table is required for the Round function which can specify the number of digits to round to
-        private static float[] roundPower10Single = new float[] {
+        private static readonly float[] roundPower10Single = new float[] {
             1e0f, 1e1f, 1e2f, 1e3f, 1e4f, 1e5f, 1e6f
         };
 
diff --git a/src/System.Private.CoreLib/shared/System/NotFiniteNumberException.cs b/src/System.Private.CoreLib/shared/System/NotFiniteNumberException.cs
index 53a5817a7dc7..471e893f3c2d 100644
--- a/src/System.Private.CoreLib/shared/System/NotFiniteNumberException.cs
+++ b/src/System.Private.CoreLib/shared/System/NotFiniteNumberException.cs
@@ -10,7 +10,7 @@ namespace System
     [System.Runtime.CompilerServices.TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
     public class NotFiniteNumberException : ArithmeticException
     {
-        private double _offendingNumber;
+        private readonly double _offendingNumber;
 
         public NotFiniteNumberException()
             : base(SR.Arg_NotFiniteNumberException)
diff --git a/src/System.Private.CoreLib/shared/System/Number.NumberToFloatingPointBits.cs b/src/System.Private.CoreLib/shared/System/Number.NumberToFloatingPointBits.cs
index ac4c7727f323..54f081ddf071 100644
--- a/src/System.Private.CoreLib/shared/System/Number.NumberToFloatingPointBits.cs
+++ b/src/System.Private.CoreLib/shared/System/Number.NumberToFloatingPointBits.cs
@@ -64,7 +64,7 @@ public FloatingPointInfo(ushort denormalMantissaBits, ushort exponentBits, int m
             }
         }
 
-        private static float[] s_Pow10SingleTable = new float[]
+        private static readonly float[] s_Pow10SingleTable = new float[]
         {
             1e0f,   // 10^0
             1e1f,   // 10^1
@@ -79,7 +79,7 @@ public FloatingPointInfo(ushort denormalMantissaBits, ushort exponentBits, int m
             1e10f,  // 10^10
         };
 
-        private static double[] s_Pow10DoubleTable = new double[]
+        private static readonly double[] s_Pow10DoubleTable = new double[]
         {
             1e0,    // 10^0
             1e1,    // 10^1
diff --git a/src/System.Private.CoreLib/shared/System/ObjectDisposedException.cs b/src/System.Private.CoreLib/shared/System/ObjectDisposedException.cs
index f2bd7e7028f4..a80522af297f 100644
--- a/src/System.Private.CoreLib/shared/System/ObjectDisposedException.cs
+++ b/src/System.Private.CoreLib/shared/System/ObjectDisposedException.cs
@@ -15,7 +15,7 @@ namespace System
     [TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
     public class ObjectDisposedException : InvalidOperationException
     {
-        private string? _objectName;
+        private readonly string? _objectName;
 
         // This constructor should only be called by the EE (COMPlusThrow)
         private ObjectDisposedException() :
diff --git a/src/System.Private.CoreLib/shared/System/ObsoleteAttribute.cs b/src/System.Private.CoreLib/shared/System/ObsoleteAttribute.cs
index ad6c4eb028b7..b195bb898b11 100644
--- a/src/System.Private.CoreLib/shared/System/ObsoleteAttribute.cs
+++ b/src/System.Private.CoreLib/shared/System/ObsoleteAttribute.cs
@@ -24,8 +24,8 @@ namespace System
         Inherited = false)]
     public sealed class ObsoleteAttribute : Attribute
     {
-        private string? _message;
-        private bool _error;
+        private readonly string? _message;
+        private readonly bool _error;
 
         public ObsoleteAttribute()
         {
diff --git a/src/System.Private.CoreLib/shared/System/Random.cs b/src/System.Private.CoreLib/shared/System/Random.cs
index 06ec46e5240a..2d5ddb0dad2c 100644
--- a/src/System.Private.CoreLib/shared/System/Random.cs
+++ b/src/System.Private.CoreLib/shared/System/Random.cs
@@ -17,7 +17,7 @@ public class Random
         //
         private int _inext;
         private int _inextp;
-        private int[] _seedArray = new int[56];
+        private readonly int[] _seedArray = new int[56];
 
         //
         // Public Constants
diff --git a/src/System.Private.CoreLib/shared/System/Reflection/AssemblyFlagsAttribute.cs b/src/System.Private.CoreLib/shared/System/Reflection/AssemblyFlagsAttribute.cs
index 1240a669e6fe..df78adb29742 100644
--- a/src/System.Private.CoreLib/shared/System/Reflection/AssemblyFlagsAttribute.cs
+++ b/src/System.Private.CoreLib/shared/System/Reflection/AssemblyFlagsAttribute.cs
@@ -7,7 +7,7 @@ namespace System.Reflection
     [AttributeUsage(AttributeTargets.Assembly, Inherited = false)]
     public sealed class AssemblyFlagsAttribute : Attribute
     {
-        private AssemblyNameFlags _flags;
+        private readonly AssemblyNameFlags _flags;
 
         [Obsolete("This constructor has been deprecated. Please use AssemblyFlagsAttribute(AssemblyNameFlags) instead. https://go.microsoft.com/fwlink/?linkid=14202")]
         [CLSCompliant(false)]
diff --git a/src/System.Private.CoreLib/shared/System/Resources/FileBasedResourceGroveler.cs b/src/System.Private.CoreLib/shared/System/Resources/FileBasedResourceGroveler.cs
index 9334e0a2fd85..afcdd293efb8 100644
--- a/src/System.Private.CoreLib/shared/System/Resources/FileBasedResourceGroveler.cs
+++ b/src/System.Private.CoreLib/shared/System/Resources/FileBasedResourceGroveler.cs
@@ -23,7 +23,7 @@ namespace System.Resources
 {
     internal class FileBasedResourceGroveler : IResourceGroveler
     {
-        private ResourceManager.ResourceManagerMediator _mediator;
+        private readonly ResourceManager.ResourceManagerMediator _mediator;
 
         public FileBasedResourceGroveler(ResourceManager.ResourceManagerMediator mediator)
         {
diff --git a/src/System.Private.CoreLib/shared/System/Resources/ManifestBasedResourceGroveler.cs b/src/System.Private.CoreLib/shared/System/Resources/ManifestBasedResourceGroveler.cs
index e23fc138db8d..865d2debb49f 100644
--- a/src/System.Private.CoreLib/shared/System/Resources/ManifestBasedResourceGroveler.cs
+++ b/src/System.Private.CoreLib/shared/System/Resources/ManifestBasedResourceGroveler.cs
@@ -41,7 +41,7 @@ namespace System.Resources
     //
     internal partial class ManifestBasedResourceGroveler : IResourceGroveler
     {
-        private ResourceManager.ResourceManagerMediator _mediator;
+        private readonly ResourceManager.ResourceManagerMediator _mediator;
 
         public ManifestBasedResourceGroveler(ResourceManager.ResourceManagerMediator mediator)
         {
diff --git a/src/System.Private.CoreLib/shared/System/Resources/MissingSatelliteAssemblyException.cs b/src/System.Private.CoreLib/shared/System/Resources/MissingSatelliteAssemblyException.cs
index c092bac47c63..139336270494 100644
--- a/src/System.Private.CoreLib/shared/System/Resources/MissingSatelliteAssemblyException.cs
+++ b/src/System.Private.CoreLib/shared/System/Resources/MissingSatelliteAssemblyException.cs
@@ -23,7 +23,7 @@ namespace System.Resources
     [System.Runtime.CompilerServices.TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
     public class MissingSatelliteAssemblyException : SystemException
     {
-        private string? _cultureName;
+        private readonly string? _cultureName;
 
         public MissingSatelliteAssemblyException()
             : base(SR.MissingSatelliteAssembly_Default)
diff --git a/src/System.Private.CoreLib/shared/System/Resources/ResourceFallbackManager.cs b/src/System.Private.CoreLib/shared/System/Resources/ResourceFallbackManager.cs
index a678e0171e52..70c8d8cc6b62 100644
--- a/src/System.Private.CoreLib/shared/System/Resources/ResourceFallbackManager.cs
+++ b/src/System.Private.CoreLib/shared/System/Resources/ResourceFallbackManager.cs
@@ -26,9 +26,9 @@ namespace System.Resources
 {
     internal class ResourceFallbackManager : IEnumerable<CultureInfo>
     {
-        private CultureInfo m_startingCulture;
-        private CultureInfo? m_neutralResourcesCulture;
-        private bool m_useParents;
+        private readonly CultureInfo m_startingCulture;
+        private readonly CultureInfo? m_neutralResourcesCulture;
+        private readonly bool m_useParents;
 
         internal ResourceFallbackManager(CultureInfo? startingCulture, CultureInfo? neutralResourcesCulture, bool useParents)
         {
diff --git a/src/System.Private.CoreLib/shared/System/Resources/ResourceManager.cs b/src/System.Private.CoreLib/shared/System/Resources/ResourceManager.cs
index a1a8b22afbb2..3a2ea1aee7d8 100644
--- a/src/System.Private.CoreLib/shared/System/Resources/ResourceManager.cs
+++ b/src/System.Private.CoreLib/shared/System/Resources/ResourceManager.cs
@@ -102,9 +102,9 @@ internal class CultureNameResourceSetPair
         protected Assembly? MainAssembly;    // Need the assembly manifest sometimes.
 
         private Dictionary<string, ResourceSet>? _resourceSets;
-        private string? _moduleDir;          // For assembly-ignorant directory location
-        private Type? _locationInfo;         // For Assembly or type-based directory layout
-        private Type? _userResourceSet;      // Which ResourceSet instance to create
+        private readonly string? _moduleDir;          // For assembly-ignorant directory location
+        private readonly Type? _locationInfo;         // For Assembly or type-based directory layout
+        private readonly Type? _userResourceSet;      // Which ResourceSet instance to create
         private CultureInfo? _neutralResourcesCulture;  // For perf optimizations.
 
         private CultureNameResourceSetPair? _lastUsedResourceCache;
@@ -761,7 +761,7 @@ internal static bool IsDefaultType(string asmTypeName,
 
         internal class ResourceManagerMediator
         {
-            private ResourceManager _rm;
+            private readonly ResourceManager _rm;
 
             internal ResourceManagerMediator(ResourceManager rm)
             {
diff --git a/src/System.Private.CoreLib/shared/System/Resources/ResourceReader.cs b/src/System.Private.CoreLib/shared/System/Resources/ResourceReader.cs
index c0b2f5d4a9df..f5caa42bfd93 100644
--- a/src/System.Private.CoreLib/shared/System/Resources/ResourceReader.cs
+++ b/src/System.Private.CoreLib/shared/System/Resources/ResourceReader.cs
@@ -993,7 +993,7 @@ internal sealed class ResourceEnumerator : IDictionaryEnumerator
             private const int ENUM_DONE = int.MinValue;
             private const int ENUM_NOT_STARTED = -1;
 
-            private ResourceReader _reader;
+            private readonly ResourceReader _reader;
             private bool _currentIsValid;
             private int _currentName;
             private int _dataPosition; // cached for case-insensitive table
diff --git a/src/System.Private.CoreLib/shared/System/Runtime/CompilerServices/DateTimeConstantAttribute.cs b/src/System.Private.CoreLib/shared/System/Runtime/CompilerServices/DateTimeConstantAttribute.cs
index ee9250b2d97c..315e9e3b9383 100644
--- a/src/System.Private.CoreLib/shared/System/Runtime/CompilerServices/DateTimeConstantAttribute.cs
+++ b/src/System.Private.CoreLib/shared/System/Runtime/CompilerServices/DateTimeConstantAttribute.cs
@@ -7,7 +7,7 @@ namespace System.Runtime.CompilerServices
     [AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter, Inherited = false)]
     public sealed class DateTimeConstantAttribute : CustomConstantAttribute
     {
-        private DateTime _date;
+        private readonly DateTime _date;
 
         public DateTimeConstantAttribute(long ticks)
         {
diff --git a/src/System.Private.CoreLib/shared/System/Runtime/CompilerServices/DecimalConstantAttribute.cs b/src/System.Private.CoreLib/shared/System/Runtime/CompilerServices/DecimalConstantAttribute.cs
index 521a3abe9cb6..abfab1cbebf0 100644
--- a/src/System.Private.CoreLib/shared/System/Runtime/CompilerServices/DecimalConstantAttribute.cs
+++ b/src/System.Private.CoreLib/shared/System/Runtime/CompilerServices/DecimalConstantAttribute.cs
@@ -9,7 +9,7 @@ namespace System.Runtime.CompilerServices
     [AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter, Inherited = false)]
     public sealed class DecimalConstantAttribute : Attribute
     {
-        private decimal _dec;
+        private readonly decimal _dec;
 
         [CLSCompliant(false)]
         public DecimalConstantAttribute(
diff --git a/src/System.Private.CoreLib/shared/System/Runtime/InteropServices/ArrayWithOffset.cs b/src/System.Private.CoreLib/shared/System/Runtime/InteropServices/ArrayWithOffset.cs
index 600b7748589e..5d7396b92c5d 100644
--- a/src/System.Private.CoreLib/shared/System/Runtime/InteropServices/ArrayWithOffset.cs
+++ b/src/System.Private.CoreLib/shared/System/Runtime/InteropServices/ArrayWithOffset.cs
@@ -13,9 +13,9 @@ namespace System.Runtime.InteropServices
 {
     public struct ArrayWithOffset
     {
-        private object? m_array;
-        private int m_offset;
-        private int m_count;
+        private readonly object? m_array;
+        private readonly int m_offset;
+        private readonly int m_count;
 
         // From MAX_SIZE_FOR_INTEROP in mlinfo.h
         private const int MaxSizeForInterop = 0x7ffffff0;
diff --git a/src/System.Private.CoreLib/shared/System/Runtime/MemoryFailPoint.cs b/src/System.Private.CoreLib/shared/System/Runtime/MemoryFailPoint.cs
index 3d1f0caafad6..9931ad75dc2d 100644
--- a/src/System.Private.CoreLib/shared/System/Runtime/MemoryFailPoint.cs
+++ b/src/System.Private.CoreLib/shared/System/Runtime/MemoryFailPoint.cs
@@ -134,7 +134,7 @@ private static long LastTimeCheckingAddressSpace
         // in the critical finalizer.
         private static long s_failPointReservedMemory;
 
-        private ulong _reservedMemory;  // The size of this request (from user)
+        private readonly ulong _reservedMemory;  // The size of this request (from user)
         private bool _mustSubtractReservation; // Did we add data to SharedStatics?
 
         // We can remove this link demand in a future version - we will
@@ -358,16 +358,16 @@ internal static ulong MemoryFailPointReservedMemory
         [Serializable]
         internal sealed class MemoryFailPointState
         {
-            private ulong _segmentSize;
-            private int _allocationSizeInMB;
-            private bool _needPageFile;
-            private bool _needAddressSpace;
-            private bool _needContiguousVASpace;
-            private ulong _availPageFile;
-            private ulong _totalFreeAddressSpace;
-            private long _lastKnownFreeAddressSpace;
-            private ulong _reservedMem;
-            private string _stackTrace;  // Where did we fail, for additional debugging.
+            private readonly ulong _segmentSize;
+            private readonly int _allocationSizeInMB;
+            private readonly bool _needPageFile;
+            private readonly bool _needAddressSpace;
+            private readonly bool _needContiguousVASpace;
+            private readonly ulong _availPageFile;
+            private readonly ulong _totalFreeAddressSpace;
+            private readonly long _lastKnownFreeAddressSpace;
+            private readonly ulong _reservedMem;
+            private readonly string _stackTrace;  // Where did we fail, for additional debugging.
 
             internal MemoryFailPointState(int allocationSizeInMB, ulong segmentSize, bool needPageFile, bool needAddressSpace, bool needContiguousVASpace, ulong availPageFile, ulong totalFreeAddressSpace, long lastKnownFreeAddressSpace, ulong reservedMem)
             {
diff --git a/src/System.Private.CoreLib/shared/System/Runtime/Remoting/ObjectHandle.cs b/src/System.Private.CoreLib/shared/System/Runtime/Remoting/ObjectHandle.cs
index 993b14c143e9..b5294ba80165 100644
--- a/src/System.Private.CoreLib/shared/System/Runtime/Remoting/ObjectHandle.cs
+++ b/src/System.Private.CoreLib/shared/System/Runtime/Remoting/ObjectHandle.cs
@@ -6,7 +6,7 @@ namespace System.Runtime.Remoting
 {
     public class ObjectHandle : MarshalByRefObject
     {
-        private object? _wrappedObject;
+        private readonly object? _wrappedObject;
 
         private ObjectHandle()
         {
diff --git a/src/System.Private.CoreLib/shared/System/Runtime/Serialization/SerializationInfo.cs b/src/System.Private.CoreLib/shared/System/Runtime/Serialization/SerializationInfo.cs
index 999bcfb9936f..3a603d252718 100644
--- a/src/System.Private.CoreLib/shared/System/Runtime/Serialization/SerializationInfo.cs
+++ b/src/System.Private.CoreLib/shared/System/Runtime/Serialization/SerializationInfo.cs
@@ -20,8 +20,8 @@ public sealed class SerializationInfo
         private object?[] _values;
         private Type[] _types;
         private int _count;
-        private Dictionary<string, int> _nameToIndex;
-        private IFormatterConverter _converter;
+        private readonly Dictionary<string, int> _nameToIndex;
+        private readonly IFormatterConverter _converter;
         private string _rootTypeName;
         private string _rootTypeAssemblyName;
         private Type _rootType;
diff --git a/src/System.Private.CoreLib/shared/System/Runtime/Versioning/TargetFrameworkAttribute.cs b/src/System.Private.CoreLib/shared/System/Runtime/Versioning/TargetFrameworkAttribute.cs
index 91b3df6ebe0c..df3011e9be22 100644
--- a/src/System.Private.CoreLib/shared/System/Runtime/Versioning/TargetFrameworkAttribute.cs
+++ b/src/System.Private.CoreLib/shared/System/Runtime/Versioning/TargetFrameworkAttribute.cs
@@ -17,7 +17,7 @@ namespace System.Runtime.Versioning
     [AttributeUsageAttribute(AttributeTargets.Assembly, AllowMultiple = false, Inherited = false)]
     public sealed class TargetFrameworkAttribute : Attribute
     {
-        private string _frameworkName;  // A target framework moniker
+        private readonly string _frameworkName;  // A target framework moniker
         private string? _frameworkDisplayName;
 
         // The frameworkName parameter is intended to be the string form of a FrameworkName instance.
diff --git a/src/System.Private.CoreLib/shared/System/StringComparer.cs b/src/System.Private.CoreLib/shared/System/StringComparer.cs
index 28242fddb53a..f55a098306c7 100644
--- a/src/System.Private.CoreLib/shared/System/StringComparer.cs
+++ b/src/System.Private.CoreLib/shared/System/StringComparer.cs
@@ -173,7 +173,7 @@ public sealed class CultureAwareComparer : StringComparer, ISerializable
         private const CompareOptions ValidCompareMaskOffFlags = ~(CompareOptions.IgnoreCase | CompareOptions.IgnoreSymbols | CompareOptions.IgnoreNonSpace | CompareOptions.IgnoreWidth | CompareOptions.IgnoreKanaType | CompareOptions.StringSort);
 
         private readonly CompareInfo _compareInfo; // Do not rename (binary serialization)
-        private CompareOptions _options;
+        private readonly CompareOptions _options;
 
         internal CultureAwareComparer(CultureInfo culture, CompareOptions options) : this(culture.CompareInfo, options) { }
 
diff --git a/src/System.Private.CoreLib/shared/System/Text/DecoderBestFitFallback.cs b/src/System.Private.CoreLib/shared/System/Text/DecoderBestFitFallback.cs
index d69d1c7f7450..2a1edefc229f 100644
--- a/src/System.Private.CoreLib/shared/System/Text/DecoderBestFitFallback.cs
+++ b/src/System.Private.CoreLib/shared/System/Text/DecoderBestFitFallback.cs
@@ -59,7 +59,7 @@ internal sealed class InternalDecoderBestFitFallbackBuffer : DecoderFallbackBuff
         private char _cBestFit = '\0';
         private int _iCount = -1;
         private int _iSize;
-        private InternalDecoderBestFitFallback _oFallback;
+        private readonly InternalDecoderBestFitFallback _oFallback;
 
         // Private object for locking instead of locking on a public type for SQL reliability work.
         private static object? s_InternalSyncObject;
diff --git a/src/System.Private.CoreLib/shared/System/Text/DecoderExceptionFallback.cs b/src/System.Private.CoreLib/shared/System/Text/DecoderExceptionFallback.cs
index 865f2ea1c1f4..a1cce71174ad 100644
--- a/src/System.Private.CoreLib/shared/System/Text/DecoderExceptionFallback.cs
+++ b/src/System.Private.CoreLib/shared/System/Text/DecoderExceptionFallback.cs
@@ -106,8 +106,8 @@ private void Throw(byte[] bytesUnknown, int index)
     [System.Runtime.CompilerServices.TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
     public sealed class DecoderFallbackException : ArgumentException
     {
-        private byte[]? _bytesUnknown = null;
-        private int _index = 0;
+        private readonly byte[]? _bytesUnknown = null;
+        private readonly int _index = 0;
 
         public DecoderFallbackException()
             : base(SR.Arg_ArgumentException)
diff --git a/src/System.Private.CoreLib/shared/System/Text/DecoderNLS.cs b/src/System.Private.CoreLib/shared/System/Text/DecoderNLS.cs
index 7ccaead4d205..20c40c35f8ec 100644
--- a/src/System.Private.CoreLib/shared/System/Text/DecoderNLS.cs
+++ b/src/System.Private.CoreLib/shared/System/Text/DecoderNLS.cs
@@ -22,7 +22,7 @@ namespace System.Text
     internal class DecoderNLS : Decoder
     {
         // Remember our encoding
-        private Encoding _encoding;
+        private readonly Encoding _encoding;
         private bool _mustFlush;
         internal bool _throwOnOverflow;
         internal int _bytesUsed;
diff --git a/src/System.Private.CoreLib/shared/System/Text/DecoderReplacementFallback.cs b/src/System.Private.CoreLib/shared/System/Text/DecoderReplacementFallback.cs
index a609b31656a5..bbb15d45f6ff 100644
--- a/src/System.Private.CoreLib/shared/System/Text/DecoderReplacementFallback.cs
+++ b/src/System.Private.CoreLib/shared/System/Text/DecoderReplacementFallback.cs
@@ -9,7 +9,7 @@ namespace System.Text
     public sealed class DecoderReplacementFallback : DecoderFallback
     {
         // Our variables
-        private string _strDefault;
+        private readonly string _strDefault;
 
         // Construction.  Default replacement fallback uses no best fit and ? replacement string
         public DecoderReplacementFallback() : this("?")
@@ -102,7 +102,7 @@ public override int GetHashCode()
     public sealed class DecoderReplacementFallbackBuffer : DecoderFallbackBuffer
     {
         // Store our default string
-        private string _strDefault;
+        private readonly string _strDefault;
         private int _fallbackCount = -1;
         private int _fallbackIndex = -1;
 
diff --git a/src/System.Private.CoreLib/shared/System/Text/EncoderBestFitFallback.cs b/src/System.Private.CoreLib/shared/System/Text/EncoderBestFitFallback.cs
index eb443e6dcd8a..2cc35e789a50 100644
--- a/src/System.Private.CoreLib/shared/System/Text/EncoderBestFitFallback.cs
+++ b/src/System.Private.CoreLib/shared/System/Text/EncoderBestFitFallback.cs
@@ -57,7 +57,7 @@ internal sealed class InternalEncoderBestFitFallbackBuffer : EncoderFallbackBuff
     {
         // Our variables
         private char _cBestFit = '\0';
-        private InternalEncoderBestFitFallback _oFallback;
+        private readonly InternalEncoderBestFitFallback _oFallback;
         private int _iCount = -1;
         private int _iSize;
 
diff --git a/src/System.Private.CoreLib/shared/System/Text/EncoderExceptionFallback.cs b/src/System.Private.CoreLib/shared/System/Text/EncoderExceptionFallback.cs
index e55cfb6c679e..3df6accef314 100644
--- a/src/System.Private.CoreLib/shared/System/Text/EncoderExceptionFallback.cs
+++ b/src/System.Private.CoreLib/shared/System/Text/EncoderExceptionFallback.cs
@@ -99,10 +99,10 @@ public override int Remaining
     [System.Runtime.CompilerServices.TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
     public sealed class EncoderFallbackException : ArgumentException
     {
-        private char _charUnknown;
-        private char _charUnknownHigh;
-        private char _charUnknownLow;
-        private int _index;
+        private readonly char _charUnknown;
+        private readonly char _charUnknownHigh;
+        private readonly char _charUnknownLow;
+        private readonly int _index;
 
         public EncoderFallbackException()
             : base(SR.Arg_ArgumentException)
diff --git a/src/System.Private.CoreLib/shared/System/Text/EncoderNLS.cs b/src/System.Private.CoreLib/shared/System/Text/EncoderNLS.cs
index 12d020c9c6b1..ea63926b94ea 100644
--- a/src/System.Private.CoreLib/shared/System/Text/EncoderNLS.cs
+++ b/src/System.Private.CoreLib/shared/System/Text/EncoderNLS.cs
@@ -24,7 +24,7 @@ internal class EncoderNLS : Encoder
     {
         // Need a place for the last left over character, most of our encodings use this
         internal char _charLeftOver;
-        private Encoding _encoding;
+        private readonly Encoding _encoding;
         private bool _mustFlush;
         internal bool _throwOnOverflow;
         internal int _charsUsed;
diff --git a/src/System.Private.CoreLib/shared/System/Text/EncoderReplacementFallback.cs b/src/System.Private.CoreLib/shared/System/Text/EncoderReplacementFallback.cs
index 5eb743bb488e..47a4204aa50f 100644
--- a/src/System.Private.CoreLib/shared/System/Text/EncoderReplacementFallback.cs
+++ b/src/System.Private.CoreLib/shared/System/Text/EncoderReplacementFallback.cs
@@ -11,7 +11,7 @@ namespace System.Text
     public sealed class EncoderReplacementFallback : EncoderFallback
     {
         // Our variables
-        private string _strDefault;
+        private readonly string _strDefault;
 
         // Construction.  Default replacement fallback uses no best fit and ? replacement string
         public EncoderReplacementFallback() : this("?")
@@ -103,7 +103,7 @@ public override int GetHashCode()
     public sealed class EncoderReplacementFallbackBuffer : EncoderFallbackBuffer
     {
         // Store our default string
-        private string _strDefault;
+        private readonly string _strDefault;
         private int _fallbackCount = -1;
         private int _fallbackIndex = -1;
 
diff --git a/src/System.Private.CoreLib/shared/System/Text/Encoding.cs b/src/System.Private.CoreLib/shared/System/Text/Encoding.cs
index b91fcbae26c5..2fba2b2fd9f1 100644
--- a/src/System.Private.CoreLib/shared/System/Text/Encoding.cs
+++ b/src/System.Private.CoreLib/shared/System/Text/Encoding.cs
@@ -1197,7 +1197,7 @@ internal void ThrowCharsOverflow(DecoderNLS? decoder, bool nothingDecoded)
 
         internal sealed class DefaultEncoder : Encoder, IObjectReference
         {
-            private Encoding _encoding;
+            private readonly Encoding _encoding;
 
             public DefaultEncoder(Encoding encoding)
             {
@@ -1262,7 +1262,7 @@ public override unsafe int GetBytes(char* chars, int charCount,
 
         internal sealed class DefaultDecoder : Decoder, IObjectReference
         {
-            private Encoding _encoding;
+            private readonly Encoding _encoding;
 
             public DefaultDecoder(Encoding encoding)
             {
@@ -1337,15 +1337,15 @@ public override unsafe int GetChars(byte* bytes, int byteCount,
         internal class EncodingCharBuffer
         {
             private unsafe char* _chars;
-            private unsafe char* _charStart;
-            private unsafe char* _charEnd;
+            private readonly unsafe char* _charStart;
+            private readonly unsafe char* _charEnd;
             private int _charCountResult = 0;
-            private Encoding _enc;
-            private DecoderNLS? _decoder;
-            private unsafe byte* _byteStart;
-            private unsafe byte* _byteEnd;
+            private readonly Encoding _enc;
+            private readonly DecoderNLS? _decoder;
+            private readonly unsafe byte* _byteStart;
+            private readonly unsafe byte* _byteEnd;
             private unsafe byte* _bytes;
-            private DecoderFallbackBuffer _fallbackBuffer;
+            private readonly DecoderFallbackBuffer _fallbackBuffer;
 
             internal unsafe EncodingCharBuffer(Encoding enc, DecoderNLS? decoder, char* charStart, int charCount,
                                                     byte* byteStart, int byteCount)
@@ -1510,14 +1510,14 @@ internal int Count
         internal class EncodingByteBuffer
         {
             private unsafe byte* _bytes;
-            private unsafe byte* _byteStart;
-            private unsafe byte* _byteEnd;
+            private readonly unsafe byte* _byteStart;
+            private readonly unsafe byte* _byteEnd;
             private unsafe char* _chars;
-            private unsafe char* _charStart;
-            private unsafe char* _charEnd;
+            private readonly unsafe char* _charStart;
+            private readonly unsafe char* _charEnd;
             private int _byteCountResult = 0;
-            private Encoding _enc;
-            private EncoderNLS? _encoder;
+            private readonly Encoding _enc;
+            private readonly EncoderNLS? _encoder;
             internal EncoderFallbackBuffer fallbackBuffer;
 
             internal unsafe EncodingByteBuffer(Encoding inEncoding, EncoderNLS? inEncoder,
diff --git a/src/System.Private.CoreLib/shared/System/Text/EncodingProvider.cs b/src/System.Private.CoreLib/shared/System/Text/EncodingProvider.cs
index d65dff932ff0..dc96417dfe48 100644
--- a/src/System.Private.CoreLib/shared/System/Text/EncodingProvider.cs
+++ b/src/System.Private.CoreLib/shared/System/Text/EncodingProvider.cs
@@ -130,7 +130,7 @@ internal static void AddProvider(EncodingProvider provider)
             return null;
         }
 
-        private static object s_InternalSyncObject = new object();
+        private static readonly object s_InternalSyncObject = new object();
         private static volatile EncodingProvider[]? s_providers;
     }
 }
diff --git a/src/System.Private.CoreLib/shared/System/Text/UTF7Encoding.cs b/src/System.Private.CoreLib/shared/System/Text/UTF7Encoding.cs
index 823a13c83b62..2e414cf8d0f4 100644
--- a/src/System.Private.CoreLib/shared/System/Text/UTF7Encoding.cs
+++ b/src/System.Private.CoreLib/shared/System/Text/UTF7Encoding.cs
@@ -42,7 +42,7 @@ public class UTF7Encoding : Encoding
         // This array has a size of 128.
         private bool[] _directEncode = null!;
 
-        private bool _allowOptionals;
+        private readonly bool _allowOptionals;
 
         private const int UTF7_CODEPAGE = 65000;
 
diff --git a/src/System.Private.CoreLib/shared/System/Threading/ReaderWriterLockSlim.cs b/src/System.Private.CoreLib/shared/System/Threading/ReaderWriterLockSlim.cs
index a06281e2a310..c1b0e301b3f4 100644
--- a/src/System.Private.CoreLib/shared/System/Threading/ReaderWriterLockSlim.cs
+++ b/src/System.Private.CoreLib/shared/System/Threading/ReaderWriterLockSlim.cs
@@ -83,7 +83,7 @@ public class ReaderWriterLockSlim : IDisposable
         // Every lock instance has a unique ID, which is used by ReaderWriterCount to associate itself with the lock
         // without holding a reference to it.
         private static long s_nextLockID;
-        private long _lockID;
+        private readonly long _lockID;
 
         // See comments on ReaderWriterCount.
         [ThreadStatic]
@@ -235,8 +235,8 @@ public void EnterReadLock()
         //
         private struct TimeoutTracker
         {
-            private int _total;
-            private int _start;
+            private readonly int _total;
+            private readonly int _start;
 
             public TimeoutTracker(TimeSpan timeout)
             {
diff --git a/src/System.Private.CoreLib/shared/System/Threading/Tasks/Future.cs b/src/System.Private.CoreLib/shared/System/Threading/Tasks/Future.cs
index 0a981a382acc..21c5eae8c5d3 100644
--- a/src/System.Private.CoreLib/shared/System/Threading/Tasks/Future.cs
+++ b/src/System.Private.CoreLib/shared/System/Threading/Tasks/Future.cs
@@ -1378,7 +1378,7 @@ internal Task<TNewResult> ContinueWith<TNewResult>(Func<Task<TResult>, object?,
     // Proxy class for better debugging experience
     internal class SystemThreadingTasks_FutureDebugView<TResult>
     {
-        private Task<TResult> m_task;
+        private readonly Task<TResult> m_task;
 
         public SystemThreadingTasks_FutureDebugView(Task<TResult> task)
         {
diff --git a/src/System.Private.CoreLib/shared/System/Threading/Tasks/FutureFactory.cs b/src/System.Private.CoreLib/shared/System/Threading/Tasks/FutureFactory.cs
index c9819ed4247c..8ad7a5a85685 100644
--- a/src/System.Private.CoreLib/shared/System/Threading/Tasks/FutureFactory.cs
+++ b/src/System.Private.CoreLib/shared/System/Threading/Tasks/FutureFactory.cs
@@ -32,10 +32,10 @@ public class TaskFactory<TResult>
         // Should we be thinking about a TaskFactoryBase class?
 
         // member variables
-        private CancellationToken m_defaultCancellationToken;
-        private TaskScheduler? m_defaultScheduler;
-        private TaskCreationOptions m_defaultCreationOptions;
-        private TaskContinuationOptions m_defaultContinuationOptions;
+        private readonly CancellationToken m_defaultCancellationToken;
+        private readonly TaskScheduler? m_defaultScheduler;
+        private readonly TaskCreationOptions m_defaultCreationOptions;
+        private readonly TaskContinuationOptions m_defaultContinuationOptions;
 
         private TaskScheduler DefaultScheduler => m_defaultScheduler ?? TaskScheduler.Current;
 
diff --git a/src/System.Private.CoreLib/shared/System/Threading/Tasks/Task.cs b/src/System.Private.CoreLib/shared/System/Threading/Tasks/Task.cs
index c56e3798d702..53d35fbdf1cb 100644
--- a/src/System.Private.CoreLib/shared/System/Threading/Tasks/Task.cs
+++ b/src/System.Private.CoreLib/shared/System/Threading/Tasks/Task.cs
@@ -6287,7 +6287,7 @@ void IThreadPoolWorkItem.Execute()
     // Proxy class for better debugging experience
     internal class SystemThreadingTasks_TaskDebugView
     {
-        private Task m_task;
+        private readonly Task m_task;
 
         public SystemThreadingTasks_TaskDebugView(Task task)
         {
diff --git a/src/System.Private.CoreLib/shared/System/Threading/Tasks/TaskScheduler.cs b/src/System.Private.CoreLib/shared/System/Threading/Tasks/TaskScheduler.cs
index f9db0675b2b7..75b8cab3ec32 100644
--- a/src/System.Private.CoreLib/shared/System/Threading/Tasks/TaskScheduler.cs
+++ b/src/System.Private.CoreLib/shared/System/Threading/Tasks/TaskScheduler.cs
@@ -569,7 +569,7 @@ public IEnumerable<Task>? ScheduledTasks
     /// </summary>
     internal sealed class SynchronizationContextTaskScheduler : TaskScheduler
     {
-        private SynchronizationContext m_synchronizationContext;
+        private readonly SynchronizationContext m_synchronizationContext;
 
         /// <summary>
         /// Constructs a SynchronizationContextTaskScheduler associated with <see cref="System.Threading.SynchronizationContext.Current"/>
@@ -653,7 +653,7 @@ public override int MaximumConcurrencyLevel
     /// </remarks>
     public class UnobservedTaskExceptionEventArgs : EventArgs
     {
-        private AggregateException? m_exception;
+        private readonly AggregateException? m_exception;
         internal bool m_observed = false;
 
         /// <summary>
diff --git a/src/System.Private.CoreLib/shared/System/Threading/ThreadLocal.cs b/src/System.Private.CoreLib/shared/System/Threading/ThreadLocal.cs
index 68c589774686..0056172e3958 100644
--- a/src/System.Private.CoreLib/shared/System/Threading/ThreadLocal.cs
+++ b/src/System.Private.CoreLib/shared/System/Threading/ThreadLocal.cs
@@ -687,7 +687,7 @@ private class IdManager
             private int _nextIdToTry = 0;
 
             // Stores whether each ID is free or not. Additionally, the object is also used as a lock for the IdManager.
-            private List<bool> _freeIds = new List<bool>();
+            private readonly List<bool> _freeIds = new List<bool>();
 
             internal int GetId()
             {
@@ -741,7 +741,7 @@ internal void ReturnId(int id)
         private class FinalizationHelper
         {
             internal LinkedSlotVolatile[] SlotArray;
-            private bool _trackAllValues;
+            private readonly bool _trackAllValues;
 
             internal FinalizationHelper(LinkedSlotVolatile[] slotArray, bool trackAllValues)
             {
diff --git a/src/System.Private.CoreLib/shared/System/Threading/ThreadPool.cs b/src/System.Private.CoreLib/shared/System/Threading/ThreadPool.cs
index 4050d03af262..6925af04265f 100644
--- a/src/System.Private.CoreLib/shared/System/Threading/ThreadPool.cs
+++ b/src/System.Private.CoreLib/shared/System/Threading/ThreadPool.cs
@@ -407,11 +407,11 @@ public int Count
         internal bool loggingEnabled;
         internal readonly ConcurrentQueue<object> workItems = new ConcurrentQueue<object>(); // SOS's ThreadPool command depends on this name
 
-        private Internal.PaddingFor32 pad1;
+        private readonly Internal.PaddingFor32 pad1;
 
         private volatile int numOutstandingThreadRequests = 0;
 
-        private Internal.PaddingFor32 pad2;
+        private readonly Internal.PaddingFor32 pad2;
 
         public ThreadPoolWorkQueue()
         {
@@ -911,9 +911,9 @@ public override void Execute()
 
     internal sealed class _ThreadPoolWaitOrTimerCallback
     {
-        private WaitOrTimerCallback _waitOrTimerCallback;
-        private ExecutionContext? _executionContext;
-        private object? _state;
+        private readonly WaitOrTimerCallback _waitOrTimerCallback;
+        private readonly ExecutionContext? _executionContext;
+        private readonly object? _state;
         private static readonly ContextCallback _ccbt = new ContextCallback(WaitOrTimerCallback_Context_t);
         private static readonly ContextCallback _ccbf = new ContextCallback(WaitOrTimerCallback_Context_f);
 
diff --git a/src/System.Private.CoreLib/shared/System/Threading/Timer.cs b/src/System.Private.CoreLib/shared/System/Threading/Timer.cs
index 722c0b3e709f..b3088c7f362b 100644
--- a/src/System.Private.CoreLib/shared/System/Threading/Timer.cs
+++ b/src/System.Private.CoreLib/shared/System/Threading/Timer.cs
@@ -124,7 +124,7 @@ private bool EnsureTimerFiresBy(uint requestedDuration)
         private const int ShortTimersThresholdMilliseconds = 333;
 
         // Time when Pause was called
-        private volatile int _pauseTicks = 0;
+        private readonly int _pauseTicks = 0;
 
         // Fire any timers that have expired, and update the native timer to schedule the rest of them.
         // We're in a thread pool work item here, and if there are multiple timers to be fired, we want
diff --git a/src/System.Private.CoreLib/shared/System/TypeInitializationException.cs b/src/System.Private.CoreLib/shared/System/TypeInitializationException.cs
index a0f5d086631b..05d424f6a24c 100644
--- a/src/System.Private.CoreLib/shared/System/TypeInitializationException.cs
+++ b/src/System.Private.CoreLib/shared/System/TypeInitializationException.cs
@@ -23,7 +23,7 @@ namespace System
     [System.Runtime.CompilerServices.TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
     public sealed class TypeInitializationException : SystemException
     {
-        private string? _typeName;
+        private readonly string? _typeName;
 
         // This exception is not creatable without specifying the
         //    inner exception.
diff --git a/src/System.Private.CoreLib/shared/System/UnhandledExceptionEventArgs.cs b/src/System.Private.CoreLib/shared/System/UnhandledExceptionEventArgs.cs
index c214afdc7113..ed4ef11c25e7 100644
--- a/src/System.Private.CoreLib/shared/System/UnhandledExceptionEventArgs.cs
+++ b/src/System.Private.CoreLib/shared/System/UnhandledExceptionEventArgs.cs
@@ -6,8 +6,8 @@ namespace System
 {
     public class UnhandledExceptionEventArgs : EventArgs
     {
-        private object _exception;
-        private bool _isTerminating;
+        private readonly object _exception;
+        private readonly bool _isTerminating;
 
         public UnhandledExceptionEventArgs(object exception, bool isTerminating)
         {
diff --git a/src/System.Private.CoreLib/src/Internal/Runtime/InteropServices/ComActivator.cs b/src/System.Private.CoreLib/src/Internal/Runtime/InteropServices/ComActivator.cs
index 1e38807c3e8f..fb052618a39b 100644
--- a/src/System.Private.CoreLib/src/Internal/Runtime/InteropServices/ComActivator.cs
+++ b/src/System.Private.CoreLib/src/Internal/Runtime/InteropServices/ComActivator.cs
@@ -573,21 +573,21 @@ internal sealed class LicenseInteropProxy
         private static readonly Type? s_licenseExceptionType = Type.GetType("System.ComponentModel.LicenseException, System.ComponentModel.TypeConverter", throwOnError: false);
 
         // LicenseManager
-        private MethodInfo _createWithContext;
+        private readonly MethodInfo _createWithContext;
 
         // LicenseInteropHelper
-        private MethodInfo _validateTypeAndReturnDetails;
-        private MethodInfo _getCurrentContextInfo;
+        private readonly MethodInfo _validateTypeAndReturnDetails;
+        private readonly MethodInfo _getCurrentContextInfo;
 
         // CLRLicenseContext
-        private MethodInfo _createDesignContext;
-        private MethodInfo _createRuntimeContext;
+        private readonly MethodInfo _createDesignContext;
+        private readonly MethodInfo _createRuntimeContext;
 
         // LicenseContext
-        private MethodInfo _setSavedLicenseKey;
+        private readonly MethodInfo _setSavedLicenseKey;
 
-        private Type _licInfoHelper;
-        private MethodInfo _licInfoHelperContains;
+        private readonly Type _licInfoHelper;
+        private readonly MethodInfo _licInfoHelperContains;
 
         // RCW Activation
         private object? _licContext;
diff --git a/src/System.Private.CoreLib/src/System/Collections/ObjectModel/ReadOnlyDictionary.cs b/src/System.Private.CoreLib/src/System/Collections/ObjectModel/ReadOnlyDictionary.cs
index c8bf060dfcfb..2a8898e008cc 100644
--- a/src/System.Private.CoreLib/src/System/Collections/ObjectModel/ReadOnlyDictionary.cs
+++ b/src/System.Private.CoreLib/src/System/Collections/ObjectModel/ReadOnlyDictionary.cs
@@ -356,7 +356,7 @@ object ICollection.SyncRoot
         private struct DictionaryEnumerator : IDictionaryEnumerator
         {
             private readonly IDictionary<TKey, TValue> m_dictionary;
-            private IEnumerator<KeyValuePair<TKey, TValue>> m_enumerator;
+            private readonly IEnumerator<KeyValuePair<TKey, TValue>> m_enumerator;
 
             public DictionaryEnumerator(IDictionary<TKey, TValue> dictionary)
             {
diff --git a/src/System.Private.CoreLib/src/System/Diagnostics/Eventing/EventPipe.cs b/src/System.Private.CoreLib/src/System/Diagnostics/Eventing/EventPipe.cs
index 1188495a6260..62585e4b569c 100644
--- a/src/System.Private.CoreLib/src/System/Diagnostics/Eventing/EventPipe.cs
+++ b/src/System.Private.CoreLib/src/System/Diagnostics/Eventing/EventPipe.cs
@@ -35,9 +35,9 @@ internal struct EventPipeSessionInfo
     internal struct EventPipeProviderConfiguration
     {
         [MarshalAs(UnmanagedType.LPWStr)]
-        private string m_providerName;
-        private ulong m_keywords;
-        private uint m_loggingLevel;
+        private readonly string m_providerName;
+        private readonly ulong m_keywords;
+        private readonly uint m_loggingLevel;
 
         [MarshalAs(UnmanagedType.LPWStr)]
         private readonly string? m_filterData;
@@ -93,10 +93,10 @@ internal sealed class EventPipeWaitHandle : WaitHandle
 
     internal sealed class EventPipeConfiguration
     {
-        private string m_outputFile;
-        private EventPipeSerializationFormat m_format;
-        private uint m_circularBufferSizeInMB;
-        private List<EventPipeProviderConfiguration> m_providers;
+        private readonly string m_outputFile;
+        private readonly EventPipeSerializationFormat m_format;
+        private readonly uint m_circularBufferSizeInMB;
+        private readonly List<EventPipeProviderConfiguration> m_providers;
         private TimeSpan m_minTimeBetweenSamples = TimeSpan.FromMilliseconds(1);
 
         internal EventPipeConfiguration(
diff --git a/src/System.Private.CoreLib/src/System/Diagnostics/Eventing/EventPipeEventDispatcher.cs b/src/System.Private.CoreLib/src/System/Diagnostics/Eventing/EventPipeEventDispatcher.cs
index ff9a7016c958..10d9c46697e0 100644
--- a/src/System.Private.CoreLib/src/System/Diagnostics/Eventing/EventPipeEventDispatcher.cs
+++ b/src/System.Private.CoreLib/src/System/Diagnostics/Eventing/EventPipeEventDispatcher.cs
@@ -25,7 +25,7 @@ internal EventListenerSubscription(EventKeywords matchAnyKeywords, EventLevel le
 
         internal static readonly EventPipeEventDispatcher Instance = new EventPipeEventDispatcher();
 
-        private IntPtr m_RuntimeProviderID;
+        private readonly IntPtr m_RuntimeProviderID;
 
         private ulong m_sessionID = 0;
         private DateTime m_syncTimeUtc;
@@ -33,12 +33,12 @@ internal EventListenerSubscription(EventKeywords matchAnyKeywords, EventLevel le
         private long m_timeQPCFrequency;
 
         private bool m_stopDispatchTask;
-        private EventPipeWaitHandle m_dispatchTaskWaitHandle = new EventPipeWaitHandle();
+        private readonly EventPipeWaitHandle m_dispatchTaskWaitHandle = new EventPipeWaitHandle();
         private Task? m_dispatchTask = null;
-        private object m_dispatchControlLock = new object();
-        private Dictionary<EventListener, EventListenerSubscription> m_subscriptions = new Dictionary<EventListener, EventListenerSubscription>();
+        private readonly object m_dispatchControlLock = new object();
+        private readonly Dictionary<EventListener, EventListenerSubscription> m_subscriptions = new Dictionary<EventListener, EventListenerSubscription>();
 
-        private static uint DefaultEventListenerCircularMBSize = 10;
+        private const uint DefaultEventListenerCircularMBSize = 10;
 
         private EventPipeEventDispatcher()
         {
diff --git a/src/System.Private.CoreLib/src/System/Exception.CoreCLR.cs b/src/System.Private.CoreLib/src/System/Exception.CoreCLR.cs
index 26b21089a3e8..979f40f2e3cd 100644
--- a/src/System.Private.CoreLib/src/System/Exception.CoreCLR.cs
+++ b/src/System.Private.CoreLib/src/System/Exception.CoreCLR.cs
@@ -69,7 +69,7 @@ internal class __RestrictedErrorObject
         {
             // Hold the error object instance but don't serialize/deserialize it
             [NonSerialized]
-            private object _realErrorObject;
+            private readonly object _realErrorObject;
 
             internal __RestrictedErrorObject(object errorObject)
             {
@@ -345,7 +345,7 @@ internal void RestoreDispatchState(in DispatchState dispatchState)
         private MethodBase? _exceptionMethod;  //Needed for serialization.
         internal string? _message;
         private IDictionary? _data;
-        private Exception? _innerException;
+        private readonly Exception? _innerException;
         private string? _helpURL;
         private object? _stackTrace;
         private object? _watsonBuckets;
@@ -357,11 +357,11 @@ internal void RestoreDispatchState(in DispatchState dispatchState)
         // DynamicMethodDescs alive for the lifetime of the exception. We do this because
         // the _stackTrace field holds MethodDescs, and a DynamicMethodDesc can be destroyed
         // unless a System.Resolver object roots it.
-        private object? _dynamicMethods;
+        private readonly object? _dynamicMethods;
         private string? _source;         // Mainly used by VB.
         private UIntPtr _ipForWatsonBuckets; // Used to persist the IP for Watson Bucketing
-        private IntPtr _xptrs;             // Internal EE stuff
-        private int _xcode = _COMPlusExceptionCode;             // Internal EE stuff
+        private readonly IntPtr _xptrs;             // Internal EE stuff
+        private readonly int _xcode = _COMPlusExceptionCode;             // Internal EE stuff
 #pragma warning restore CA1823
 #pragma warning restore 414
 
diff --git a/src/System.Private.CoreLib/src/System/Runtime/CompilerServices/TypeDependencyAttribute.cs b/src/System.Private.CoreLib/src/System/Runtime/CompilerServices/TypeDependencyAttribute.cs
index 7801ffca439e..66128fc7910f 100644
--- a/src/System.Private.CoreLib/src/System/Runtime/CompilerServices/TypeDependencyAttribute.cs
+++ b/src/System.Private.CoreLib/src/System/Runtime/CompilerServices/TypeDependencyAttribute.cs
@@ -9,7 +9,7 @@ namespace System.Runtime.CompilerServices
     [AttributeUsage(AttributeTargets.Struct | AttributeTargets.Class | AttributeTargets.Interface, AllowMultiple = true, Inherited = false)]
     internal sealed class TypeDependencyAttribute : Attribute
     {
-        private string typeName;
+        private readonly string typeName;
 
         public TypeDependencyAttribute(string typeName)
         {
diff --git a/src/System.Private.CoreLib/src/System/Runtime/InteropServices/ComEventsInfo.cs b/src/System.Private.CoreLib/src/System/Runtime/InteropServices/ComEventsInfo.cs
index 5a3bc042dd50..e65614b92fea 100644
--- a/src/System.Private.CoreLib/src/System/Runtime/InteropServices/ComEventsInfo.cs
+++ b/src/System.Private.CoreLib/src/System/Runtime/InteropServices/ComEventsInfo.cs
@@ -7,7 +7,7 @@ namespace System.Runtime.InteropServices
     internal class ComEventsInfo
     {
         private ComEventsSink? _sinks;
-        private object _rcw;
+        private readonly object _rcw;
 
         private ComEventsInfo(object rcw)
         {
diff --git a/src/System.Private.CoreLib/src/System/Runtime/InteropServices/ComEventsMethod.cs b/src/System.Private.CoreLib/src/System/Runtime/InteropServices/ComEventsMethod.cs
index 10ef6e5db930..53d7a5d71681 100644
--- a/src/System.Private.CoreLib/src/System/Runtime/InteropServices/ComEventsMethod.cs
+++ b/src/System.Private.CoreLib/src/System/Runtime/InteropServices/ComEventsMethod.cs
@@ -100,7 +100,7 @@ private void PreProcessSignature()
         /// Since multicast delegate's built-in chaining supports only chaining instances of the same type,
         /// we need to complement this design by using an explicit linked list data structure.
         /// </summary>
-        private List<DelegateWrapper> _delegateWrappers = new List<DelegateWrapper>();
+        private readonly List<DelegateWrapper> _delegateWrappers = new List<DelegateWrapper>();
 
         private readonly int _dispid;
         private ComEventsMethod? _next;
diff --git a/src/System.Private.CoreLib/src/System/Runtime/InteropServices/CustomMarshalers/EnumeratorViewOfEnumVariant.cs b/src/System.Private.CoreLib/src/System/Runtime/InteropServices/CustomMarshalers/EnumeratorViewOfEnumVariant.cs
index 6b1c418862f0..ecb583f2b9c5 100644
--- a/src/System.Private.CoreLib/src/System/Runtime/InteropServices/CustomMarshalers/EnumeratorViewOfEnumVariant.cs
+++ b/src/System.Private.CoreLib/src/System/Runtime/InteropServices/CustomMarshalers/EnumeratorViewOfEnumVariant.cs
@@ -11,7 +11,7 @@ internal class EnumeratorViewOfEnumVariant : ICustomAdapter, IEnumerator
     {
         private readonly IEnumVARIANT _enumVariantObject;
         private bool _fetchedLastObject;
-        private object[] _nextArray = new object[1];
+        private readonly object[] _nextArray = new object[1];
         private object? _current;
 
         public EnumeratorViewOfEnumVariant(IEnumVARIANT enumVariantObject)
diff --git a/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/Attributes.cs b/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/Attributes.cs
index 8fe16d11faff..3c3f853b1de1 100644
--- a/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/Attributes.cs
+++ b/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/Attributes.cs
@@ -8,7 +8,7 @@ namespace System.Runtime.InteropServices.WindowsRuntime
     [AttributeUsage(AttributeTargets.Class | AttributeTargets.Interface, AllowMultiple = false, Inherited = false)]
     public sealed class DefaultInterfaceAttribute : Attribute
     {
-        private Type m_defaultInterface;
+        private readonly Type m_defaultInterface;
 
         public DefaultInterfaceAttribute(Type defaultInterface)
         {
@@ -55,11 +55,11 @@ public InterfaceImplementedInVersionAttribute(Type interfaceType, byte majorVers
 
         public byte RevisionVersion => m_revisionVersion;
 
-        private Type m_interfaceType;
-        private byte m_majorVersion;
-        private byte m_minorVersion;
-        private byte m_buildVersion;
-        private byte m_revisionVersion;
+        private readonly Type m_interfaceType;
+        private readonly byte m_majorVersion;
+        private readonly byte m_minorVersion;
+        private readonly byte m_buildVersion;
+        private readonly byte m_revisionVersion;
     }
 
     // Applies to read-only array parameters
@@ -82,7 +82,7 @@ public WriteOnlyArrayAttribute() { }
     [AttributeUsage(AttributeTargets.ReturnValue | AttributeTargets.Delegate, AllowMultiple = false, Inherited = false)]
     public sealed class ReturnValueNameAttribute : Attribute
     {
-        private string m_Name;
+        private readonly string m_Name;
 
         public ReturnValueNameAttribute(string name)
         {
diff --git a/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/CLRIPropertyValueImpl.cs b/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/CLRIPropertyValueImpl.cs
index b4b1b59670f0..edad524b4c30 100644
--- a/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/CLRIPropertyValueImpl.cs
+++ b/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/CLRIPropertyValueImpl.cs
@@ -11,8 +11,8 @@ namespace System.Runtime.InteropServices.WindowsRuntime
 {
     internal class CLRIPropertyValueImpl : IPropertyValue
     {
-        private PropertyType _type;
-        private object _data;
+        private readonly PropertyType _type;
+        private readonly object _data;
 
         // Numeric scalar types which participate in coersion
         private static volatile Tuple<Type, PropertyType>[]? s_numericScalarTypes;
diff --git a/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/CLRIReferenceImpl.cs b/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/CLRIReferenceImpl.cs
index 8ee1ff778366..23867cdaea84 100644
--- a/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/CLRIReferenceImpl.cs
+++ b/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/CLRIReferenceImpl.cs
@@ -9,7 +9,7 @@ namespace System.Runtime.InteropServices.WindowsRuntime
 {
     internal sealed class CLRIReferenceImpl<T> : CLRIPropertyValueImpl, IReference<T>, IGetProxyTarget
     {
-        private T _value;
+        private readonly T _value;
 
         public CLRIReferenceImpl(PropertyType type, T obj)
             : base(type, obj!)
@@ -60,8 +60,8 @@ internal sealed class CLRIReferenceArrayImpl<T> : CLRIPropertyValueImpl,
                                                       IReferenceArray<T>,
                                                       IList                     // Jupiter data binding needs IList/IEnumerable
     {
-        private T[] _value;
-        private IList _list;
+        private readonly T[] _value;
+        private readonly IList _list;
 
         public CLRIReferenceArrayImpl(PropertyType type, T[] obj)
             : base(type, obj)
diff --git a/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/ConstantSplittableMap.cs b/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/ConstantSplittableMap.cs
index fbab70b97461..49bdab07add9 100644
--- a/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/ConstantSplittableMap.cs
+++ b/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/ConstantSplittableMap.cs
@@ -167,9 +167,9 @@ public bool TryGetValue(TKey key, [MaybeNullWhen(false)] out TValue value)
 
         internal struct IKeyValuePairEnumerator : IEnumerator<IKeyValuePair<TKey, TValue>>
         {
-            private KeyValuePair<TKey, TValue>[] _array;
-            private int _start;
-            private int _end;
+            private readonly KeyValuePair<TKey, TValue>[] _array;
+            private readonly int _start;
+            private readonly int _end;
             private int _current;
 
             internal IKeyValuePairEnumerator(KeyValuePair<TKey, TValue>[] items, int first, int end)
diff --git a/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/CustomPropertyImpl.cs b/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/CustomPropertyImpl.cs
index 24fbaa3bd54b..d5a8a01e3da7 100644
--- a/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/CustomPropertyImpl.cs
+++ b/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/CustomPropertyImpl.cs
@@ -11,7 +11,7 @@ namespace System.Runtime.InteropServices.WindowsRuntime
     //
     internal sealed class CustomPropertyImpl : ICustomProperty
     {
-        private PropertyInfo m_property;
+        private readonly PropertyInfo m_property;
 
         //
         // Constructor
diff --git a/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/EnumeratorToIteratorAdapter.cs b/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/EnumeratorToIteratorAdapter.cs
index a2772d98c45f..096df0be7b6a 100644
--- a/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/EnumeratorToIteratorAdapter.cs
+++ b/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/EnumeratorToIteratorAdapter.cs
@@ -41,7 +41,7 @@ private EnumerableToBindableIterableAdapter()
 
         internal sealed class NonGenericToGenericEnumerator : IEnumerator<object?>
         {
-            private IEnumerator enumerator;
+            private readonly IEnumerator enumerator;
 
             public NonGenericToGenericEnumerator(IEnumerator enumerator)
             { this.enumerator = enumerator; }
@@ -63,7 +63,7 @@ internal IBindableIterator First_Stub()
     // Adapter class which holds a managed IEnumerator<T>, exposing it as a Windows Runtime IIterator<T>
     internal sealed class EnumeratorToIteratorAdapter<T> : IIterator<T>, IBindableIterator
     {
-        private IEnumerator<T> m_enumerator;
+        private readonly IEnumerator<T> m_enumerator;
         private bool m_firstItem = true;
         private bool m_hasCurrent;
 
diff --git a/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/EventRegistrationTokenTable.cs b/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/EventRegistrationTokenTable.cs
index afb93ea24f27..705379f5249d 100644
--- a/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/EventRegistrationTokenTable.cs
+++ b/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/EventRegistrationTokenTable.cs
@@ -14,7 +14,7 @@ namespace System.Runtime.InteropServices.WindowsRuntime
     public sealed class EventRegistrationTokenTable<T> where T : class
     {
         // Note this dictionary is also used as the synchronization object for this table
-        private Dictionary<EventRegistrationToken, T> m_tokens = new Dictionary<EventRegistrationToken, T>();
+        private readonly Dictionary<EventRegistrationToken, T> m_tokens = new Dictionary<EventRegistrationToken, T>();
 
         // Cached multicast delegate which will invoke all of the currently registered delegates.  This
         // will be accessed frequently in common coding paterns, so we don't want to calculate it repeatedly.
diff --git a/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/ICustomPropertyProvider.cs b/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/ICustomPropertyProvider.cs
index 6191bc9d558b..22e3c8880486 100644
--- a/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/ICustomPropertyProvider.cs
+++ b/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/ICustomPropertyProvider.cs
@@ -127,8 +127,8 @@ internal class ICustomPropertyProviderProxy<T1, T2> : IGetProxyTarget,
                                                           IBindableVector,      // IBindableVector -> IBindableVector/IVector<T>
                                                           IBindableVectorView   // IBindableVectorView -> IBindableVectorView/IVectorView<T>
     {
-        private object _target;
-        private InterfaceForwardingSupport _flags;
+        private readonly object _target;
+        private readonly InterfaceForwardingSupport _flags;
 
         internal ICustomPropertyProviderProxy(object target, InterfaceForwardingSupport flags)
         {
@@ -285,7 +285,7 @@ IBindableVectorView IBindableVector.GetView()
 
         private sealed class IVectorViewToIBindableVectorViewAdapter<T> : IBindableVectorView
         {
-            private IVectorView<T> _vectorView;
+            private readonly IVectorView<T> _vectorView;
 
             public IVectorViewToIBindableVectorViewAdapter(IVectorView<T> vectorView)
             {
@@ -482,7 +482,7 @@ IBindableIterator IBindableIterable.First()
 
         private sealed class IteratorOfTToIteratorAdapter<T> : IBindableIterator
         {
-            private IIterator<T> _iterator;
+            private readonly IIterator<T> _iterator;
 
             public IteratorOfTToIteratorAdapter(IIterator<T> iterator)
             { _iterator = iterator; }
diff --git a/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/IteratorToEnumeratorAdapter.cs b/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/IteratorToEnumeratorAdapter.cs
index b26e9f2da709..1fb47a1e1808 100644
--- a/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/IteratorToEnumeratorAdapter.cs
+++ b/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/IteratorToEnumeratorAdapter.cs
@@ -68,7 +68,7 @@ private BindableIterableToEnumerableAdapter()
 
         private sealed class NonGenericToGenericIterator : IIterator<object?>
         {
-            private IBindableIterator iterator;
+            private readonly IBindableIterator iterator;
 
             public NonGenericToGenericIterator(IBindableIterator iterator)
             { this.iterator = iterator; }
@@ -96,7 +96,7 @@ internal IEnumerator GetEnumerator_Stub()
     // Also IEnumerator throws an exception when we call Current after reaching the end of collection.
     internal sealed class IteratorToEnumeratorAdapter<T> : IEnumerator<T>
     {
-        private IIterator<T> m_iterator;
+        private readonly IIterator<T> m_iterator;
         private bool m_hadCurrent;
         private T m_current = default!;
         private bool m_isInitialized;
diff --git a/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/ManagedActivationFactory.cs b/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/ManagedActivationFactory.cs
index 01e3d28d8524..bf3392b6ad4e 100644
--- a/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/ManagedActivationFactory.cs
+++ b/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/ManagedActivationFactory.cs
@@ -25,7 +25,7 @@ internal interface IManagedActivationFactory
     [ClassInterface(ClassInterfaceType.None)]
     internal sealed class ManagedActivationFactory : IActivationFactory, IManagedActivationFactory
     {
-        private Type m_type;
+        private readonly Type m_type;
 
         internal ManagedActivationFactory(Type type)
         {
diff --git a/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/WindowsRuntimeMarshal.cs b/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/WindowsRuntimeMarshal.cs
index 8d1607602075..9a009c86e62a 100644
--- a/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/WindowsRuntimeMarshal.cs
+++ b/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/WindowsRuntimeMarshal.cs
@@ -118,7 +118,7 @@ internal static int GetRegistrationTokenCacheSize()
         //
         internal struct EventRegistrationTokenList
         {
-            private EventRegistrationToken firstToken;     // Optimization for common case where there is only one token
+            private readonly EventRegistrationToken firstToken;     // Optimization for common case where there is only one token
             private List<EventRegistrationToken>? restTokens;     // Rest of the tokens
 
             internal EventRegistrationTokenList(EventRegistrationToken token)
@@ -384,7 +384,7 @@ public int GetHashCode(EventCacheKey key)
             //
             internal class EventRegistrationTokenListWithCount
             {
-                private TokenListCount _tokenListCount;
+                private readonly TokenListCount _tokenListCount;
                 private EventRegistrationTokenList _tokenList;
 
                 internal EventRegistrationTokenListWithCount(TokenListCount tokenListCount, EventRegistrationToken token)
@@ -517,7 +517,7 @@ internal struct EventCacheEntry
                 new Dictionary<EventCacheKey, EventCacheEntry>();
 
             // Prevent add/remove handler code to run at the same with with cache cleanup code
-            private static volatile MyReaderWriterLock s_eventCacheRWLock = new MyReaderWriterLock();
+            private static readonly MyReaderWriterLock s_eventCacheRWLock = new MyReaderWriterLock();
 
             // Get InstanceKey to use in the cache
             private static object GetInstanceKey(Action<EventRegistrationToken> removeMethod)
diff --git a/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/WindowsRuntimeMetadata.cs b/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/WindowsRuntimeMetadata.cs
index 273b270f3778..b0ae3b957509 100644
--- a/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/WindowsRuntimeMetadata.cs
+++ b/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/WindowsRuntimeMetadata.cs
@@ -8,7 +8,7 @@ namespace System.Runtime.InteropServices.WindowsRuntime
 {
     internal static class WindowsRuntimeMetadata
     {
-        private static EventHandler<DesignerNamespaceResolveEventArgs>? DesignerNamespaceResolve;
+        private static readonly EventHandler<DesignerNamespaceResolveEventArgs>? DesignerNamespaceResolve;
 
         internal static string[]? OnDesignerNamespaceResolve(string namespaceName)
         {
@@ -48,8 +48,8 @@ internal static class WindowsRuntimeMetadata
 
     internal class DesignerNamespaceResolveEventArgs : EventArgs
     {
-        private string _NamespaceName;
-        private Collection<string> _ResolvedAssemblyFiles;
+        private readonly string _NamespaceName;
+        private readonly Collection<string> _ResolvedAssemblyFiles;
 
         public Collection<string> ResolvedAssemblyFiles
         {
diff --git a/src/System.Private.CoreLib/src/System/RuntimeType.CoreCLR.cs b/src/System.Private.CoreLib/src/System/RuntimeType.CoreCLR.cs
index 6be41e45762a..5e8b378ebd04 100644
--- a/src/System.Private.CoreLib/src/System/RuntimeType.CoreCLR.cs
+++ b/src/System.Private.CoreLib/src/System/RuntimeType.CoreCLR.cs
@@ -223,7 +223,7 @@ private class MemberInfoCache<T> where T : MemberInfo
                 private bool m_cacheComplete;
 
                 // This is the strong reference back to the cache
-                private RuntimeTypeCache m_runtimeTypeCache;
+                private readonly RuntimeTypeCache m_runtimeTypeCache;
                 #endregion
 
                 #region Constructor
@@ -1401,14 +1401,14 @@ internal T[] GetMemberList(MemberListType listType, string? name, CacheType cach
             #endregion
 
             #region Private Data Members
-            private RuntimeType m_runtimeType;
+            private readonly RuntimeType m_runtimeType;
             private RuntimeType? m_enclosingType;
             private TypeCode m_typeCode;
             private string? m_name;
             private string? m_fullname;
             private string? m_toString;
             private string? m_namespace;
-            private bool m_isGlobal;
+            private readonly bool m_isGlobal;
             private bool m_bIsDomainInitialized;
             private MemberInfoCache<RuntimeMethodInfo>? m_methodInfoCache;
             private MemberInfoCache<RuntimeConstructorInfo>? m_constructorInfoCache;
@@ -2354,7 +2354,7 @@ private static bool FilterApplyMethodBase(
         #region Private Data Members
 
 #pragma warning disable CA1823
-        private object m_keepalive; // This will be filled with a LoaderAllocator reference when this RuntimeType represents a collectible type
+        private readonly object m_keepalive; // This will be filled with a LoaderAllocator reference when this RuntimeType represents a collectible type
 #pragma warning restore CA1823
         private IntPtr m_cache;
         internal IntPtr m_handle;
@@ -3378,7 +3378,7 @@ public override StructLayoutAttribute? StructLayoutAttribute
         private const BindingFlags BinderGetSetProperty = BindingFlags.GetProperty | BindingFlags.SetProperty;
         private const BindingFlags BinderGetSetField = BindingFlags.GetField | BindingFlags.SetField;
         private const BindingFlags BinderNonFieldGetSet = (BindingFlags)0x00FFF300;
-        private static RuntimeType s_typedRef = (RuntimeType)typeof(TypedReference);
+        private static readonly RuntimeType s_typedRef = (RuntimeType)typeof(TypedReference);
 
         [MethodImpl(MethodImplOptions.InternalCall)]
         private static extern bool CanValueSpecialCast(RuntimeType valueType, RuntimeType targetType);
diff --git a/src/System.Private.CoreLib/src/System/Threading/Overlapped.cs b/src/System.Private.CoreLib/src/System/Threading/Overlapped.cs
index 3eefadb4cfd5..81dbe64216bf 100644
--- a/src/System.Private.CoreLib/src/System/Threading/Overlapped.cs
+++ b/src/System.Private.CoreLib/src/System/Threading/Overlapped.cs
@@ -30,8 +30,8 @@ namespace System.Threading
 
     internal unsafe class _IOCompletionCallback
     {
-        private IOCompletionCallback _ioCompletionCallback;
-        private ExecutionContext _executionContext;
+        private readonly IOCompletionCallback _ioCompletionCallback;
+        private readonly ExecutionContext _executionContext;
         private uint _errorCode; // Error code
         private uint _numBytes; // No. of bytes transferred
         private NativeOverlapped* _pNativeOverlapped;
@@ -93,7 +93,7 @@ internal sealed unsafe class OverlappedData
         internal object? _callback; // IOCompletionCallback or _IOCompletionCallback
         internal readonly Overlapped _overlapped;
         private object? _userObject;
-        private NativeOverlapped * _pNativeOverlapped;
+        private readonly NativeOverlapped* _pNativeOverlapped;
         private IntPtr _eventHandle;
         private int _offsetLow;
         private int _offsetHigh;
diff --git a/src/System.Private.CoreLib/src/System/Threading/Tasks/AsyncCausalityTracer.cs b/src/System.Private.CoreLib/src/System/Threading/Tasks/AsyncCausalityTracer.cs
index bad420454c83..23ed7ee59e77 100644
--- a/src/System.Private.CoreLib/src/System/Threading/Tasks/AsyncCausalityTracer.cs
+++ b/src/System.Private.CoreLib/src/System/Threading/Tasks/AsyncCausalityTracer.cs
@@ -33,7 +33,7 @@ internal static bool LoggingOn
         //Indicates this information comes from the BCL Library
         private const WFD.CausalitySource s_CausalitySource = WFD.CausalitySource.Library;
 
-        private static WFD.IAsyncCausalityTracerStatics s_TracerFactory = null!;
+        private static readonly WFD.IAsyncCausalityTracerStatics s_TracerFactory = null!;
 
         // The loggers that this Tracer knows about.
         [Flags]
diff --git a/src/System.Private.CoreLib/src/System/TypeNameParser.cs b/src/System.Private.CoreLib/src/System/TypeNameParser.cs
index 06eab1938a23..4eff4e36c40d 100644
--- a/src/System.Private.CoreLib/src/System/TypeNameParser.cs
+++ b/src/System.Private.CoreLib/src/System/TypeNameParser.cs
@@ -89,7 +89,7 @@ internal sealed class TypeNameParser : IDisposable
         #endregion
 
         #region Private Data Members
-        private SafeTypeNameParserHandle m_NativeParser;
+        private readonly SafeTypeNameParserHandle m_NativeParser;
         private static readonly char[] SPECIAL_CHARS = { ',', '[', ']', '&', '*', '+', '\\' }; /* see typeparse.h */
         #endregion
 

From bdaf7fa74e3596fb9e635c3d6c9f171a56e44735 Mon Sep 17 00:00:00 2001
From: Stephen Toub <stoub@microsoft.com>
Date: Fri, 9 Aug 2019 11:55:23 -0400
Subject: [PATCH 2/6] Contribute to IDE0019 (use pattern matching) and IDE0066
 (use switch expression)

---
 .../shared/System/AggregateException.cs       |   2 +-
 .../shared/System/Array.cs                    |   6 +-
 .../Text/Utf8Formatter/Utf8Formatter.Date.cs  |  46 +-
 .../Text/Utf8Parser/Utf8Parser.Date.cs        |  27 +-
 .../shared/System/Collections/ArrayList.cs    |   3 +-
 .../shared/System/Collections/Comparer.cs     |   3 +-
 .../shared/System/Convert.cs                  |  61 +-
 .../shared/System/DateTime.Windows.cs         |  11 +-
 .../shared/System/DateTime.cs                 |  13 +-
 .../Diagnostics/Tracing/EventProvider.cs      |  18 +-
 .../Tracing/TraceLogging/Statics.cs           | 125 +--
 .../shared/System/Enum.cs                     | 323 ++-----
 .../shared/System/Environment.cs              |   3 +-
 .../shared/System/Globalization/Calendar.cs   |  18 +-
 .../System/Globalization/CultureInfo.cs       |  13 +-
 .../Globalization/DateTimeFormatInfo.cs       |  17 +-
 .../System/Globalization/HebrewCalendar.cs    |  16 +-
 .../System/Globalization/TimeSpanParse.cs     |  40 +-
 .../shared/System/Guid.cs                     |  55 +-
 .../System/Runtime/Intrinsics/X86/Avx2.cs     | 912 +++++++-----------
 .../shared/System/Security/SecurityElement.cs |   3 +-
 .../shared/System/StringComparer.cs           |  25 +-
 .../shared/System/Text/UTF7Encoding.cs        |   3 +-
 .../Threading/CancellationTokenSource.cs      |  21 +-
 .../shared/System/Threading/Tasks/Task.cs     |   7 +-
 .../System/Threading/Tasks/TaskToApm.cs       |   6 +-
 .../shared/System/Tuple.cs                    | 152 ++-
 .../shared/System/ValueTuple.cs               | 174 ++--
 .../src/System/Attribute.CoreCLR.cs           |  45 +-
 .../src/System/Reflection/CustomAttribute.cs  |  77 +-
 .../src/System/Reflection/MdConstant.cs       |  75 +-
 .../src/System/Reflection/MdFieldInfo.cs      |   9 +-
 .../src/System/Reflection/RtFieldInfo.cs      |   7 +-
 .../Reflection/RuntimeConstructorInfo.cs      |   7 +-
 .../src/System/Reflection/RuntimeEventInfo.cs |   9 +-
 .../System/Reflection/RuntimeMethodInfo.cs    |   7 +-
 .../System/Reflection/RuntimePropertyInfo.cs  |   9 +-
 .../src/System/RuntimeType.CoreCLR.cs         |   6 +-
 .../src/System/StubHelpers.cs                 |   7 +-
 .../src/System/Variant.cs                     | 237 ++---
 40 files changed, 875 insertions(+), 1723 deletions(-)

diff --git a/src/System.Private.CoreLib/shared/System/AggregateException.cs b/src/System.Private.CoreLib/shared/System/AggregateException.cs
index e0f079fe2390..85277d97ac4b 100644
--- a/src/System.Private.CoreLib/shared/System/AggregateException.cs
+++ b/src/System.Private.CoreLib/shared/System/AggregateException.cs
@@ -258,7 +258,7 @@ protected AggregateException(SerializationInfo info, StreamingContext context) :
             }
 
             Exception[]? innerExceptions = info.GetValue("InnerExceptions", typeof(Exception[])) as Exception[];
-            if (innerExceptions == null)
+            if (innerExceptions is null)
             {
                 throw new SerializationException(SR.AggregateException_DeserializationFailure);
             }
diff --git a/src/System.Private.CoreLib/shared/System/Array.cs b/src/System.Private.CoreLib/shared/System/Array.cs
index 247b1ea98d12..5a338c3a8008 100644
--- a/src/System.Private.CoreLib/shared/System/Array.cs
+++ b/src/System.Private.CoreLib/shared/System/Array.cs
@@ -936,9 +936,8 @@ public static int IndexOf(Array array, object? value, int startIndex, int count)
                 return retVal;
 #endif
 
-            object[]? objArray = array as object[];
             int endIndex = startIndex + count;
-            if (objArray != null)
+            if (array is object[] objArray)
             {
                 if (value == null)
                 {
@@ -1120,9 +1119,8 @@ public static int LastIndexOf(Array array, object? value, int startIndex, int co
                 return retVal;
 #endif
 
-            object[]? objArray = array as object[];
             int endIndex = startIndex - count + 1;
-            if (objArray != null)
+            if (array is object[] objArray)
             {
                 if (value == null)
                 {
diff --git a/src/System.Private.CoreLib/shared/System/Buffers/Text/Utf8Formatter/Utf8Formatter.Date.cs b/src/System.Private.CoreLib/shared/System/Buffers/Text/Utf8Formatter/Utf8Formatter.Date.cs
index 74d607634bbe..122c490ccaa2 100644
--- a/src/System.Private.CoreLib/shared/System/Buffers/Text/Utf8Formatter/Utf8Formatter.Date.cs
+++ b/src/System.Private.CoreLib/shared/System/Buffers/Text/Utf8Formatter/Utf8Formatter.Date.cs
@@ -105,23 +105,14 @@ public static bool TryFormat(DateTimeOffset value, Span<byte> destination, out i
                 offset = value.Offset;
             }
 
-            switch (symbol)
+            return symbol switch
             {
-                case 'R':
-                    return TryFormatDateTimeR(value.UtcDateTime, destination, out bytesWritten);
-
-                case 'l':
-                    return TryFormatDateTimeL(value.UtcDateTime, destination, out bytesWritten);
-
-                case 'O':
-                    return TryFormatDateTimeO(value.DateTime, value.Offset, destination, out bytesWritten);
-
-                case 'G':
-                    return TryFormatDateTimeG(value.DateTime, offset, destination, out bytesWritten);
-
-                default:
-                    return FormattingHelpers.TryFormatThrowFormatException(out bytesWritten);
-            }
+                'R' => TryFormatDateTimeR(value.UtcDateTime, destination, out bytesWritten),
+                'l' => TryFormatDateTimeL(value.UtcDateTime, destination, out bytesWritten),
+                'O' => TryFormatDateTimeO(value.DateTime, value.Offset, destination, out bytesWritten),
+                'G' => TryFormatDateTimeG(value.DateTime, offset, destination, out bytesWritten),
+                _ => FormattingHelpers.TryFormatThrowFormatException(out bytesWritten),
+            };
         }
 
         /// <summary>
@@ -149,23 +140,14 @@ public static bool TryFormat(DateTime value, Span<byte> destination, out int byt
         {
             char symbol = FormattingHelpers.GetSymbolOrDefault(format, 'G');
 
-            switch (symbol)
+            return symbol switch
             {
-                case 'R':
-                    return TryFormatDateTimeR(value, destination, out bytesWritten);
-
-                case 'l':
-                    return TryFormatDateTimeL(value, destination, out bytesWritten);
-
-                case 'O':
-                    return TryFormatDateTimeO(value, Utf8Constants.NullUtcOffset, destination, out bytesWritten);
-
-                case 'G':
-                    return TryFormatDateTimeG(value, Utf8Constants.NullUtcOffset, destination, out bytesWritten);
-
-                default:
-                    return FormattingHelpers.TryFormatThrowFormatException(out bytesWritten);
-            }
+                'R' => TryFormatDateTimeR(value, destination, out bytesWritten),
+                'l' => TryFormatDateTimeL(value, destination, out bytesWritten),
+                'O' => TryFormatDateTimeO(value, Utf8Constants.NullUtcOffset, destination, out bytesWritten),
+                'G' => TryFormatDateTimeG(value, Utf8Constants.NullUtcOffset, destination, out bytesWritten),
+                _ => FormattingHelpers.TryFormatThrowFormatException(out bytesWritten),
+            };
         }
     }
 }
diff --git a/src/System.Private.CoreLib/shared/System/Buffers/Text/Utf8Parser/Utf8Parser.Date.cs b/src/System.Private.CoreLib/shared/System/Buffers/Text/Utf8Parser/Utf8Parser.Date.cs
index 16721850bb78..7fb6f0c60810 100644
--- a/src/System.Private.CoreLib/shared/System/Buffers/Text/Utf8Parser/Utf8Parser.Date.cs
+++ b/src/System.Private.CoreLib/shared/System/Buffers/Text/Utf8Parser/Utf8Parser.Date.cs
@@ -121,26 +121,15 @@ public static bool TryParse(ReadOnlySpan<byte> source, out DateTime value, out i
         /// </exceptions>
         public static bool TryParse(ReadOnlySpan<byte> source, out DateTimeOffset value, out int bytesConsumed, char standardFormat = default)
         {
-            switch (standardFormat)
+            return standardFormat switch
             {
-                case 'R':
-                    return TryParseDateTimeOffsetR(source, NoFlipCase, out value, out bytesConsumed);
-
-                case 'l':
-                    return TryParseDateTimeOffsetR(source, FlipCase, out value, out bytesConsumed);
-
-                case 'O':
-                    return TryParseDateTimeOffsetO(source, out value, out bytesConsumed, out _);
-
-                case default(char):
-                    return TryParseDateTimeOffsetDefault(source, out value, out bytesConsumed);
-
-                case 'G':
-                    return TryParseDateTimeG(source, out DateTime _, out value, out bytesConsumed);
-
-                default:
-                    return ParserHelpers.TryParseThrowFormatException(out value, out bytesConsumed);
-            }
+                'R' => TryParseDateTimeOffsetR(source, NoFlipCase, out value, out bytesConsumed),
+                'l' => TryParseDateTimeOffsetR(source, FlipCase, out value, out bytesConsumed),
+                'O' => TryParseDateTimeOffsetO(source, out value, out bytesConsumed, out _),
+                default(char) => TryParseDateTimeOffsetDefault(source, out value, out bytesConsumed),
+                'G' => TryParseDateTimeG(source, out DateTime _, out value, out bytesConsumed),
+                _ => ParserHelpers.TryParseThrowFormatException(out value, out bytesConsumed),
+            };
         }
 
         private const uint FlipCase = 0x00000020u;  // XOR mask to flip the case of a letter.
diff --git a/src/System.Private.CoreLib/shared/System/Collections/ArrayList.cs b/src/System.Private.CoreLib/shared/System/Collections/ArrayList.cs
index d4ae3da5350d..52bc99fb20b3 100644
--- a/src/System.Private.CoreLib/shared/System/Collections/ArrayList.cs
+++ b/src/System.Private.CoreLib/shared/System/Collections/ArrayList.cs
@@ -983,8 +983,7 @@ public override void InsertRange(int index, ICollection c)
 
                 if (c.Count > 0)
                 {
-                    ArrayList? al = _list as ArrayList;
-                    if (al != null)
+                    if (_list is ArrayList al)
                     {
                         // We need to special case ArrayList.
                         // When c is a range of _list, we need to handle this in a special way.
diff --git a/src/System.Private.CoreLib/shared/System/Collections/Comparer.cs b/src/System.Private.CoreLib/shared/System/Collections/Comparer.cs
index e8f431be6db3..102d2c9f8b64 100644
--- a/src/System.Private.CoreLib/shared/System/Collections/Comparer.cs
+++ b/src/System.Private.CoreLib/shared/System/Collections/Comparer.cs
@@ -58,8 +58,7 @@ public int Compare(object? a, object? b)
             if (a == null) return -1;
             if (b == null) return 1;
 
-            string? sa = a as string;
-            if (sa != null && b is string sb)
+            if (a is string sa && b is string sb)
                 return _compareInfo.Compare(sa, sb);
 
             if (a is IComparable ia)
diff --git a/src/System.Private.CoreLib/shared/System/Convert.cs b/src/System.Private.CoreLib/shared/System/Convert.cs
index 8616cd946058..e9c230c2022b 100644
--- a/src/System.Private.CoreLib/shared/System/Convert.cs
+++ b/src/System.Private.CoreLib/shared/System/Convert.cs
@@ -208,47 +208,28 @@ public static bool IsDBNull(object? value)
             // This line is invalid for things like Enums that return a TypeCode
             // of int, but the object can't actually be cast to an int.
             //            if (v.GetTypeCode() == typeCode) return value;
-            switch (typeCode)
+            return typeCode switch
             {
-                case TypeCode.Boolean:
-                    return v.ToBoolean(provider);
-                case TypeCode.Char:
-                    return v.ToChar(provider);
-                case TypeCode.SByte:
-                    return v.ToSByte(provider);
-                case TypeCode.Byte:
-                    return v.ToByte(provider);
-                case TypeCode.Int16:
-                    return v.ToInt16(provider);
-                case TypeCode.UInt16:
-                    return v.ToUInt16(provider);
-                case TypeCode.Int32:
-                    return v.ToInt32(provider);
-                case TypeCode.UInt32:
-                    return v.ToUInt32(provider);
-                case TypeCode.Int64:
-                    return v.ToInt64(provider);
-                case TypeCode.UInt64:
-                    return v.ToUInt64(provider);
-                case TypeCode.Single:
-                    return v.ToSingle(provider);
-                case TypeCode.Double:
-                    return v.ToDouble(provider);
-                case TypeCode.Decimal:
-                    return v.ToDecimal(provider);
-                case TypeCode.DateTime:
-                    return v.ToDateTime(provider);
-                case TypeCode.String:
-                    return v.ToString(provider);
-                case TypeCode.Object:
-                    return value;
-                case TypeCode.DBNull:
-                    throw new InvalidCastException(SR.InvalidCast_DBNull);
-                case TypeCode.Empty:
-                    throw new InvalidCastException(SR.InvalidCast_Empty);
-                default:
-                    throw new ArgumentException(SR.Arg_UnknownTypeCode);
-            }
+                TypeCode.Boolean => v.ToBoolean(provider),
+                TypeCode.Char => v.ToChar(provider),
+                TypeCode.SByte => v.ToSByte(provider),
+                TypeCode.Byte => v.ToByte(provider),
+                TypeCode.Int16 => v.ToInt16(provider),
+                TypeCode.UInt16 => v.ToUInt16(provider),
+                TypeCode.Int32 => v.ToInt32(provider),
+                TypeCode.UInt32 => v.ToUInt32(provider),
+                TypeCode.Int64 => v.ToInt64(provider),
+                TypeCode.UInt64 => v.ToUInt64(provider),
+                TypeCode.Single => v.ToSingle(provider),
+                TypeCode.Double => v.ToDouble(provider),
+                TypeCode.Decimal => v.ToDecimal(provider),
+                TypeCode.DateTime => v.ToDateTime(provider),
+                TypeCode.String => v.ToString(provider),
+                TypeCode.Object => value,
+                TypeCode.DBNull => throw new InvalidCastException(SR.InvalidCast_DBNull),
+                TypeCode.Empty => throw new InvalidCastException(SR.InvalidCast_Empty),
+                _ => throw new ArgumentException(SR.Arg_UnknownTypeCode),
+            };
         }
 
         internal static object DefaultToType(IConvertible value, Type targetType, IFormatProvider? provider)
diff --git a/src/System.Private.CoreLib/shared/System/DateTime.Windows.cs b/src/System.Private.CoreLib/shared/System/DateTime.Windows.cs
index 468c3691ccd5..ebfed3c99e86 100644
--- a/src/System.Private.CoreLib/shared/System/DateTime.Windows.cs
+++ b/src/System.Private.CoreLib/shared/System/DateTime.Windows.cs
@@ -31,13 +31,12 @@ internal static unsafe bool IsValidTimeWithLeapSeconds(int year, int month, int
             DateTime dt = new DateTime(year, month, day);
             FullSystemTime time = new FullSystemTime(year, month, dt.DayOfWeek, day, hour, minute, second);
 
-            switch (kind)
+            return kind switch
             {
-                case DateTimeKind.Local: return ValidateSystemTime(&time.systemTime, localTime: true);
-                case DateTimeKind.Utc:   return ValidateSystemTime(&time.systemTime, localTime: false);
-                default:
-                    return ValidateSystemTime(&time.systemTime, localTime: true) || ValidateSystemTime(&time.systemTime, localTime: false);
-            }
+                DateTimeKind.Local => ValidateSystemTime(&time.systemTime, localTime: true),
+                DateTimeKind.Utc => ValidateSystemTime(&time.systemTime, localTime: false),
+                _ => ValidateSystemTime(&time.systemTime, localTime: true) || ValidateSystemTime(&time.systemTime, localTime: false),
+            };
         }
 
         private static unsafe DateTime FromFileTimeLeapSecondsAware(long fileTime)
diff --git a/src/System.Private.CoreLib/shared/System/DateTime.cs b/src/System.Private.CoreLib/shared/System/DateTime.cs
index 4bdbaf077caf..6cb1c1ecfc8a 100644
--- a/src/System.Private.CoreLib/shared/System/DateTime.cs
+++ b/src/System.Private.CoreLib/shared/System/DateTime.cs
@@ -1061,15 +1061,12 @@ public DateTimeKind Kind
         {
             get
             {
-                switch (InternalKind)
+                return InternalKind switch
                 {
-                    case KindUnspecified:
-                        return DateTimeKind.Unspecified;
-                    case KindUtc:
-                        return DateTimeKind.Utc;
-                    default:
-                        return DateTimeKind.Local;
-                }
+                    KindUnspecified => DateTimeKind.Unspecified,
+                    KindUtc => DateTimeKind.Utc,
+                    _ => DateTimeKind.Local,
+                };
             }
         }
 
diff --git a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/EventProvider.cs b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/EventProvider.cs
index 8460b8b2a314..4b22fc0c9a7c 100644
--- a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/EventProvider.cs
+++ b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/EventProvider.cs
@@ -134,25 +134,15 @@ public enum WriteEventErrorCode : int
         // EventSource has special logic to do this, no one else should be calling EventProvider.
         internal EventProvider(EventProviderType providerType)
         {
-            switch (providerType)
+            m_eventProvider = providerType switch
             {
-                case EventProviderType.ETW:
 #if PLATFORM_WINDOWS
-                    m_eventProvider = new EtwEventProvider();
-#else
-                    m_eventProvider = new NoOpEventProvider();
+                EventProviderType.ETW => new EtwEventProvider(),
 #endif
-                    break;
-                case EventProviderType.EventPipe:
 #if FEATURE_PERFTRACING
-                    m_eventProvider = new EventPipeEventProvider();
-#else
-                    m_eventProvider = new NoOpEventProvider();
+                EventProviderType.EventPipe => new EventPipeEventProvider(),
 #endif
-                    break;
-                default:
-                    m_eventProvider = new NoOpEventProvider();
-                    break;
+                _ => new NoOpEventProvider(),
             };
         }
 
diff --git a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/TraceLogging/Statics.cs b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/TraceLogging/Statics.cs
index 0131a00237ec..e32b3b827222 100644
--- a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/TraceLogging/Statics.cs
+++ b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/TraceLogging/Statics.cs
@@ -203,25 +203,18 @@ public static TraceLoggingDataType Format8(
             EventFieldFormat format,
             TraceLoggingDataType native)
         {
-            switch (format)
+            return format switch
             {
-                case EventFieldFormat.Default:
-                    return native;
-                case EventFieldFormat.String:
-                    return TraceLoggingDataType.Char8;
-                case EventFieldFormat.Boolean:
-                    return TraceLoggingDataType.Boolean8;
-                case EventFieldFormat.Hexadecimal:
-                    return TraceLoggingDataType.HexInt8;
+                EventFieldFormat.Default => native,
+                EventFieldFormat.String => TraceLoggingDataType.Char8,
+                EventFieldFormat.Boolean => TraceLoggingDataType.Boolean8,
+                EventFieldFormat.Hexadecimal => TraceLoggingDataType.HexInt8,
 #if false
-                case EventSourceFieldFormat.Signed:
-                    return TraceLoggingDataType.Int8;
-                case EventSourceFieldFormat.Unsigned:
-                    return TraceLoggingDataType.UInt8;
+                EventSourceFieldFormat.Signed => TraceLoggingDataType.Int8,
+                EventSourceFieldFormat.Unsigned => TraceLoggingDataType.UInt8,
 #endif
-                default:
-                    return MakeDataType(native, format);
-            }
+                _ => MakeDataType(native, format),
+            };
         }
 
         /// <summary>
@@ -234,25 +227,18 @@ public static TraceLoggingDataType Format16(
             EventFieldFormat format,
             TraceLoggingDataType native)
         {
-            switch (format)
+            return format switch
             {
-                case EventFieldFormat.Default:
-                    return native;
-                case EventFieldFormat.String:
-                    return TraceLoggingDataType.Char16;
-                case EventFieldFormat.Hexadecimal:
-                    return TraceLoggingDataType.HexInt16;
+                EventFieldFormat.Default => native,
+                EventFieldFormat.String => TraceLoggingDataType.Char16,
+                EventFieldFormat.Hexadecimal => TraceLoggingDataType.HexInt16,
 #if false
-                case EventSourceFieldFormat.Port:
-                    return TraceLoggingDataType.Port;
-                case EventSourceFieldFormat.Signed:
-                    return TraceLoggingDataType.Int16;
-                case EventSourceFieldFormat.Unsigned:
-                    return TraceLoggingDataType.UInt16;
+                EventSourceFieldFormat.Port => TraceLoggingDataType.Port,
+                EventSourceFieldFormat.Signed => TraceLoggingDataType.Int16,
+                EventSourceFieldFormat.Unsigned => TraceLoggingDataType.UInt16,
 #endif
-                default:
-                    return MakeDataType(native, format);
-            }
+                _ => MakeDataType(native, format),
+            };
         }
 
         /// <summary>
@@ -265,37 +251,27 @@ public static TraceLoggingDataType Format32(
             EventFieldFormat format,
             TraceLoggingDataType native)
         {
-            switch (format)
+            return format switch
             {
-                case EventFieldFormat.Default:
-                    return native;
-                case EventFieldFormat.Boolean:
-                    return TraceLoggingDataType.Boolean32;
-                case EventFieldFormat.Hexadecimal:
-                    return TraceLoggingDataType.HexInt32;
+                EventFieldFormat.Default => native,
+                EventFieldFormat.Boolean => TraceLoggingDataType.Boolean32,
+                EventFieldFormat.Hexadecimal => TraceLoggingDataType.HexInt32,
 #if false
-                case EventSourceFieldFormat.Ipv4Address:
-                    return TraceLoggingDataType.Ipv4Address;
-                case EventSourceFieldFormat.ProcessId:
-                    return TraceLoggingDataType.ProcessId;
-                case EventSourceFieldFormat.ThreadId:
-                    return TraceLoggingDataType.ThreadId;
-                case EventSourceFieldFormat.Win32Error:
-                    return TraceLoggingDataType.Win32Error;
-                case EventSourceFieldFormat.NTStatus:
-                    return TraceLoggingDataType.NTStatus;
+                EventSourceFieldFormat.Ipv4Address => TraceLoggingDataType.Ipv4Address,
+                EventSourceFieldFormat.ProcessId => TraceLoggingDataType.ProcessId,
+                EventSourceFieldFormat.ThreadId => TraceLoggingDataType.ThreadId,
+                EventSourceFieldFormat.Win32Error => TraceLoggingDataType.Win32Error,
+                EventSourceFieldFormat.NTStatus => TraceLoggingDataType.NTStatus,
 #endif
-                case EventFieldFormat.HResult:
-                    return TraceLoggingDataType.HResult;
+                EventFieldFormat.HResult => TraceLoggingDataType.HResult,
 #if false
                 case EventSourceFieldFormat.Signed:
                     return TraceLoggingDataType.Int32;
                 case EventSourceFieldFormat.Unsigned:
                     return TraceLoggingDataType.UInt32;
 #endif
-                default:
-                    return MakeDataType(native, format);
-            }
+                _ => MakeDataType(native, format),
+            };
         }
 
         /// <summary>
@@ -308,23 +284,17 @@ public static TraceLoggingDataType Format64(
             EventFieldFormat format,
             TraceLoggingDataType native)
         {
-            switch (format)
+            return format switch
             {
-                case EventFieldFormat.Default:
-                    return native;
-                case EventFieldFormat.Hexadecimal:
-                    return TraceLoggingDataType.HexInt64;
+                EventFieldFormat.Default => native,
+                EventFieldFormat.Hexadecimal => TraceLoggingDataType.HexInt64,
 #if false
-                case EventSourceFieldFormat.FileTime:
-                    return TraceLoggingDataType.FileTime;
-                case EventSourceFieldFormat.Signed:
-                    return TraceLoggingDataType.Int64;
-                case EventSourceFieldFormat.Unsigned:
-                    return TraceLoggingDataType.UInt64;
+                EventSourceFieldFormat.FileTime => TraceLoggingDataType.FileTime,
+                EventSourceFieldFormat.Signed => TraceLoggingDataType.Int64,
+                EventSourceFieldFormat.Unsigned => TraceLoggingDataType.UInt64,
 #endif
-                default:
-                    return MakeDataType(native, format);
-            }
+                _ => MakeDataType(native, format),
+            };
         }
 
         /// <summary>
@@ -337,21 +307,16 @@ public static TraceLoggingDataType FormatPtr(
             EventFieldFormat format,
             TraceLoggingDataType native)
         {
-            switch (format)
+            return format switch
             {
-                case EventFieldFormat.Default:
-                    return native;
-                case EventFieldFormat.Hexadecimal:
-                    return HexIntPtrType;
+                EventFieldFormat.Default => native,
+                EventFieldFormat.Hexadecimal => HexIntPtrType,
 #if false
-                case EventSourceFieldFormat.Signed:
-                    return IntPtrType;
-                case EventSourceFieldFormat.Unsigned:
-                    return UIntPtrType;
+                EventSourceFieldFormat.Signed => IntPtrType,
+                EventSourceFieldFormat.Unsigned => UIntPtrType,
 #endif
-                default:
-                    return MakeDataType(native, format);
-            }
+                _ => MakeDataType(native, format),
+            };
         }
 
         #endregion
diff --git a/src/System.Private.CoreLib/shared/System/Enum.cs b/src/System.Private.CoreLib/shared/System/Enum.cs
index df7c1eeadd8d..206b1e3fd517 100644
--- a/src/System.Private.CoreLib/shared/System/Enum.cs
+++ b/src/System.Private.CoreLib/shared/System/Enum.cs
@@ -42,39 +42,24 @@ public abstract partial class Enum : ValueType, IComparable, IFormattable, IConv
         private string ValueToString()
         {
             ref byte data = ref this.GetRawData();
-            switch (InternalGetCorElementType())
+            return (InternalGetCorElementType()) switch
             {
-                case CorElementType.ELEMENT_TYPE_I1:
-                    return Unsafe.As<byte, sbyte>(ref data).ToString();
-                case CorElementType.ELEMENT_TYPE_U1:
-                    return data.ToString();
-                case CorElementType.ELEMENT_TYPE_BOOLEAN:
-                    return Unsafe.As<byte, bool>(ref data).ToString();
-                case CorElementType.ELEMENT_TYPE_I2:
-                    return Unsafe.As<byte, short>(ref data).ToString();
-                case CorElementType.ELEMENT_TYPE_U2:
-                    return Unsafe.As<byte, ushort>(ref data).ToString();
-                case CorElementType.ELEMENT_TYPE_CHAR:
-                    return Unsafe.As<byte, char>(ref data).ToString();
-                case CorElementType.ELEMENT_TYPE_I4:
-                    return Unsafe.As<byte, int>(ref data).ToString();
-                case CorElementType.ELEMENT_TYPE_U4:
-                    return Unsafe.As<byte, uint>(ref data).ToString();
-                case CorElementType.ELEMENT_TYPE_R4:
-                    return Unsafe.As<byte, float>(ref data).ToString();
-                case CorElementType.ELEMENT_TYPE_I8:
-                    return Unsafe.As<byte, long>(ref data).ToString();
-                case CorElementType.ELEMENT_TYPE_U8:
-                    return Unsafe.As<byte, ulong>(ref data).ToString();
-                case CorElementType.ELEMENT_TYPE_R8:
-                    return Unsafe.As<byte, double>(ref data).ToString();
-                case CorElementType.ELEMENT_TYPE_I:
-                    return Unsafe.As<byte, IntPtr>(ref data).ToString();
-                case CorElementType.ELEMENT_TYPE_U:
-                    return Unsafe.As<byte, UIntPtr>(ref data).ToString();
-                default:
-                    throw new InvalidOperationException(SR.InvalidOperation_UnknownEnumType);
-            }
+                CorElementType.ELEMENT_TYPE_I1 => Unsafe.As<byte, sbyte>(ref data).ToString(),
+                CorElementType.ELEMENT_TYPE_U1 => data.ToString(),
+                CorElementType.ELEMENT_TYPE_BOOLEAN => Unsafe.As<byte, bool>(ref data).ToString(),
+                CorElementType.ELEMENT_TYPE_I2 => Unsafe.As<byte, short>(ref data).ToString(),
+                CorElementType.ELEMENT_TYPE_U2 => Unsafe.As<byte, ushort>(ref data).ToString(),
+                CorElementType.ELEMENT_TYPE_CHAR => Unsafe.As<byte, char>(ref data).ToString(),
+                CorElementType.ELEMENT_TYPE_I4 => Unsafe.As<byte, int>(ref data).ToString(),
+                CorElementType.ELEMENT_TYPE_U4 => Unsafe.As<byte, uint>(ref data).ToString(),
+                CorElementType.ELEMENT_TYPE_R4 => Unsafe.As<byte, float>(ref data).ToString(),
+                CorElementType.ELEMENT_TYPE_I8 => Unsafe.As<byte, long>(ref data).ToString(),
+                CorElementType.ELEMENT_TYPE_U8 => Unsafe.As<byte, ulong>(ref data).ToString(),
+                CorElementType.ELEMENT_TYPE_R8 => Unsafe.As<byte, double>(ref data).ToString(),
+                CorElementType.ELEMENT_TYPE_I => Unsafe.As<byte, IntPtr>(ref data).ToString(),
+                CorElementType.ELEMENT_TYPE_U => Unsafe.As<byte, UIntPtr>(ref data).ToString(),
+                _ => throw new InvalidOperationException(SR.InvalidOperation_UnknownEnumType),
+            };
         }
 
         private string ValueToHexString()
@@ -104,33 +89,20 @@ private string ValueToHexString()
 
         private static string ValueToHexString(object value)
         {
-            switch (Convert.GetTypeCode(value))
+            return (Convert.GetTypeCode(value)) switch
             {
-                case TypeCode.SByte:
-                    return ((byte)(sbyte)value).ToString("X2", null);
-                case TypeCode.Byte:
-                    return ((byte)value).ToString("X2", null);
-                case TypeCode.Boolean:
-                    // direct cast from bool to byte is not allowed
-                    return Convert.ToByte((bool)value).ToString("X2", null);
-                case TypeCode.Int16:
-                    return ((ushort)(short)value).ToString("X4", null);
-                case TypeCode.UInt16:
-                    return ((ushort)value).ToString("X4", null);
-                case TypeCode.Char:
-                    return ((ushort)(char)value).ToString("X4", null);
-                case TypeCode.UInt32:
-                    return ((uint)value).ToString("X8", null);
-                case TypeCode.Int32:
-                    return ((uint)(int)value).ToString("X8", null);
-                case TypeCode.UInt64:
-                    return ((ulong)value).ToString("X16", null);
-                case TypeCode.Int64:
-                    return ((ulong)(long)value).ToString("X16", null);
-                // All unsigned types will be directly cast
-                default:
-                    throw new InvalidOperationException(SR.InvalidOperation_UnknownEnumType);
-            }
+                TypeCode.SByte => ((byte)(sbyte)value).ToString("X2", null),
+                TypeCode.Byte => ((byte)value).ToString("X2", null),
+                TypeCode.Boolean => Convert.ToByte((bool)value).ToString("X2", null), // direct cast from bool to byte is not allowed
+                TypeCode.Int16 => ((ushort)(short)value).ToString("X4", null),
+                TypeCode.UInt16 => ((ushort)value).ToString("X4", null),
+                TypeCode.Char => ((ushort)(char)value).ToString("X4", null),
+                TypeCode.UInt32 => ((uint)value).ToString("X8", null),
+                TypeCode.Int32 => ((uint)(int)value).ToString("X8", null),
+                TypeCode.UInt64 => ((ulong)value).ToString("X16", null),
+                TypeCode.Int64 => ((ulong)(long)value).ToString("X16", null),
+                _ => throw new InvalidOperationException(SR.InvalidOperation_UnknownEnumType),
+            };
         }
 
         internal static string? GetEnumName(RuntimeType enumType, ulong ulValue)
@@ -271,46 +243,20 @@ internal static ulong ToUInt64(object value)
             // Helper function to silently convert the value to UInt64 from the other base types for enum without throwing an exception.
             // This is need since the Convert functions do overflow checks.
             TypeCode typeCode = Convert.GetTypeCode(value);
-
-            ulong result;
-            switch (typeCode)
+            var result = typeCode switch
             {
-                case TypeCode.SByte:
-                    result = (ulong)(sbyte)value;
-                    break;
-                case TypeCode.Byte:
-                    result = (byte)value;
-                    break;
-                case TypeCode.Boolean:
-                    // direct cast from bool to byte is not allowed
-                    result = Convert.ToByte((bool)value);
-                    break;
-                case TypeCode.Int16:
-                    result = (ulong)(short)value;
-                    break;
-                case TypeCode.UInt16:
-                    result = (ushort)value;
-                    break;
-                case TypeCode.Char:
-                    result = (ushort)(char)value;
-                    break;
-                case TypeCode.UInt32:
-                    result = (uint)value;
-                    break;
-                case TypeCode.Int32:
-                    result = (ulong)(int)value;
-                    break;
-                case TypeCode.UInt64:
-                    result = (ulong)value;
-                    break;
-                case TypeCode.Int64:
-                    result = (ulong)(long)value;
-                    break;
-                // All unsigned types will be directly cast
-                default:
-                    throw new InvalidOperationException(SR.InvalidOperation_UnknownEnumType);
-            }
-
+                TypeCode.SByte => (ulong)(sbyte)value,
+                TypeCode.Byte => (byte)value,
+                TypeCode.Boolean => Convert.ToByte((bool)value), // direct cast from bool to byte is not allowed
+                TypeCode.Int16 => (ulong)(short)value,
+                TypeCode.UInt16 => (ushort)value,
+                TypeCode.Char => (ushort)(char)value,
+                TypeCode.UInt32 => (uint)value,
+                TypeCode.Int32 => (ulong)(int)value,
+                TypeCode.UInt64 => (ulong)value,
+                TypeCode.Int64 => (ulong)(long)value,
+                _ => throw new InvalidOperationException(SR.InvalidOperation_UnknownEnumType),
+            };
             return result;
         }
 
@@ -786,42 +732,20 @@ public static object ToObject(Type enumType, object value)
             // Delegate rest of error checking to the other functions
             TypeCode typeCode = Convert.GetTypeCode(value);
 
-            switch (typeCode)
+            return typeCode switch
             {
-                case TypeCode.Int32:
-                    return ToObject(enumType, (int)value);
-
-                case TypeCode.SByte:
-                    return ToObject(enumType, (sbyte)value);
-
-                case TypeCode.Int16:
-                    return ToObject(enumType, (short)value);
-
-                case TypeCode.Int64:
-                    return ToObject(enumType, (long)value);
-
-                case TypeCode.UInt32:
-                    return ToObject(enumType, (uint)value);
-
-                case TypeCode.Byte:
-                    return ToObject(enumType, (byte)value);
-
-                case TypeCode.UInt16:
-                    return ToObject(enumType, (ushort)value);
-
-                case TypeCode.UInt64:
-                    return ToObject(enumType, (ulong)value);
-
-                case TypeCode.Char:
-                    return ToObject(enumType, (char)value);
-
-                case TypeCode.Boolean:
-                    return ToObject(enumType, (bool)value);
-
-                default:
-                    // All unsigned types will be directly cast
-                    throw new ArgumentException(SR.Arg_MustBeEnumBaseTypeOrEnum, nameof(value));
-            }
+                TypeCode.Int32 => ToObject(enumType, (int)value),
+                TypeCode.SByte => ToObject(enumType, (sbyte)value),
+                TypeCode.Int16 => ToObject(enumType, (short)value),
+                TypeCode.Int64 => ToObject(enumType, (long)value),
+                TypeCode.UInt32 => ToObject(enumType, (uint)value),
+                TypeCode.Byte => ToObject(enumType, (byte)value),
+                TypeCode.UInt16 => ToObject(enumType, (ushort)value),
+                TypeCode.UInt64 => ToObject(enumType, (ulong)value),
+                TypeCode.Char => ToObject(enumType, (char)value),
+                TypeCode.Boolean => ToObject(enumType, (bool)value),
+                _ => throw new ArgumentException(SR.Arg_MustBeEnumBaseTypeOrEnum, nameof(value)),
+            };
         }
 
         public static string Format(Type enumType, object value, string format)
@@ -886,39 +810,24 @@ public static string Format(Type enumType, object value, string format)
         internal object GetValue()
         {
             ref byte data = ref this.GetRawData();
-            switch (InternalGetCorElementType())
+            return (InternalGetCorElementType()) switch
             {
-                case CorElementType.ELEMENT_TYPE_I1:
-                    return Unsafe.As<byte, sbyte>(ref data);
-                case CorElementType.ELEMENT_TYPE_U1:
-                    return data;
-                case CorElementType.ELEMENT_TYPE_BOOLEAN:
-                    return Unsafe.As<byte, bool>(ref data);
-                case CorElementType.ELEMENT_TYPE_I2:
-                    return Unsafe.As<byte, short>(ref data);
-                case CorElementType.ELEMENT_TYPE_U2:
-                    return Unsafe.As<byte, ushort>(ref data);
-                case CorElementType.ELEMENT_TYPE_CHAR:
-                    return Unsafe.As<byte, char>(ref data);
-                case CorElementType.ELEMENT_TYPE_I4:
-                    return Unsafe.As<byte, int>(ref data);
-                case CorElementType.ELEMENT_TYPE_U4:
-                    return Unsafe.As<byte, uint>(ref data);
-                case CorElementType.ELEMENT_TYPE_R4:
-                    return Unsafe.As<byte, float>(ref data);
-                case CorElementType.ELEMENT_TYPE_I8:
-                    return Unsafe.As<byte, long>(ref data);
-                case CorElementType.ELEMENT_TYPE_U8:
-                    return Unsafe.As<byte, ulong>(ref data);
-                case CorElementType.ELEMENT_TYPE_R8:
-                    return Unsafe.As<byte, double>(ref data);
-                case CorElementType.ELEMENT_TYPE_I:
-                    return Unsafe.As<byte, IntPtr>(ref data);
-                case CorElementType.ELEMENT_TYPE_U:
-                    return Unsafe.As<byte, UIntPtr>(ref data);
-                default:
-                    throw new InvalidOperationException(SR.InvalidOperation_UnknownEnumType);
-            }
+                CorElementType.ELEMENT_TYPE_I1 => Unsafe.As<byte, sbyte>(ref data),
+                CorElementType.ELEMENT_TYPE_U1 => data,
+                CorElementType.ELEMENT_TYPE_BOOLEAN => Unsafe.As<byte, bool>(ref data),
+                CorElementType.ELEMENT_TYPE_I2 => Unsafe.As<byte, short>(ref data),
+                CorElementType.ELEMENT_TYPE_U2 => Unsafe.As<byte, ushort>(ref data),
+                CorElementType.ELEMENT_TYPE_CHAR => Unsafe.As<byte, char>(ref data),
+                CorElementType.ELEMENT_TYPE_I4 => Unsafe.As<byte, int>(ref data),
+                CorElementType.ELEMENT_TYPE_U4 => Unsafe.As<byte, uint>(ref data),
+                CorElementType.ELEMENT_TYPE_R4 => Unsafe.As<byte, float>(ref data),
+                CorElementType.ELEMENT_TYPE_I8 => Unsafe.As<byte, long>(ref data),
+                CorElementType.ELEMENT_TYPE_U8 => Unsafe.As<byte, ulong>(ref data),
+                CorElementType.ELEMENT_TYPE_R8 => Unsafe.As<byte, double>(ref data),
+                CorElementType.ELEMENT_TYPE_I => Unsafe.As<byte, IntPtr>(ref data),
+                CorElementType.ELEMENT_TYPE_U => Unsafe.As<byte, UIntPtr>(ref data),
+                _ => throw new InvalidOperationException(SR.InvalidOperation_UnknownEnumType),
+            };
         }
 
         private ulong ToUInt64()
@@ -966,39 +875,24 @@ public override int GetHashCode()
             // The runtime can bypass calls to Enum::GetHashCode and call the underlying type's GetHashCode directly
             // to avoid boxing the enum.
             ref byte data = ref this.GetRawData();
-            switch (InternalGetCorElementType())
+            return (InternalGetCorElementType()) switch
             {
-                case CorElementType.ELEMENT_TYPE_I1:
-                    return Unsafe.As<byte, sbyte>(ref data).GetHashCode();
-                case CorElementType.ELEMENT_TYPE_U1:
-                    return data.GetHashCode();
-                case CorElementType.ELEMENT_TYPE_BOOLEAN:
-                    return Unsafe.As<byte, bool>(ref data).GetHashCode();
-                case CorElementType.ELEMENT_TYPE_I2:
-                    return Unsafe.As<byte, short>(ref data).GetHashCode();
-                case CorElementType.ELEMENT_TYPE_U2:
-                    return Unsafe.As<byte, ushort>(ref data).GetHashCode();
-                case CorElementType.ELEMENT_TYPE_CHAR:
-                    return Unsafe.As<byte, char>(ref data).GetHashCode();
-                case CorElementType.ELEMENT_TYPE_I4:
-                    return Unsafe.As<byte, int>(ref data).GetHashCode();
-                case CorElementType.ELEMENT_TYPE_U4:
-                    return Unsafe.As<byte, uint>(ref data).GetHashCode();
-                case CorElementType.ELEMENT_TYPE_R4:
-                    return Unsafe.As<byte, float>(ref data).GetHashCode();
-                case CorElementType.ELEMENT_TYPE_I8:
-                    return Unsafe.As<byte, long>(ref data).GetHashCode();
-                case CorElementType.ELEMENT_TYPE_U8:
-                    return Unsafe.As<byte, ulong>(ref data).GetHashCode();
-                case CorElementType.ELEMENT_TYPE_R8:
-                    return Unsafe.As<byte, double>(ref data).GetHashCode();
-                case CorElementType.ELEMENT_TYPE_I:
-                    return Unsafe.As<byte, IntPtr>(ref data).GetHashCode();
-                case CorElementType.ELEMENT_TYPE_U:
-                    return Unsafe.As<byte, UIntPtr>(ref data).GetHashCode();
-                default:
-                    throw new InvalidOperationException(SR.InvalidOperation_UnknownEnumType);
-            }
+                CorElementType.ELEMENT_TYPE_I1 => Unsafe.As<byte, sbyte>(ref data).GetHashCode(),
+                CorElementType.ELEMENT_TYPE_U1 => data.GetHashCode(),
+                CorElementType.ELEMENT_TYPE_BOOLEAN => Unsafe.As<byte, bool>(ref data).GetHashCode(),
+                CorElementType.ELEMENT_TYPE_I2 => Unsafe.As<byte, short>(ref data).GetHashCode(),
+                CorElementType.ELEMENT_TYPE_U2 => Unsafe.As<byte, ushort>(ref data).GetHashCode(),
+                CorElementType.ELEMENT_TYPE_CHAR => Unsafe.As<byte, char>(ref data).GetHashCode(),
+                CorElementType.ELEMENT_TYPE_I4 => Unsafe.As<byte, int>(ref data).GetHashCode(),
+                CorElementType.ELEMENT_TYPE_U4 => Unsafe.As<byte, uint>(ref data).GetHashCode(),
+                CorElementType.ELEMENT_TYPE_R4 => Unsafe.As<byte, float>(ref data).GetHashCode(),
+                CorElementType.ELEMENT_TYPE_I8 => Unsafe.As<byte, long>(ref data).GetHashCode(),
+                CorElementType.ELEMENT_TYPE_U8 => Unsafe.As<byte, ulong>(ref data).GetHashCode(),
+                CorElementType.ELEMENT_TYPE_R8 => Unsafe.As<byte, double>(ref data).GetHashCode(),
+                CorElementType.ELEMENT_TYPE_I => Unsafe.As<byte, IntPtr>(ref data).GetHashCode(),
+                CorElementType.ELEMENT_TYPE_U => Unsafe.As<byte, UIntPtr>(ref data).GetHashCode(),
+                _ => throw new InvalidOperationException(SR.InvalidOperation_UnknownEnumType),
+            };
         }
 
         public override string ToString()
@@ -1066,31 +960,20 @@ public string ToString(IFormatProvider? provider)
         #region IConvertible
         public TypeCode GetTypeCode()
         {
-            switch (InternalGetCorElementType())
+            return (InternalGetCorElementType()) switch
             {
-                case CorElementType.ELEMENT_TYPE_I1:
-                    return TypeCode.SByte;
-                case CorElementType.ELEMENT_TYPE_U1:
-                    return TypeCode.Byte;
-                case CorElementType.ELEMENT_TYPE_BOOLEAN:
-                    return TypeCode.Boolean;
-                case CorElementType.ELEMENT_TYPE_I2:
-                    return TypeCode.Int16;
-                case CorElementType.ELEMENT_TYPE_U2:
-                    return TypeCode.UInt16;
-                case CorElementType.ELEMENT_TYPE_CHAR:
-                    return TypeCode.Char;
-                case CorElementType.ELEMENT_TYPE_I4:
-                    return TypeCode.Int32;
-                case CorElementType.ELEMENT_TYPE_U4:
-                    return TypeCode.UInt32;
-                case CorElementType.ELEMENT_TYPE_I8:
-                    return TypeCode.Int64;
-                case CorElementType.ELEMENT_TYPE_U8:
-                    return TypeCode.UInt64;
-                default:
-                    throw new InvalidOperationException(SR.InvalidOperation_UnknownEnumType);
-            }
+                CorElementType.ELEMENT_TYPE_I1 => TypeCode.SByte,
+                CorElementType.ELEMENT_TYPE_U1 => TypeCode.Byte,
+                CorElementType.ELEMENT_TYPE_BOOLEAN => TypeCode.Boolean,
+                CorElementType.ELEMENT_TYPE_I2 => TypeCode.Int16,
+                CorElementType.ELEMENT_TYPE_U2 => TypeCode.UInt16,
+                CorElementType.ELEMENT_TYPE_CHAR => TypeCode.Char,
+                CorElementType.ELEMENT_TYPE_I4 => TypeCode.Int32,
+                CorElementType.ELEMENT_TYPE_U4 => TypeCode.UInt32,
+                CorElementType.ELEMENT_TYPE_I8 => TypeCode.Int64,
+                CorElementType.ELEMENT_TYPE_U8 => TypeCode.UInt64,
+                _ => throw new InvalidOperationException(SR.InvalidOperation_UnknownEnumType),
+            };
         }
 
         bool IConvertible.ToBoolean(IFormatProvider? provider)
diff --git a/src/System.Private.CoreLib/shared/System/Environment.cs b/src/System.Private.CoreLib/shared/System/Environment.cs
index 2b638fde9eb3..0f602030c528 100644
--- a/src/System.Private.CoreLib/shared/System/Environment.cs
+++ b/src/System.Private.CoreLib/shared/System/Environment.cs
@@ -162,8 +162,7 @@ public static long WorkingSet
                 // present in Process.  If it proves important, we could look at separating that functionality out of Process into
                 // Common files which could also be included here.
                 Type? processType = Type.GetType("System.Diagnostics.Process, System.Diagnostics.Process, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a", throwOnError: false);
-                IDisposable? currentProcess = processType?.GetMethod("GetCurrentProcess")?.Invoke(null, BindingFlags.DoNotWrapExceptions, null, null, null) as IDisposable;
-                if (currentProcess != null)
+                if (processType?.GetMethod("GetCurrentProcess")?.Invoke(null, BindingFlags.DoNotWrapExceptions, null, null, null) is IDisposable currentProcess)
                 {
                     using (currentProcess)
                     {
diff --git a/src/System.Private.CoreLib/shared/System/Globalization/Calendar.cs b/src/System.Private.CoreLib/shared/System/Globalization/Calendar.cs
index 4460d3d06a22..86e630180c34 100644
--- a/src/System.Private.CoreLib/shared/System/Globalization/Calendar.cs
+++ b/src/System.Private.CoreLib/shared/System/Globalization/Calendar.cs
@@ -520,19 +520,17 @@ public virtual int GetWeekOfYear(DateTime time, CalendarWeekRule rule, DayOfWeek
                     firstDayOfWeek,
                     SR.Format(SR.ArgumentOutOfRange_Range, DayOfWeek.Sunday, DayOfWeek.Saturday));
             }
-            switch (rule)
+
+            return rule switch
             {
-                case CalendarWeekRule.FirstDay:
-                    return GetFirstDayWeekOfYear(time, (int)firstDayOfWeek);
-                case CalendarWeekRule.FirstFullWeek:
-                    return GetWeekOfYearFullDays(time, (int)firstDayOfWeek, 7);
-                case CalendarWeekRule.FirstFourDayWeek:
-                    return GetWeekOfYearFullDays(time, (int)firstDayOfWeek, 4);
-                default:
-                    throw new ArgumentOutOfRangeException(
+                CalendarWeekRule.FirstDay => GetFirstDayWeekOfYear(time, (int)firstDayOfWeek),
+                CalendarWeekRule.FirstFullWeek => GetWeekOfYearFullDays(time, (int)firstDayOfWeek, 7),
+                CalendarWeekRule.FirstFourDayWeek => GetWeekOfYearFullDays(time, (int)firstDayOfWeek, 4),
+                _ => throw new ArgumentOutOfRangeException(
                         nameof(rule),
                         rule,
-                        SR.Format(SR.ArgumentOutOfRange_Range, CalendarWeekRule.FirstDay, CalendarWeekRule.FirstFourDayWeek));            }
+                        SR.Format(SR.ArgumentOutOfRange_Range, CalendarWeekRule.FirstDay, CalendarWeekRule.FirstFourDayWeek)),
+            };
         }
 
         /// <summary>
diff --git a/src/System.Private.CoreLib/shared/System/Globalization/CultureInfo.cs b/src/System.Private.CoreLib/shared/System/Globalization/CultureInfo.cs
index 63dd62c7053e..76ddc4c69330 100644
--- a/src/System.Private.CoreLib/shared/System/Globalization/CultureInfo.cs
+++ b/src/System.Private.CoreLib/shared/System/Globalization/CultureInfo.cs
@@ -649,15 +649,12 @@ public string IetfLanguageTag
             get
             {
                 // special case the compatibility cultures
-                switch (this.Name)
+                return Name switch
                 {
-                    case "zh-CHT":
-                        return "zh-Hant";
-                    case "zh-CHS":
-                        return "zh-Hans";
-                    default:
-                        return this.Name;
-                }
+                    "zh-CHT" => "zh-Hant",
+                    "zh-CHS" => "zh-Hans",
+                    _ => Name,
+                };
             }
         }
 
diff --git a/src/System.Private.CoreLib/shared/System/Globalization/DateTimeFormatInfo.cs b/src/System.Private.CoreLib/shared/System/Globalization/DateTimeFormatInfo.cs
index 67a7b47c5e89..07327ff902d8 100644
--- a/src/System.Private.CoreLib/shared/System/Globalization/DateTimeFormatInfo.cs
+++ b/src/System.Private.CoreLib/shared/System/Globalization/DateTimeFormatInfo.cs
@@ -1316,19 +1316,12 @@ public string[] MonthNames
         /// </summary>
         internal string InternalGetMonthName(int month, MonthNameStyles style, bool abbreviated)
         {
-            string[] monthNamesArray;
-            switch (style)
+            string[] monthNamesArray = style switch
             {
-                case MonthNameStyles.Genitive:
-                    monthNamesArray = InternalGetGenitiveMonthNames(abbreviated);
-                    break;
-                case MonthNameStyles.LeapYear:
-                    monthNamesArray = InternalGetLeapYearMonthNames();
-                    break;
-                default:
-                    monthNamesArray = (abbreviated ? InternalGetAbbreviatedMonthNames() : InternalGetMonthNames());
-                    break;
-            }
+                MonthNameStyles.Genitive => InternalGetGenitiveMonthNames(abbreviated),
+                MonthNameStyles.LeapYear => InternalGetLeapYearMonthNames(),
+                _ => (abbreviated ? InternalGetAbbreviatedMonthNames() : InternalGetMonthNames()),
+            };
 
             // The month range is from 1 ~ m_monthNames.Length
             // (actually is 13 right now for all cases)
diff --git a/src/System.Private.CoreLib/shared/System/Globalization/HebrewCalendar.cs b/src/System.Private.CoreLib/shared/System/Globalization/HebrewCalendar.cs
index d72e3817b19f..667721dc1627 100644
--- a/src/System.Private.CoreLib/shared/System/Globalization/HebrewCalendar.cs
+++ b/src/System.Private.CoreLib/shared/System/Globalization/HebrewCalendar.cs
@@ -363,17 +363,13 @@ private static void CheckTicksRange(long ticks)
 
         private static int GetResult(DateBuffer result, int part)
         {
-            switch (part)
+            return part switch
             {
-                case DatePartYear:
-                    return result.year;
-                case DatePartMonth:
-                    return result.month;
-                case DatePartDay:
-                    return result.day;
-            }
-
-            throw new InvalidOperationException(SR.InvalidOperation_DateTimeParsing);
+                DatePartYear => result.year,
+                DatePartMonth => result.month,
+                DatePartDay => result.day,
+                _ => throw new InvalidOperationException(SR.InvalidOperation_DateTimeParsing),
+            };
         }
 
         /// <summary>
diff --git a/src/System.Private.CoreLib/shared/System/Globalization/TimeSpanParse.cs b/src/System.Private.CoreLib/shared/System/Globalization/TimeSpanParse.cs
index 7dfac05aaddc..4f492b755efb 100644
--- a/src/System.Private.CoreLib/shared/System/Globalization/TimeSpanParse.cs
+++ b/src/System.Private.CoreLib/shared/System/Globalization/TimeSpanParse.cs
@@ -568,18 +568,18 @@ internal bool SetBadFormatSpecifierFailure(char? formatSpecifierCharacter = null
 
         internal static long Pow10(int pow)
         {
-            switch (pow)
-            {
-                case 0:  return 1;
-                case 1:  return 10;
-                case 2:  return 100;
-                case 3:  return 1000;
-                case 4:  return 10000;
-                case 5:  return 100000;
-                case 6:  return 1000000;
-                case 7:  return 10000000;
-                default: return (long)Math.Pow(10, pow);
-            }
+            return pow switch
+            {
+                0 => 1,
+                1 => 10,
+                2 => 100,
+                3 => 1000,
+                4 => 10000,
+                5 => 100000,
+                6 => 1000000,
+                7 => 10000000,
+                _ => (long)Math.Pow(10, pow),
+            };
         }
 
         private static bool TryTimeToTicks(bool positive, TimeSpanToken days, TimeSpanToken hours, TimeSpanToken minutes, TimeSpanToken seconds, TimeSpanToken fraction, out long result)
@@ -740,15 +740,15 @@ private static bool ProcessTerminalState(ref TimeSpanRawInfo raw, TimeSpanStanda
                 }
             }
 
-            switch (raw._numCount)
+            return raw._numCount switch
             {
-                case 1: return ProcessTerminal_D(ref raw, style, ref result);
-                case 2: return ProcessTerminal_HM(ref raw, style, ref result);
-                case 3: return ProcessTerminal_HM_S_D(ref raw, style, ref result);
-                case 4: return ProcessTerminal_HMS_F_D(ref raw, style, ref result);
-                case 5: return ProcessTerminal_DHMSF(ref raw, style, ref result);
-                default: return result.SetBadTimeSpanFailure();
-            }
+                1 => ProcessTerminal_D(ref raw, style, ref result),
+                2 => ProcessTerminal_HM(ref raw, style, ref result),
+                3 => ProcessTerminal_HM_S_D(ref raw, style, ref result),
+                4 => ProcessTerminal_HMS_F_D(ref raw, style, ref result),
+                5 => ProcessTerminal_DHMSF(ref raw, style, ref result),
+                _ => result.SetBadTimeSpanFailure(),
+            };
         }
 
         /// <summary>Validate the 5-number "Days.Hours:Minutes:Seconds.Fraction" terminal case.</summary>
diff --git a/src/System.Private.CoreLib/shared/System/Guid.cs b/src/System.Private.CoreLib/shared/System/Guid.cs
index ba92c19381cc..1a9aee78a566 100644
--- a/src/System.Private.CoreLib/shared/System/Guid.cs
+++ b/src/System.Private.CoreLib/shared/System/Guid.cs
@@ -238,33 +238,15 @@ public static Guid ParseExact(ReadOnlySpan<char> input, ReadOnlySpan<char> forma
             input = input.Trim();
 
             var result = new GuidResult(GuidParseThrowStyle.AllButOverflow);
-            bool success;
-            switch ((char)(format[0] | 0x20))
-            {
-                case 'd':
-                    success = TryParseExactD(input, ref result);
-                    break;
-
-                case 'n':
-                    success = TryParseExactN(input, ref result);
-                    break;
-
-                case 'b':
-                    success = TryParseExactB(input, ref result);
-                    break;
-
-                case 'p':
-                    success = TryParseExactP(input, ref result);
-                    break;
-
-                case 'x':
-                    success = TryParseExactX(input, ref result);
-                    break;
-
-                default:
-                    throw new FormatException(SR.Format_InvalidGuidFormatSpecification);
-            }
-
+            bool success = ((char)(format[0] | 0x20)) switch
+            {
+                'd' => TryParseExactD(input, ref result),
+                'n' => TryParseExactN(input, ref result),
+                'b' => TryParseExactB(input, ref result),
+                'p' => TryParseExactP(input, ref result),
+                'x' => TryParseExactX(input, ref result),
+                _ => throw new FormatException(SR.Format_InvalidGuidFormatSpecification),
+            };
             Debug.Assert(success, "GuidParseThrowStyle.AllButOverflow means throw on all failures");
             return result._parsedGuid;
         }
@@ -337,21 +319,16 @@ private static bool TryParseGuid(ReadOnlySpan<char> guidString, ref GuidResult r
                 return false;
             }
 
-            switch (guidString[0])
+            return (guidString[0]) switch
             {
-                case '(':
-                    return TryParseExactP(guidString, ref result);
-
-                case '{':
-                    return guidString.Contains('-') ?
+                '(' => TryParseExactP(guidString, ref result),
+                '{' => guidString.Contains('-') ?
                         TryParseExactB(guidString, ref result) :
-                        TryParseExactX(guidString, ref result);
-
-                default:
-                    return guidString.Contains('-') ?
+                        TryParseExactX(guidString, ref result),
+                _ => guidString.Contains('-') ?
                         TryParseExactD(guidString, ref result) :
-                        TryParseExactN(guidString, ref result);
-            }
+                        TryParseExactN(guidString, ref result),
+            };
         }
 
         // Two helpers used for parsing components:
diff --git a/src/System.Private.CoreLib/shared/System/Runtime/Intrinsics/X86/Avx2.cs b/src/System.Private.CoreLib/shared/System/Runtime/Intrinsics/X86/Avx2.cs
index 3a066bb38ccd..f3e5877447db 100644
--- a/src/System.Private.CoreLib/shared/System/Runtime/Intrinsics/X86/Avx2.cs
+++ b/src/System.Private.CoreLib/shared/System/Runtime/Intrinsics/X86/Avx2.cs
@@ -848,19 +848,14 @@ internal Avx2() { }
         /// </summary>
         public static unsafe Vector128<int> GatherVector128(int* baseAddress, Vector128<int> index, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherVector128(baseAddress, index, 1);
-                case 2:
-                    return GatherVector128(baseAddress, index, 2);
-                case 4:
-                    return GatherVector128(baseAddress, index, 4);
-                case 8:
-                    return GatherVector128(baseAddress, index, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherVector128(baseAddress, index, 1),
+                2 => GatherVector128(baseAddress, index, 2),
+                4 => GatherVector128(baseAddress, index, 4),
+                8 => GatherVector128(baseAddress, index, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m128i _mm_i32gather_epi32 (int const* base_addr, __m128i vindex, const int scale)
@@ -869,19 +864,14 @@ public static unsafe Vector128<int> GatherVector128(int* baseAddress, Vector128<
         /// </summary>
         public static unsafe Vector128<uint> GatherVector128(uint* baseAddress, Vector128<int> index, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherVector128(baseAddress, index, 1);
-                case 2:
-                    return GatherVector128(baseAddress, index, 2);
-                case 4:
-                    return GatherVector128(baseAddress, index, 4);
-                case 8:
-                    return GatherVector128(baseAddress, index, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherVector128(baseAddress, index, 1),
+                2 => GatherVector128(baseAddress, index, 2),
+                4 => GatherVector128(baseAddress, index, 4),
+                8 => GatherVector128(baseAddress, index, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m128i _mm_i32gather_epi64 (__int64 const* base_addr, __m128i vindex, const int scale)
@@ -890,19 +880,14 @@ public static unsafe Vector128<uint> GatherVector128(uint* baseAddress, Vector12
         /// </summary>
         public static unsafe Vector128<long> GatherVector128(long* baseAddress, Vector128<int> index, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherVector128(baseAddress, index, 1);
-                case 2:
-                    return GatherVector128(baseAddress, index, 2);
-                case 4:
-                    return GatherVector128(baseAddress, index, 4);
-                case 8:
-                    return GatherVector128(baseAddress, index, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherVector128(baseAddress, index, 1),
+                2 => GatherVector128(baseAddress, index, 2),
+                4 => GatherVector128(baseAddress, index, 4),
+                8 => GatherVector128(baseAddress, index, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m128i _mm_i32gather_epi64 (__int64 const* base_addr, __m128i vindex, const int scale)
@@ -911,19 +896,14 @@ public static unsafe Vector128<long> GatherVector128(long* baseAddress, Vector12
         /// </summary>
         public static unsafe Vector128<ulong> GatherVector128(ulong* baseAddress, Vector128<int> index, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherVector128(baseAddress, index, 1);
-                case 2:
-                    return GatherVector128(baseAddress, index, 2);
-                case 4:
-                    return GatherVector128(baseAddress, index, 4);
-                case 8:
-                    return GatherVector128(baseAddress, index, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherVector128(baseAddress, index, 1),
+                2 => GatherVector128(baseAddress, index, 2),
+                4 => GatherVector128(baseAddress, index, 4),
+                8 => GatherVector128(baseAddress, index, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m128 _mm_i32gather_ps (float const* base_addr, __m128i vindex, const int scale)
@@ -932,19 +912,14 @@ public static unsafe Vector128<ulong> GatherVector128(ulong* baseAddress, Vector
         /// </summary>
         public static unsafe Vector128<float> GatherVector128(float* baseAddress, Vector128<int> index, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherVector128(baseAddress, index, 1);
-                case 2:
-                    return GatherVector128(baseAddress, index, 2);
-                case 4:
-                    return GatherVector128(baseAddress, index, 4);
-                case 8:
-                    return GatherVector128(baseAddress, index, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherVector128(baseAddress, index, 1),
+                2 => GatherVector128(baseAddress, index, 2),
+                4 => GatherVector128(baseAddress, index, 4),
+                8 => GatherVector128(baseAddress, index, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m128d _mm_i32gather_pd (double const* base_addr, __m128i vindex, const int scale)
@@ -953,19 +928,14 @@ public static unsafe Vector128<float> GatherVector128(float* baseAddress, Vector
         /// </summary>
         public static unsafe Vector128<double> GatherVector128(double* baseAddress, Vector128<int> index, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherVector128(baseAddress, index, 1);
-                case 2:
-                    return GatherVector128(baseAddress, index, 2);
-                case 4:
-                    return GatherVector128(baseAddress, index, 4);
-                case 8:
-                    return GatherVector128(baseAddress, index, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherVector128(baseAddress, index, 1),
+                2 => GatherVector128(baseAddress, index, 2),
+                4 => GatherVector128(baseAddress, index, 4),
+                8 => GatherVector128(baseAddress, index, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m128i _mm_i64gather_epi32 (int const* base_addr, __m128i vindex, const int scale)
@@ -974,19 +944,14 @@ public static unsafe Vector128<double> GatherVector128(double* baseAddress, Vect
         /// </summary>
         public static unsafe Vector128<int> GatherVector128(int* baseAddress, Vector128<long> index, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherVector128(baseAddress, index, 1);
-                case 2:
-                    return GatherVector128(baseAddress, index, 2);
-                case 4:
-                    return GatherVector128(baseAddress, index, 4);
-                case 8:
-                    return GatherVector128(baseAddress, index, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherVector128(baseAddress, index, 1),
+                2 => GatherVector128(baseAddress, index, 2),
+                4 => GatherVector128(baseAddress, index, 4),
+                8 => GatherVector128(baseAddress, index, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m128i _mm_i64gather_epi32 (int const* base_addr, __m128i vindex, const int scale)
@@ -995,19 +960,14 @@ public static unsafe Vector128<int> GatherVector128(int* baseAddress, Vector128<
         /// </summary>
         public static unsafe Vector128<uint> GatherVector128(uint* baseAddress, Vector128<long> index, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherVector128(baseAddress, index, 1);
-                case 2:
-                    return GatherVector128(baseAddress, index, 2);
-                case 4:
-                    return GatherVector128(baseAddress, index, 4);
-                case 8:
-                    return GatherVector128(baseAddress, index, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherVector128(baseAddress, index, 1),
+                2 => GatherVector128(baseAddress, index, 2),
+                4 => GatherVector128(baseAddress, index, 4),
+                8 => GatherVector128(baseAddress, index, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m128i _mm_i64gather_epi64 (__int64 const* base_addr, __m128i vindex, const int scale)
@@ -1016,19 +976,14 @@ public static unsafe Vector128<uint> GatherVector128(uint* baseAddress, Vector12
         /// </summary>
         public static unsafe Vector128<long> GatherVector128(long* baseAddress, Vector128<long> index, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherVector128(baseAddress, index, 1);
-                case 2:
-                    return GatherVector128(baseAddress, index, 2);
-                case 4:
-                    return GatherVector128(baseAddress, index, 4);
-                case 8:
-                    return GatherVector128(baseAddress, index, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherVector128(baseAddress, index, 1),
+                2 => GatherVector128(baseAddress, index, 2),
+                4 => GatherVector128(baseAddress, index, 4),
+                8 => GatherVector128(baseAddress, index, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m128i _mm_i64gather_epi64 (__int64 const* base_addr, __m128i vindex, const int scale)
@@ -1037,19 +992,14 @@ public static unsafe Vector128<long> GatherVector128(long* baseAddress, Vector12
         /// </summary>
         public static unsafe Vector128<ulong> GatherVector128(ulong* baseAddress, Vector128<long> index, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherVector128(baseAddress, index, 1);
-                case 2:
-                    return GatherVector128(baseAddress, index, 2);
-                case 4:
-                    return GatherVector128(baseAddress, index, 4);
-                case 8:
-                    return GatherVector128(baseAddress, index, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherVector128(baseAddress, index, 1),
+                2 => GatherVector128(baseAddress, index, 2),
+                4 => GatherVector128(baseAddress, index, 4),
+                8 => GatherVector128(baseAddress, index, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m128 _mm_i64gather_ps (float const* base_addr, __m128i vindex, const int scale)
@@ -1058,19 +1008,14 @@ public static unsafe Vector128<ulong> GatherVector128(ulong* baseAddress, Vector
         /// </summary>
         public static unsafe Vector128<float> GatherVector128(float* baseAddress, Vector128<long> index, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherVector128(baseAddress, index, 1);
-                case 2:
-                    return GatherVector128(baseAddress, index, 2);
-                case 4:
-                    return GatherVector128(baseAddress, index, 4);
-                case 8:
-                    return GatherVector128(baseAddress, index, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherVector128(baseAddress, index, 1),
+                2 => GatherVector128(baseAddress, index, 2),
+                4 => GatherVector128(baseAddress, index, 4),
+                8 => GatherVector128(baseAddress, index, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m128d _mm_i64gather_pd (double const* base_addr, __m128i vindex, const int scale)
@@ -1079,19 +1024,14 @@ public static unsafe Vector128<float> GatherVector128(float* baseAddress, Vector
         /// </summary>
         public static unsafe Vector128<double> GatherVector128(double* baseAddress, Vector128<long> index, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherVector128(baseAddress, index, 1);
-                case 2:
-                    return GatherVector128(baseAddress, index, 2);
-                case 4:
-                    return GatherVector128(baseAddress, index, 4);
-                case 8:
-                    return GatherVector128(baseAddress, index, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherVector128(baseAddress, index, 1),
+                2 => GatherVector128(baseAddress, index, 2),
+                4 => GatherVector128(baseAddress, index, 4),
+                8 => GatherVector128(baseAddress, index, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m256i _mm256_i32gather_epi32 (int const* base_addr, __m256i vindex, const int scale)
@@ -1100,19 +1040,14 @@ public static unsafe Vector128<double> GatherVector128(double* baseAddress, Vect
         /// </summary>
         public static unsafe Vector256<int> GatherVector256(int* baseAddress, Vector256<int> index, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherVector256(baseAddress, index, 1);
-                case 2:
-                    return GatherVector256(baseAddress, index, 2);
-                case 4:
-                    return GatherVector256(baseAddress, index, 4);
-                case 8:
-                    return GatherVector256(baseAddress, index, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherVector256(baseAddress, index, 1),
+                2 => GatherVector256(baseAddress, index, 2),
+                4 => GatherVector256(baseAddress, index, 4),
+                8 => GatherVector256(baseAddress, index, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m256i _mm256_i32gather_epi32 (int const* base_addr, __m256i vindex, const int scale)
@@ -1121,19 +1056,14 @@ public static unsafe Vector256<int> GatherVector256(int* baseAddress, Vector256<
         /// </summary>
         public static unsafe Vector256<uint> GatherVector256(uint* baseAddress, Vector256<int> index, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherVector256(baseAddress, index, 1);
-                case 2:
-                    return GatherVector256(baseAddress, index, 2);
-                case 4:
-                    return GatherVector256(baseAddress, index, 4);
-                case 8:
-                    return GatherVector256(baseAddress, index, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherVector256(baseAddress, index, 1),
+                2 => GatherVector256(baseAddress, index, 2),
+                4 => GatherVector256(baseAddress, index, 4),
+                8 => GatherVector256(baseAddress, index, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m256i _mm256_i32gather_epi64 (__int64 const* base_addr, __m128i vindex, const int scale)
@@ -1142,19 +1072,14 @@ public static unsafe Vector256<uint> GatherVector256(uint* baseAddress, Vector25
         /// </summary>
         public static unsafe Vector256<long> GatherVector256(long* baseAddress, Vector128<int> index, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherVector256(baseAddress, index, 1);
-                case 2:
-                    return GatherVector256(baseAddress, index, 2);
-                case 4:
-                    return GatherVector256(baseAddress, index, 4);
-                case 8:
-                    return GatherVector256(baseAddress, index, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherVector256(baseAddress, index, 1),
+                2 => GatherVector256(baseAddress, index, 2),
+                4 => GatherVector256(baseAddress, index, 4),
+                8 => GatherVector256(baseAddress, index, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m256i _mm256_i32gather_epi64 (__int64 const* base_addr, __m128i vindex, const int scale)
@@ -1163,19 +1088,14 @@ public static unsafe Vector256<long> GatherVector256(long* baseAddress, Vector12
         /// </summary>
         public static unsafe Vector256<ulong> GatherVector256(ulong* baseAddress, Vector128<int> index, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherVector256(baseAddress, index, 1);
-                case 2:
-                    return GatherVector256(baseAddress, index, 2);
-                case 4:
-                    return GatherVector256(baseAddress, index, 4);
-                case 8:
-                    return GatherVector256(baseAddress, index, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherVector256(baseAddress, index, 1),
+                2 => GatherVector256(baseAddress, index, 2),
+                4 => GatherVector256(baseAddress, index, 4),
+                8 => GatherVector256(baseAddress, index, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m256 _mm256_i32gather_ps (float const* base_addr, __m256i vindex, const int scale)
@@ -1184,19 +1104,14 @@ public static unsafe Vector256<ulong> GatherVector256(ulong* baseAddress, Vector
         /// </summary>
         public static unsafe Vector256<float> GatherVector256(float* baseAddress, Vector256<int> index, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherVector256(baseAddress, index, 1);
-                case 2:
-                    return GatherVector256(baseAddress, index, 2);
-                case 4:
-                    return GatherVector256(baseAddress, index, 4);
-                case 8:
-                    return GatherVector256(baseAddress, index, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherVector256(baseAddress, index, 1),
+                2 => GatherVector256(baseAddress, index, 2),
+                4 => GatherVector256(baseAddress, index, 4),
+                8 => GatherVector256(baseAddress, index, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m256d _mm256_i32gather_pd (double const* base_addr, __m128i vindex, const int scale)
@@ -1205,19 +1120,14 @@ public static unsafe Vector256<float> GatherVector256(float* baseAddress, Vector
         /// </summary>
         public static unsafe Vector256<double> GatherVector256(double* baseAddress, Vector128<int> index, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherVector256(baseAddress, index, 1);
-                case 2:
-                    return GatherVector256(baseAddress, index, 2);
-                case 4:
-                    return GatherVector256(baseAddress, index, 4);
-                case 8:
-                    return GatherVector256(baseAddress, index, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherVector256(baseAddress, index, 1),
+                2 => GatherVector256(baseAddress, index, 2),
+                4 => GatherVector256(baseAddress, index, 4),
+                8 => GatherVector256(baseAddress, index, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m128i _mm256_i64gather_epi32 (int const* base_addr, __m256i vindex, const int scale)
@@ -1226,19 +1136,14 @@ public static unsafe Vector256<double> GatherVector256(double* baseAddress, Vect
         /// </summary>
         public static unsafe Vector128<int> GatherVector128(int* baseAddress, Vector256<long> index, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherVector128(baseAddress, index, 1);
-                case 2:
-                    return GatherVector128(baseAddress, index, 2);
-                case 4:
-                    return GatherVector128(baseAddress, index, 4);
-                case 8:
-                    return GatherVector128(baseAddress, index, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherVector128(baseAddress, index, 1),
+                2 => GatherVector128(baseAddress, index, 2),
+                4 => GatherVector128(baseAddress, index, 4),
+                8 => GatherVector128(baseAddress, index, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m128i _mm256_i64gather_epi32 (int const* base_addr, __m256i vindex, const int scale)
@@ -1247,19 +1152,14 @@ public static unsafe Vector128<int> GatherVector128(int* baseAddress, Vector256<
         /// </summary>
         public static unsafe Vector128<uint> GatherVector128(uint* baseAddress, Vector256<long> index, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherVector128(baseAddress, index, 1);
-                case 2:
-                    return GatherVector128(baseAddress, index, 2);
-                case 4:
-                    return GatherVector128(baseAddress, index, 4);
-                case 8:
-                    return GatherVector128(baseAddress, index, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherVector128(baseAddress, index, 1),
+                2 => GatherVector128(baseAddress, index, 2),
+                4 => GatherVector128(baseAddress, index, 4),
+                8 => GatherVector128(baseAddress, index, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m256i _mm256_i64gather_epi64 (__int64 const* base_addr, __m256i vindex, const int scale)
@@ -1268,19 +1168,14 @@ public static unsafe Vector128<uint> GatherVector128(uint* baseAddress, Vector25
         /// </summary>
         public static unsafe Vector256<long> GatherVector256(long* baseAddress, Vector256<long> index, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherVector256(baseAddress, index, 1);
-                case 2:
-                    return GatherVector256(baseAddress, index, 2);
-                case 4:
-                    return GatherVector256(baseAddress, index, 4);
-                case 8:
-                    return GatherVector256(baseAddress, index, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherVector256(baseAddress, index, 1),
+                2 => GatherVector256(baseAddress, index, 2),
+                4 => GatherVector256(baseAddress, index, 4),
+                8 => GatherVector256(baseAddress, index, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m256i _mm256_i64gather_epi64 (__int64 const* base_addr, __m256i vindex, const int scale)
@@ -1289,19 +1184,14 @@ public static unsafe Vector256<long> GatherVector256(long* baseAddress, Vector25
         /// </summary>
         public static unsafe Vector256<ulong> GatherVector256(ulong* baseAddress, Vector256<long> index, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherVector256(baseAddress, index, 1);
-                case 2:
-                    return GatherVector256(baseAddress, index, 2);
-                case 4:
-                    return GatherVector256(baseAddress, index, 4);
-                case 8:
-                    return GatherVector256(baseAddress, index, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherVector256(baseAddress, index, 1),
+                2 => GatherVector256(baseAddress, index, 2),
+                4 => GatherVector256(baseAddress, index, 4),
+                8 => GatherVector256(baseAddress, index, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m128 _mm256_i64gather_ps (float const* base_addr, __m256i vindex, const int scale)
@@ -1310,19 +1200,14 @@ public static unsafe Vector256<ulong> GatherVector256(ulong* baseAddress, Vector
         /// </summary>
         public static unsafe Vector128<float> GatherVector128(float* baseAddress, Vector256<long> index, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherVector128(baseAddress, index, 1);
-                case 2:
-                    return GatherVector128(baseAddress, index, 2);
-                case 4:
-                    return GatherVector128(baseAddress, index, 4);
-                case 8:
-                    return GatherVector128(baseAddress, index, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherVector128(baseAddress, index, 1),
+                2 => GatherVector128(baseAddress, index, 2),
+                4 => GatherVector128(baseAddress, index, 4),
+                8 => GatherVector128(baseAddress, index, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m256d _mm256_i64gather_pd (double const* base_addr, __m256i vindex, const int scale)
@@ -1331,19 +1216,14 @@ public static unsafe Vector128<float> GatherVector128(float* baseAddress, Vector
         /// </summary>
         public static unsafe Vector256<double> GatherVector256(double* baseAddress, Vector256<long> index, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherVector256(baseAddress, index, 1);
-                case 2:
-                    return GatherVector256(baseAddress, index, 2);
-                case 4:
-                    return GatherVector256(baseAddress, index, 4);
-                case 8:
-                    return GatherVector256(baseAddress, index, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherVector256(baseAddress, index, 1),
+                2 => GatherVector256(baseAddress, index, 2),
+                4 => GatherVector256(baseAddress, index, 4),
+                8 => GatherVector256(baseAddress, index, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
 
         /// <summary>
@@ -1353,19 +1233,14 @@ public static unsafe Vector256<double> GatherVector256(double* baseAddress, Vect
         /// </summary>
         public static unsafe Vector128<int> GatherMaskVector128(Vector128<int> source, int* baseAddress, Vector128<int> index, Vector128<int> mask, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 1);
-                case 2:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 2);
-                case 4:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 4);
-                case 8:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherMaskVector128(source, baseAddress, index, mask, 1),
+                2 => GatherMaskVector128(source, baseAddress, index, mask, 2),
+                4 => GatherMaskVector128(source, baseAddress, index, mask, 4),
+                8 => GatherMaskVector128(source, baseAddress, index, mask, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m128i _mm_mask_i32gather_epi32 (__m128i src, int const* base_addr, __m128i vindex, __m128i mask, const int scale)
@@ -1374,19 +1249,14 @@ public static unsafe Vector128<int> GatherMaskVector128(Vector128<int> source, i
         /// </summary>
         public static unsafe Vector128<uint> GatherMaskVector128(Vector128<uint> source, uint* baseAddress, Vector128<int> index, Vector128<uint> mask, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 1);
-                case 2:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 2);
-                case 4:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 4);
-                case 8:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherMaskVector128(source, baseAddress, index, mask, 1),
+                2 => GatherMaskVector128(source, baseAddress, index, mask, 2),
+                4 => GatherMaskVector128(source, baseAddress, index, mask, 4),
+                8 => GatherMaskVector128(source, baseAddress, index, mask, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m128i _mm_mask_i32gather_epi64 (__m128i src, __int64 const* base_addr, __m128i vindex, __m128i mask, const int scale)
@@ -1395,19 +1265,14 @@ public static unsafe Vector128<uint> GatherMaskVector128(Vector128<uint> source,
         /// </summary>
         public static unsafe Vector128<long> GatherMaskVector128(Vector128<long> source, long* baseAddress, Vector128<int> index, Vector128<long> mask, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 1);
-                case 2:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 2);
-                case 4:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 4);
-                case 8:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherMaskVector128(source, baseAddress, index, mask, 1),
+                2 => GatherMaskVector128(source, baseAddress, index, mask, 2),
+                4 => GatherMaskVector128(source, baseAddress, index, mask, 4),
+                8 => GatherMaskVector128(source, baseAddress, index, mask, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m128i _mm_mask_i32gather_epi64 (__m128i src, __int64 const* base_addr, __m128i vindex, __m128i mask, const int scale)
@@ -1416,19 +1281,14 @@ public static unsafe Vector128<long> GatherMaskVector128(Vector128<long> source,
         /// </summary>
         public static unsafe Vector128<ulong> GatherMaskVector128(Vector128<ulong> source, ulong* baseAddress, Vector128<int> index, Vector128<ulong> mask, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 1);
-                case 2:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 2);
-                case 4:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 4);
-                case 8:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherMaskVector128(source, baseAddress, index, mask, 1),
+                2 => GatherMaskVector128(source, baseAddress, index, mask, 2),
+                4 => GatherMaskVector128(source, baseAddress, index, mask, 4),
+                8 => GatherMaskVector128(source, baseAddress, index, mask, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m128 _mm_mask_i32gather_ps (__m128 src, float const* base_addr, __m128i vindex, __m128 mask, const int scale)
@@ -1437,19 +1297,14 @@ public static unsafe Vector128<ulong> GatherMaskVector128(Vector128<ulong> sourc
         /// </summary>
         public static unsafe Vector128<float> GatherMaskVector128(Vector128<float> source, float* baseAddress, Vector128<int> index, Vector128<float> mask, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 1);
-                case 2:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 2);
-                case 4:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 4);
-                case 8:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherMaskVector128(source, baseAddress, index, mask, 1),
+                2 => GatherMaskVector128(source, baseAddress, index, mask, 2),
+                4 => GatherMaskVector128(source, baseAddress, index, mask, 4),
+                8 => GatherMaskVector128(source, baseAddress, index, mask, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m128d _mm_mask_i32gather_pd (__m128d src, double const* base_addr, __m128i vindex, __m128d mask, const int scale)
@@ -1458,19 +1313,14 @@ public static unsafe Vector128<float> GatherMaskVector128(Vector128<float> sourc
         /// </summary>
         public static unsafe Vector128<double> GatherMaskVector128(Vector128<double> source, double* baseAddress, Vector128<int> index, Vector128<double> mask, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 1);
-                case 2:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 2);
-                case 4:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 4);
-                case 8:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherMaskVector128(source, baseAddress, index, mask, 1),
+                2 => GatherMaskVector128(source, baseAddress, index, mask, 2),
+                4 => GatherMaskVector128(source, baseAddress, index, mask, 4),
+                8 => GatherMaskVector128(source, baseAddress, index, mask, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m128i _mm_mask_i64gather_epi32 (__m128i src, int const* base_addr, __m128i vindex, __m128i mask, const int scale)
@@ -1479,19 +1329,14 @@ public static unsafe Vector128<double> GatherMaskVector128(Vector128<double> sou
         /// </summary>
         public static unsafe Vector128<int> GatherMaskVector128(Vector128<int> source, int* baseAddress, Vector128<long> index, Vector128<int> mask, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 1);
-                case 2:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 2);
-                case 4:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 4);
-                case 8:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherMaskVector128(source, baseAddress, index, mask, 1),
+                2 => GatherMaskVector128(source, baseAddress, index, mask, 2),
+                4 => GatherMaskVector128(source, baseAddress, index, mask, 4),
+                8 => GatherMaskVector128(source, baseAddress, index, mask, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m128i _mm_mask_i64gather_epi32 (__m128i src, int const* base_addr, __m128i vindex, __m128i mask, const int scale)
@@ -1500,19 +1345,14 @@ public static unsafe Vector128<int> GatherMaskVector128(Vector128<int> source, i
         /// </summary>
         public static unsafe Vector128<uint> GatherMaskVector128(Vector128<uint> source, uint* baseAddress, Vector128<long> index, Vector128<uint> mask, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 1);
-                case 2:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 2);
-                case 4:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 4);
-                case 8:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherMaskVector128(source, baseAddress, index, mask, 1),
+                2 => GatherMaskVector128(source, baseAddress, index, mask, 2),
+                4 => GatherMaskVector128(source, baseAddress, index, mask, 4),
+                8 => GatherMaskVector128(source, baseAddress, index, mask, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m128i _mm_mask_i64gather_epi64 (__m128i src, __int64 const* base_addr, __m128i vindex, __m128i mask, const int scale)
@@ -1521,19 +1361,14 @@ public static unsafe Vector128<uint> GatherMaskVector128(Vector128<uint> source,
         /// </summary>
         public static unsafe Vector128<long> GatherMaskVector128(Vector128<long> source, long* baseAddress, Vector128<long> index, Vector128<long> mask, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 1);
-                case 2:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 2);
-                case 4:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 4);
-                case 8:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherMaskVector128(source, baseAddress, index, mask, 1),
+                2 => GatherMaskVector128(source, baseAddress, index, mask, 2),
+                4 => GatherMaskVector128(source, baseAddress, index, mask, 4),
+                8 => GatherMaskVector128(source, baseAddress, index, mask, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m128i _mm_mask_i64gather_epi64 (__m128i src, __int64 const* base_addr, __m128i vindex, __m128i mask, const int scale)
@@ -1542,19 +1377,14 @@ public static unsafe Vector128<long> GatherMaskVector128(Vector128<long> source,
         /// </summary>
         public static unsafe Vector128<ulong> GatherMaskVector128(Vector128<ulong> source, ulong* baseAddress, Vector128<long> index, Vector128<ulong> mask, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 1);
-                case 2:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 2);
-                case 4:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 4);
-                case 8:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherMaskVector128(source, baseAddress, index, mask, 1),
+                2 => GatherMaskVector128(source, baseAddress, index, mask, 2),
+                4 => GatherMaskVector128(source, baseAddress, index, mask, 4),
+                8 => GatherMaskVector128(source, baseAddress, index, mask, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m128 _mm_mask_i64gather_ps (__m128 src, float const* base_addr, __m128i vindex, __m128 mask, const int scale)
@@ -1563,19 +1393,14 @@ public static unsafe Vector128<ulong> GatherMaskVector128(Vector128<ulong> sourc
         /// </summary>
         public static unsafe Vector128<float> GatherMaskVector128(Vector128<float> source, float* baseAddress, Vector128<long> index, Vector128<float> mask, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 1);
-                case 2:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 2);
-                case 4:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 4);
-                case 8:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherMaskVector128(source, baseAddress, index, mask, 1),
+                2 => GatherMaskVector128(source, baseAddress, index, mask, 2),
+                4 => GatherMaskVector128(source, baseAddress, index, mask, 4),
+                8 => GatherMaskVector128(source, baseAddress, index, mask, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m128d _mm_mask_i64gather_pd (__m128d src, double const* base_addr, __m128i vindex, __m128d mask, const int scale)
@@ -1584,19 +1409,14 @@ public static unsafe Vector128<float> GatherMaskVector128(Vector128<float> sourc
         /// </summary>
         public static unsafe Vector128<double> GatherMaskVector128(Vector128<double> source, double* baseAddress, Vector128<long> index, Vector128<double> mask, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 1);
-                case 2:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 2);
-                case 4:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 4);
-                case 8:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherMaskVector128(source, baseAddress, index, mask, 1),
+                2 => GatherMaskVector128(source, baseAddress, index, mask, 2),
+                4 => GatherMaskVector128(source, baseAddress, index, mask, 4),
+                8 => GatherMaskVector128(source, baseAddress, index, mask, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m256i _mm256_mask_i32gather_epi32 (__m256i src, int const* base_addr, __m256i vindex, __m256i mask, const int scale)
@@ -1605,19 +1425,14 @@ public static unsafe Vector128<double> GatherMaskVector128(Vector128<double> sou
         /// </summary>
         public static unsafe Vector256<int> GatherMaskVector256(Vector256<int> source, int* baseAddress, Vector256<int> index, Vector256<int> mask, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherMaskVector256(source, baseAddress, index, mask, 1);
-                case 2:
-                    return GatherMaskVector256(source, baseAddress, index, mask, 2);
-                case 4:
-                    return GatherMaskVector256(source, baseAddress, index, mask, 4);
-                case 8:
-                    return GatherMaskVector256(source, baseAddress, index, mask, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherMaskVector256(source, baseAddress, index, mask, 1),
+                2 => GatherMaskVector256(source, baseAddress, index, mask, 2),
+                4 => GatherMaskVector256(source, baseAddress, index, mask, 4),
+                8 => GatherMaskVector256(source, baseAddress, index, mask, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m256i _mm256_mask_i32gather_epi32 (__m256i src, int const* base_addr, __m256i vindex, __m256i mask, const int scale)
@@ -1626,19 +1441,14 @@ public static unsafe Vector256<int> GatherMaskVector256(Vector256<int> source, i
         /// </summary>
         public static unsafe Vector256<uint> GatherMaskVector256(Vector256<uint> source, uint* baseAddress, Vector256<int> index, Vector256<uint> mask, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherMaskVector256(source, baseAddress, index, mask, 1);
-                case 2:
-                    return GatherMaskVector256(source, baseAddress, index, mask, 2);
-                case 4:
-                    return GatherMaskVector256(source, baseAddress, index, mask, 4);
-                case 8:
-                    return GatherMaskVector256(source, baseAddress, index, mask, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherMaskVector256(source, baseAddress, index, mask, 1),
+                2 => GatherMaskVector256(source, baseAddress, index, mask, 2),
+                4 => GatherMaskVector256(source, baseAddress, index, mask, 4),
+                8 => GatherMaskVector256(source, baseAddress, index, mask, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m256i _mm256_mask_i32gather_epi64 (__m256i src, __int64 const* base_addr, __m128i vindex, __m256i mask, const int scale)
@@ -1647,19 +1457,14 @@ public static unsafe Vector256<uint> GatherMaskVector256(Vector256<uint> source,
         /// </summary>
         public static unsafe Vector256<long> GatherMaskVector256(Vector256<long> source, long* baseAddress, Vector128<int> index, Vector256<long> mask, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherMaskVector256(source, baseAddress, index, mask, 1);
-                case 2:
-                    return GatherMaskVector256(source, baseAddress, index, mask, 2);
-                case 4:
-                    return GatherMaskVector256(source, baseAddress, index, mask, 4);
-                case 8:
-                    return GatherMaskVector256(source, baseAddress, index, mask, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherMaskVector256(source, baseAddress, index, mask, 1),
+                2 => GatherMaskVector256(source, baseAddress, index, mask, 2),
+                4 => GatherMaskVector256(source, baseAddress, index, mask, 4),
+                8 => GatherMaskVector256(source, baseAddress, index, mask, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m256i _mm256_mask_i32gather_epi64 (__m256i src, __int64 const* base_addr, __m128i vindex, __m256i mask, const int scale)
@@ -1668,19 +1473,14 @@ public static unsafe Vector256<long> GatherMaskVector256(Vector256<long> source,
         /// </summary>
         public static unsafe Vector256<ulong> GatherMaskVector256(Vector256<ulong> source, ulong* baseAddress, Vector128<int> index, Vector256<ulong> mask, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherMaskVector256(source, baseAddress, index, mask, 1);
-                case 2:
-                    return GatherMaskVector256(source, baseAddress, index, mask, 2);
-                case 4:
-                    return GatherMaskVector256(source, baseAddress, index, mask, 4);
-                case 8:
-                    return GatherMaskVector256(source, baseAddress, index, mask, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherMaskVector256(source, baseAddress, index, mask, 1),
+                2 => GatherMaskVector256(source, baseAddress, index, mask, 2),
+                4 => GatherMaskVector256(source, baseAddress, index, mask, 4),
+                8 => GatherMaskVector256(source, baseAddress, index, mask, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m256 _mm256_mask_i32gather_ps (__m256 src, float const* base_addr, __m256i vindex, __m256 mask, const int scale)
@@ -1689,19 +1489,14 @@ public static unsafe Vector256<ulong> GatherMaskVector256(Vector256<ulong> sourc
         /// </summary>
         public static unsafe Vector256<float> GatherMaskVector256(Vector256<float> source, float* baseAddress, Vector256<int> index, Vector256<float> mask, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherMaskVector256(source, baseAddress, index, mask, 1);
-                case 2:
-                    return GatherMaskVector256(source, baseAddress, index, mask, 2);
-                case 4:
-                    return GatherMaskVector256(source, baseAddress, index, mask, 4);
-                case 8:
-                    return GatherMaskVector256(source, baseAddress, index, mask, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherMaskVector256(source, baseAddress, index, mask, 1),
+                2 => GatherMaskVector256(source, baseAddress, index, mask, 2),
+                4 => GatherMaskVector256(source, baseAddress, index, mask, 4),
+                8 => GatherMaskVector256(source, baseAddress, index, mask, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m256d _mm256_mask_i32gather_pd (__m256d src, double const* base_addr, __m128i vindex, __m256d mask, const int scale)
@@ -1710,19 +1505,14 @@ public static unsafe Vector256<float> GatherMaskVector256(Vector256<float> sourc
         /// </summary>
         public static unsafe Vector256<double> GatherMaskVector256(Vector256<double> source, double* baseAddress, Vector128<int> index, Vector256<double> mask, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherMaskVector256(source, baseAddress, index, mask, 1);
-                case 2:
-                    return GatherMaskVector256(source, baseAddress, index, mask, 2);
-                case 4:
-                    return GatherMaskVector256(source, baseAddress, index, mask, 4);
-                case 8:
-                    return GatherMaskVector256(source, baseAddress, index, mask, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherMaskVector256(source, baseAddress, index, mask, 1),
+                2 => GatherMaskVector256(source, baseAddress, index, mask, 2),
+                4 => GatherMaskVector256(source, baseAddress, index, mask, 4),
+                8 => GatherMaskVector256(source, baseAddress, index, mask, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m128i _mm256_mask_i64gather_epi32 (__m128i src, int const* base_addr, __m256i vindex, __m128i mask, const int scale)
@@ -1731,19 +1521,14 @@ public static unsafe Vector256<double> GatherMaskVector256(Vector256<double> sou
         /// </summary>
         public static unsafe Vector128<int> GatherMaskVector128(Vector128<int> source, int* baseAddress, Vector256<long> index, Vector128<int> mask, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 1);
-                case 2:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 2);
-                case 4:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 4);
-                case 8:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherMaskVector128(source, baseAddress, index, mask, 1),
+                2 => GatherMaskVector128(source, baseAddress, index, mask, 2),
+                4 => GatherMaskVector128(source, baseAddress, index, mask, 4),
+                8 => GatherMaskVector128(source, baseAddress, index, mask, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m128i _mm256_mask_i64gather_epi32 (__m128i src, int const* base_addr, __m256i vindex, __m128i mask, const int scale)
@@ -1752,19 +1537,14 @@ public static unsafe Vector128<int> GatherMaskVector128(Vector128<int> source, i
         /// </summary>
         public static unsafe Vector128<uint> GatherMaskVector128(Vector128<uint> source, uint* baseAddress, Vector256<long> index, Vector128<uint> mask, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 1);
-                case 2:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 2);
-                case 4:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 4);
-                case 8:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherMaskVector128(source, baseAddress, index, mask, 1),
+                2 => GatherMaskVector128(source, baseAddress, index, mask, 2),
+                4 => GatherMaskVector128(source, baseAddress, index, mask, 4),
+                8 => GatherMaskVector128(source, baseAddress, index, mask, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m256i _mm256_mask_i64gather_epi64 (__m256i src, __int64 const* base_addr, __m256i vindex, __m256i mask, const int scale)
@@ -1773,19 +1553,14 @@ public static unsafe Vector128<uint> GatherMaskVector128(Vector128<uint> source,
         /// </summary>
         public static unsafe Vector256<long> GatherMaskVector256(Vector256<long> source, long* baseAddress, Vector256<long> index, Vector256<long> mask, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherMaskVector256(source, baseAddress, index, mask, 1);
-                case 2:
-                    return GatherMaskVector256(source, baseAddress, index, mask, 2);
-                case 4:
-                    return GatherMaskVector256(source, baseAddress, index, mask, 4);
-                case 8:
-                    return GatherMaskVector256(source, baseAddress, index, mask, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherMaskVector256(source, baseAddress, index, mask, 1),
+                2 => GatherMaskVector256(source, baseAddress, index, mask, 2),
+                4 => GatherMaskVector256(source, baseAddress, index, mask, 4),
+                8 => GatherMaskVector256(source, baseAddress, index, mask, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m256i _mm256_mask_i64gather_epi64 (__m256i src, __int64 const* base_addr, __m256i vindex, __m256i mask, const int scale)
@@ -1794,19 +1569,14 @@ public static unsafe Vector256<long> GatherMaskVector256(Vector256<long> source,
         /// </summary>
         public static unsafe Vector256<ulong> GatherMaskVector256(Vector256<ulong> source, ulong* baseAddress, Vector256<long> index, Vector256<ulong> mask, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherMaskVector256(source, baseAddress, index, mask, 1);
-                case 2:
-                    return GatherMaskVector256(source, baseAddress, index, mask, 2);
-                case 4:
-                    return GatherMaskVector256(source, baseAddress, index, mask, 4);
-                case 8:
-                    return GatherMaskVector256(source, baseAddress, index, mask, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherMaskVector256(source, baseAddress, index, mask, 1),
+                2 => GatherMaskVector256(source, baseAddress, index, mask, 2),
+                4 => GatherMaskVector256(source, baseAddress, index, mask, 4),
+                8 => GatherMaskVector256(source, baseAddress, index, mask, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m128 _mm256_mask_i64gather_ps (__m128 src, float const* base_addr, __m256i vindex, __m128 mask, const int scale)
@@ -1815,19 +1585,14 @@ public static unsafe Vector256<ulong> GatherMaskVector256(Vector256<ulong> sourc
         /// </summary>
         public static unsafe Vector128<float> GatherMaskVector128(Vector128<float> source, float* baseAddress, Vector256<long> index, Vector128<float> mask, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 1);
-                case 2:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 2);
-                case 4:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 4);
-                case 8:
-                    return GatherMaskVector128(source, baseAddress, index, mask, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherMaskVector128(source, baseAddress, index, mask, 1),
+                2 => GatherMaskVector128(source, baseAddress, index, mask, 2),
+                4 => GatherMaskVector128(source, baseAddress, index, mask, 4),
+                8 => GatherMaskVector128(source, baseAddress, index, mask, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
         /// <summary>
         /// __m256d _mm256_mask_i64gather_pd (__m256d src, double const* base_addr, __m256i vindex, __m256d mask, const int scale)
@@ -1836,19 +1601,14 @@ public static unsafe Vector128<float> GatherMaskVector128(Vector128<float> sourc
         /// </summary>
         public static unsafe Vector256<double> GatherMaskVector256(Vector256<double> source, double* baseAddress, Vector256<long> index, Vector256<double> mask, byte scale)
         {
-            switch (scale)
+            return scale switch
             {
-                case 1:
-                    return GatherMaskVector256(source, baseAddress, index, mask, 1);
-                case 2:
-                    return GatherMaskVector256(source, baseAddress, index, mask, 2);
-                case 4:
-                    return GatherMaskVector256(source, baseAddress, index, mask, 4);
-                case 8:
-                    return GatherMaskVector256(source, baseAddress, index, mask, 8);
-                default:
-                    throw new ArgumentOutOfRangeException(nameof(scale));
-            }
+                1 => GatherMaskVector256(source, baseAddress, index, mask, 1),
+                2 => GatherMaskVector256(source, baseAddress, index, mask, 2),
+                4 => GatherMaskVector256(source, baseAddress, index, mask, 4),
+                8 => GatherMaskVector256(source, baseAddress, index, mask, 8),
+                _ => throw new ArgumentOutOfRangeException(nameof(scale)),
+            };
         }
 
         /// <summary>
diff --git a/src/System.Private.CoreLib/shared/System/Security/SecurityElement.cs b/src/System.Private.CoreLib/shared/System/Security/SecurityElement.cs
index 8965aefe6787..f9e4f686d81e 100644
--- a/src/System.Private.CoreLib/shared/System/Security/SecurityElement.cs
+++ b/src/System.Private.CoreLib/shared/System/Security/SecurityElement.cs
@@ -202,8 +202,7 @@ internal void ConvertSecurityElementFactories()
 
             for (int i = 0; i < _children.Count; ++i)
             {
-                ISecurityElementFactory? iseFactory = _children[i] as ISecurityElementFactory;
-                if (iseFactory != null && !(_children[i] is SecurityElement))
+                if (_children[i] is ISecurityElementFactory iseFactory && !(_children[i] is SecurityElement))
                     _children[i] = iseFactory.CreateSecurityElement();
             }
         }
diff --git a/src/System.Private.CoreLib/shared/System/StringComparer.cs b/src/System.Private.CoreLib/shared/System/StringComparer.cs
index f55a098306c7..f9bd66f286e1 100644
--- a/src/System.Private.CoreLib/shared/System/StringComparer.cs
+++ b/src/System.Private.CoreLib/shared/System/StringComparer.cs
@@ -69,23 +69,16 @@ public static StringComparer OrdinalIgnoreCase
         // Convert a StringComparison to a StringComparer
         public static StringComparer FromComparison(StringComparison comparisonType)
         {
-            switch (comparisonType)
+            return comparisonType switch
             {
-                case StringComparison.CurrentCulture:
-                    return CurrentCulture;
-                case StringComparison.CurrentCultureIgnoreCase:
-                    return CurrentCultureIgnoreCase;
-                case StringComparison.InvariantCulture:
-                    return InvariantCulture;
-                case StringComparison.InvariantCultureIgnoreCase:
-                    return InvariantCultureIgnoreCase;
-                case StringComparison.Ordinal:
-                    return Ordinal;
-                case StringComparison.OrdinalIgnoreCase:
-                    return OrdinalIgnoreCase;
-                default:
-                    throw new ArgumentException(SR.NotSupported_StringComparison, nameof(comparisonType));
-            }
+                StringComparison.CurrentCulture => CurrentCulture,
+                StringComparison.CurrentCultureIgnoreCase => CurrentCultureIgnoreCase,
+                StringComparison.InvariantCulture => InvariantCulture,
+                StringComparison.InvariantCultureIgnoreCase => InvariantCultureIgnoreCase,
+                StringComparison.Ordinal => Ordinal,
+                StringComparison.OrdinalIgnoreCase => OrdinalIgnoreCase,
+                _ => throw new ArgumentException(SR.NotSupported_StringComparison, nameof(comparisonType)),
+            };
         }
 
         public static StringComparer Create(CultureInfo culture, bool ignoreCase)
diff --git a/src/System.Private.CoreLib/shared/System/Text/UTF7Encoding.cs b/src/System.Private.CoreLib/shared/System/Text/UTF7Encoding.cs
index 2e414cf8d0f4..aaec29694785 100644
--- a/src/System.Private.CoreLib/shared/System/Text/UTF7Encoding.cs
+++ b/src/System.Private.CoreLib/shared/System/Text/UTF7Encoding.cs
@@ -873,8 +873,7 @@ public override int MaxCharCount
 
             public override bool Equals(object? value)
             {
-                DecoderUTF7Fallback? that = value as DecoderUTF7Fallback;
-                if (that != null)
+                if (value is DecoderUTF7Fallback)
                 {
                     return true;
                 }
diff --git a/src/System.Private.CoreLib/shared/System/Threading/CancellationTokenSource.cs b/src/System.Private.CoreLib/shared/System/Threading/CancellationTokenSource.cs
index 704e2c1d7478..dec0eab801a2 100644
--- a/src/System.Private.CoreLib/shared/System/Threading/CancellationTokenSource.cs
+++ b/src/System.Private.CoreLib/shared/System/Threading/CancellationTokenSource.cs
@@ -773,19 +773,16 @@ public static CancellationTokenSource CreateLinkedTokenSource(params Cancellatio
                 throw new ArgumentNullException(nameof(tokens));
             }
 
-            switch (tokens.Length)
+            return tokens.Length switch
             {
-                case 0:
-                    throw new ArgumentException(SR.CancellationToken_CreateLinkedToken_TokensIsEmpty);
-                case 1:
-                    return CreateLinkedTokenSource(tokens[0]);
-                case 2:
-                    return CreateLinkedTokenSource(tokens[0], tokens[1]);
-                default:
-                    // a defensive copy is not required as the array has value-items that have only a single reference field,
-                    // hence each item cannot be null itself, and reads of the payloads cannot be torn.
-                    return new LinkedNCancellationTokenSource(tokens);
-            }
+                0 => throw new ArgumentException(SR.CancellationToken_CreateLinkedToken_TokensIsEmpty),
+                1 => CreateLinkedTokenSource(tokens[0]),
+                2 => CreateLinkedTokenSource(tokens[0], tokens[1]),
+
+                // a defensive copy is not required as the array has value-items that have only a single reference field,
+                // hence each item cannot be null itself, and reads of the payloads cannot be torn.
+                _ => new LinkedNCancellationTokenSource(tokens),
+            };
         }
 
         /// <summary>
diff --git a/src/System.Private.CoreLib/shared/System/Threading/Tasks/Task.cs b/src/System.Private.CoreLib/shared/System/Threading/Tasks/Task.cs
index 53d35fbdf1cb..c1a5ac3ed9fb 100644
--- a/src/System.Private.CoreLib/shared/System/Threading/Tasks/Task.cs
+++ b/src/System.Private.CoreLib/shared/System/Threading/Tasks/Task.cs
@@ -6659,10 +6659,9 @@ private bool TrySetFromTask(Task task, bool lookForOce)
                 case TaskStatus.Faulted:
                     var edis = task.GetExceptionDispatchInfos();
                     ExceptionDispatchInfo oceEdi;
-                    OperationCanceledException? oce;
                     if (lookForOce && edis.Count > 0 &&
                         (oceEdi = edis[0]) != null &&
-                        (oce = oceEdi.SourceException as OperationCanceledException) != null)
+                        oceEdi.SourceException is OperationCanceledException oce)
                     {
                         result = TrySetCanceled(oce.CancellationToken, oceEdi);
                     }
@@ -6673,15 +6672,13 @@ private bool TrySetFromTask(Task task, bool lookForOce)
                     break;
 
                 case TaskStatus.RanToCompletion:
-                    var taskTResult = task as Task<TResult>;
-
                     if (AsyncCausalityTracer.LoggingOn)
                         AsyncCausalityTracer.TraceOperationCompletion(this, AsyncCausalityStatus.Completed);
 
                     if (Task.s_asyncDebuggingEnabled)
                         RemoveFromActiveTasks(this);
 
-                    result = TrySetResult(taskTResult != null ? taskTResult.Result : default);
+                    result = TrySetResult(task is Task<TResult> taskTResult ? taskTResult.Result : default);
                     break;
             }
             return result;
diff --git a/src/System.Private.CoreLib/shared/System/Threading/Tasks/TaskToApm.cs b/src/System.Private.CoreLib/shared/System/Threading/Tasks/TaskToApm.cs
index aaa072fe6055..0343fbaefaab 100644
--- a/src/System.Private.CoreLib/shared/System/Threading/Tasks/TaskToApm.cs
+++ b/src/System.Private.CoreLib/shared/System/Threading/Tasks/TaskToApm.cs
@@ -67,8 +67,7 @@ public static void End(IAsyncResult asyncResult)
             Task? task;
 
             // If the IAsyncResult is our task-wrapping IAsyncResult, extract the Task.
-            var twar = asyncResult as TaskWrapperAsyncResult;
-            if (twar != null)
+            if (asyncResult is TaskWrapperAsyncResult twar)
             {
                 task = twar.Task;
                 Debug.Assert(task != null, "TaskWrapperAsyncResult should never wrap a null Task.");
@@ -95,8 +94,7 @@ public static TResult End<TResult>(IAsyncResult asyncResult)
             Task<TResult>? task;
 
             // If the IAsyncResult is our task-wrapping IAsyncResult, extract the Task.
-            var twar = asyncResult as TaskWrapperAsyncResult;
-            if (twar != null)
+            if (asyncResult is TaskWrapperAsyncResult twar)
             {
                 task = twar.Task as Task<TResult>;
                 Debug.Assert(twar.Task != null, "TaskWrapperAsyncResult should never wrap a null Task.");
diff --git a/src/System.Private.CoreLib/shared/System/Tuple.cs b/src/System.Private.CoreLib/shared/System/Tuple.cs
index 9c77f0245dba..1d3f045845f9 100644
--- a/src/System.Private.CoreLib/shared/System/Tuple.cs
+++ b/src/System.Private.CoreLib/shared/System/Tuple.cs
@@ -296,15 +296,12 @@ string ITupleInternal.ToString(StringBuilder sb)
         {
             get
             {
-                switch (index)
+                return index switch
                 {
-                    case 0:
-                        return Item1;
-                    case 1:
-                        return Item2;
-                    default:
-                        throw new IndexOutOfRangeException();
-                }
+                    0 => Item1,
+                    1 => Item2,
+                    _ => throw new IndexOutOfRangeException(),
+                };
             }
         }
     }
@@ -416,17 +413,13 @@ string ITupleInternal.ToString(StringBuilder sb)
         {
             get
             {
-                switch (index)
+                return index switch
                 {
-                    case 0:
-                        return Item1;
-                    case 1:
-                        return Item2;
-                    case 2:
-                        return Item3;
-                    default:
-                        throw new IndexOutOfRangeException();
-                }
+                    0 => Item1,
+                    1 => Item2,
+                    2 => Item3,
+                    _ => throw new IndexOutOfRangeException(),
+                };
             }
         }
     }
@@ -547,19 +540,14 @@ string ITupleInternal.ToString(StringBuilder sb)
         {
             get
             {
-                switch (index)
+                return index switch
                 {
-                    case 0:
-                        return Item1;
-                    case 1:
-                        return Item2;
-                    case 2:
-                        return Item3;
-                    case 3:
-                        return Item4;
-                    default:
-                        throw new IndexOutOfRangeException();
-                }
+                    0 => Item1,
+                    1 => Item2,
+                    2 => Item3,
+                    3 => Item4,
+                    _ => throw new IndexOutOfRangeException(),
+                };
             }
         }
     }
@@ -689,21 +677,15 @@ string ITupleInternal.ToString(StringBuilder sb)
         {
             get
             {
-                switch (index)
+                return index switch
                 {
-                    case 0:
-                        return Item1;
-                    case 1:
-                        return Item2;
-                    case 2:
-                        return Item3;
-                    case 3:
-                        return Item4;
-                    case 4:
-                        return Item5;
-                    default:
-                        throw new IndexOutOfRangeException();
-                }
+                    0 => Item1,
+                    1 => Item2,
+                    2 => Item3,
+                    3 => Item4,
+                    4 => Item5,
+                    _ => throw new IndexOutOfRangeException(),
+                };
             }
         }
     }
@@ -842,23 +824,16 @@ string ITupleInternal.ToString(StringBuilder sb)
         {
             get
             {
-                switch (index)
+                return index switch
                 {
-                    case 0:
-                        return Item1;
-                    case 1:
-                        return Item2;
-                    case 2:
-                        return Item3;
-                    case 3:
-                        return Item4;
-                    case 4:
-                        return Item5;
-                    case 5:
-                        return Item6;
-                    default:
-                        throw new IndexOutOfRangeException();
-                }
+                    0 => Item1,
+                    1 => Item2,
+                    2 => Item3,
+                    3 => Item4,
+                    4 => Item5,
+                    5 => Item6,
+                    _ => throw new IndexOutOfRangeException(),
+                };
             }
         }
     }
@@ -1006,25 +981,17 @@ string ITupleInternal.ToString(StringBuilder sb)
         {
             get
             {
-                switch (index)
+                return index switch
                 {
-                    case 0:
-                        return Item1;
-                    case 1:
-                        return Item2;
-                    case 2:
-                        return Item3;
-                    case 3:
-                        return Item4;
-                    case 4:
-                        return Item5;
-                    case 5:
-                        return Item6;
-                    case 6:
-                        return Item7;
-                    default:
-                        throw new IndexOutOfRangeException();
-                }
+                    0 => Item1,
+                    1 => Item2,
+                    2 => Item3,
+                    3 => Item4,
+                    4 => Item5,
+                    5 => Item6,
+                    6 => Item7,
+                    _ => throw new IndexOutOfRangeException(),
+                };
             }
         }
     }
@@ -1214,25 +1181,18 @@ int ITuple.Length
         {
             get
             {
-                switch (index)
+                return index switch
                 {
-                    case 0:
-                        return Item1;
-                    case 1:
-                        return Item2;
-                    case 2:
-                        return Item3;
-                    case 3:
-                        return Item4;
-                    case 4:
-                        return Item5;
-                    case 5:
-                        return Item6;
-                    case 6:
-                        return Item7;
-                }
-
-                return ((ITupleInternal)Rest)[index - 7];
+                    0 => Item1,
+                    1 => Item2,
+                    2 => Item3,
+                    3 => Item4,
+                    4 => Item5,
+                    5 => Item6,
+                    6 => Item7,
+
+                    _ => ((ITupleInternal)Rest)[index - 7],
+                };
             }
         }
     }
diff --git a/src/System.Private.CoreLib/shared/System/ValueTuple.cs b/src/System.Private.CoreLib/shared/System/ValueTuple.cs
index 6fd28db31e16..216ef5c697b0 100644
--- a/src/System.Private.CoreLib/shared/System/ValueTuple.cs
+++ b/src/System.Private.CoreLib/shared/System/ValueTuple.cs
@@ -656,15 +656,12 @@ string IValueTupleInternal.ToStringEnd()
         {
             get
             {
-                switch (index)
+                return index switch
                 {
-                    case 0:
-                        return Item1;
-                    case 1:
-                        return Item2;
-                    default:
-                        throw new IndexOutOfRangeException();
-                }
+                    0 => Item1,
+                    1 => Item2,
+                    _ => throw new IndexOutOfRangeException(),
+                };
             }
         }
     }
@@ -862,17 +859,13 @@ string IValueTupleInternal.ToStringEnd()
         {
             get
             {
-                switch (index)
+                return index switch
                 {
-                    case 0:
-                        return Item1;
-                    case 1:
-                        return Item2;
-                    case 2:
-                        return Item3;
-                    default:
-                        throw new IndexOutOfRangeException();
-                }
+                    0 => Item1,
+                    1 => Item2,
+                    2 => Item3,
+                    _ => throw new IndexOutOfRangeException(),
+                };
             }
         }
     }
@@ -1087,19 +1080,14 @@ string IValueTupleInternal.ToStringEnd()
         {
             get
             {
-                switch (index)
+                return index switch
                 {
-                    case 0:
-                        return Item1;
-                    case 1:
-                        return Item2;
-                    case 2:
-                        return Item3;
-                    case 3:
-                        return Item4;
-                    default:
-                        throw new IndexOutOfRangeException();
-                }
+                    0 => Item1,
+                    1 => Item2,
+                    2 => Item3,
+                    3 => Item4,
+                    _ => throw new IndexOutOfRangeException(),
+                };
             }
         }
     }
@@ -1331,21 +1319,15 @@ string IValueTupleInternal.ToStringEnd()
         {
             get
             {
-                switch (index)
+                return index switch
                 {
-                    case 0:
-                        return Item1;
-                    case 1:
-                        return Item2;
-                    case 2:
-                        return Item3;
-                    case 3:
-                        return Item4;
-                    case 4:
-                        return Item5;
-                    default:
-                        throw new IndexOutOfRangeException();
-                }
+                    0 => Item1,
+                    1 => Item2,
+                    2 => Item3,
+                    3 => Item4,
+                    4 => Item5,
+                    _ => throw new IndexOutOfRangeException(),
+                };
             }
         }
     }
@@ -1594,23 +1576,16 @@ string IValueTupleInternal.ToStringEnd()
         {
             get
             {
-                switch (index)
+                return index switch
                 {
-                    case 0:
-                        return Item1;
-                    case 1:
-                        return Item2;
-                    case 2:
-                        return Item3;
-                    case 3:
-                        return Item4;
-                    case 4:
-                        return Item5;
-                    case 5:
-                        return Item6;
-                    default:
-                        throw new IndexOutOfRangeException();
-                }
+                    0 => Item1,
+                    1 => Item2,
+                    2 => Item3,
+                    3 => Item4,
+                    4 => Item5,
+                    5 => Item6,
+                    _ => throw new IndexOutOfRangeException(),
+                };
             }
         }
     }
@@ -1876,25 +1851,17 @@ string IValueTupleInternal.ToStringEnd()
         {
             get
             {
-                switch (index)
+                return index switch
                 {
-                    case 0:
-                        return Item1;
-                    case 1:
-                        return Item2;
-                    case 2:
-                        return Item3;
-                    case 3:
-                        return Item4;
-                    case 4:
-                        return Item5;
-                    case 5:
-                        return Item6;
-                    case 6:
-                        return Item7;
-                    default:
-                        throw new IndexOutOfRangeException();
-                }
+                    0 => Item1,
+                    1 => Item2,
+                    2 => Item3,
+                    3 => Item4,
+                    4 => Item5,
+                    5 => Item6,
+                    6 => Item7,
+                    _ => throw new IndexOutOfRangeException(),
+                };
             }
         }
     }
@@ -2122,8 +2089,7 @@ int IStructuralComparable.CompareTo(object? other, IComparer comparer)
         public override int GetHashCode()
         {
             // We want to have a limited hash in this case.  We'll use the last 8 elements of the tuple
-            IValueTupleInternal? rest = Rest as IValueTupleInternal;
-            if (rest == null)
+            if (!(Rest is IValueTupleInternal rest))
             {
                 return ValueTuple.CombineHashCodes(Item1?.GetHashCode() ?? 0,
                                                    Item2?.GetHashCode() ?? 0,
@@ -2198,8 +2164,7 @@ int IStructuralEquatable.GetHashCode(IEqualityComparer comparer)
         private int GetHashCodeCore(IEqualityComparer comparer)
         {
             // We want to have a limited hash in this case.  We'll use the last 8 elements of the tuple
-            IValueTupleInternal? rest = Rest as IValueTupleInternal;
-            if (rest == null)
+            if (!(Rest is IValueTupleInternal rest))
             {
                 return ValueTuple.CombineHashCodes(comparer.GetHashCode(Item1!), comparer.GetHashCode(Item2!), comparer.GetHashCode(Item3!),
                                                    comparer.GetHashCode(Item4!), comparer.GetHashCode(Item5!), comparer.GetHashCode(Item6!),
@@ -2256,41 +2221,28 @@ int IValueTupleInternal.GetHashCode(IEqualityComparer comparer)
         /// </remarks>
         public override string ToString()
         {
-            IValueTupleInternal? rest = Rest as IValueTupleInternal;
-            if (rest == null)
-            {
-                return "(" + Item1?.ToString() + ", " + Item2?.ToString() + ", " + Item3?.ToString() + ", " + Item4?.ToString() + ", " + Item5?.ToString() + ", " + Item6?.ToString() + ", " + Item7?.ToString() + ", " + Rest.ToString() + ")";
-            }
-            else
+            if (Rest is IValueTupleInternal rest)
             {
                 return "(" + Item1?.ToString() + ", " + Item2?.ToString() + ", " + Item3?.ToString() + ", " + Item4?.ToString() + ", " + Item5?.ToString() + ", " + Item6?.ToString() + ", " + Item7?.ToString() + ", " + rest.ToStringEnd();
             }
+
+            return "(" + Item1?.ToString() + ", " + Item2?.ToString() + ", " + Item3?.ToString() + ", " + Item4?.ToString() + ", " + Item5?.ToString() + ", " + Item6?.ToString() + ", " + Item7?.ToString() + ", " + Rest.ToString() + ")";
         }
 
         string IValueTupleInternal.ToStringEnd()
         {
-            IValueTupleInternal? rest = Rest as IValueTupleInternal;
-            if (rest == null)
-            {
-                return Item1?.ToString() + ", " + Item2?.ToString() + ", " + Item3?.ToString() + ", " + Item4?.ToString() + ", " + Item5?.ToString() + ", " + Item6?.ToString() + ", " + Item7?.ToString() + ", " + Rest.ToString() + ")";
-            }
-            else
+            if (Rest is IValueTupleInternal rest)
             {
                 return Item1?.ToString() + ", " + Item2?.ToString() + ", " + Item3?.ToString() + ", " + Item4?.ToString() + ", " + Item5?.ToString() + ", " + Item6?.ToString() + ", " + Item7?.ToString() + ", " + rest.ToStringEnd();
             }
+
+            return Item1?.ToString() + ", " + Item2?.ToString() + ", " + Item3?.ToString() + ", " + Item4?.ToString() + ", " + Item5?.ToString() + ", " + Item6?.ToString() + ", " + Item7?.ToString() + ", " + Rest.ToString() + ")";
         }
 
         /// <summary>
         /// The number of positions in this data structure.
         /// </summary>
-        int ITuple.Length
-        {
-            get
-            {
-                IValueTupleInternal? rest = Rest as IValueTupleInternal;
-                return rest == null ? 8 : 7 + rest.Length;
-            }
-        }
+        int ITuple.Length => Rest is IValueTupleInternal rest ? 7 + rest.Length : 8;
 
         /// <summary>
         /// Get the element at position <param name="index"/>.
@@ -2317,16 +2269,18 @@ int ITuple.Length
                         return Item7;
                 }
 
-                IValueTupleInternal? rest = Rest as IValueTupleInternal;
-                if (rest == null)
+                if (Rest is IValueTupleInternal rest)
                 {
-                    if (index == 7)
-                    {
-                        return Rest;
-                    }
-                    throw new IndexOutOfRangeException();
+                    return rest[index - 7];
+                }
+
+
+                if (index == 7)
+                {
+                    return Rest;
                 }
-                return rest[index - 7];
+
+                throw new IndexOutOfRangeException();
             }
         }
     }
diff --git a/src/System.Private.CoreLib/src/System/Attribute.CoreCLR.cs b/src/System.Private.CoreLib/src/System/Attribute.CoreCLR.cs
index 75c40f05f601..1a71dcbcecf1 100644
--- a/src/System.Private.CoreLib/src/System/Attribute.CoreCLR.cs
+++ b/src/System.Private.CoreLib/src/System/Attribute.CoreCLR.cs
@@ -452,17 +452,12 @@ public static Attribute[] GetCustomAttributes(MemberInfo element, Type type, boo
             if (!type.IsSubclassOf(typeof(Attribute)) && type != typeof(Attribute))
                 throw new ArgumentException(SR.Argument_MustHaveAttributeBaseClass);
 
-            switch (element.MemberType)
+            return element.MemberType switch
             {
-                case MemberTypes.Property:
-                    return InternalGetCustomAttributes((PropertyInfo)element, type, inherit);
-
-                case MemberTypes.Event:
-                    return InternalGetCustomAttributes((EventInfo)element, type, inherit);
-
-                default:
-                    return (element.GetCustomAttributes(type, inherit) as Attribute[])!;
-            }
+                MemberTypes.Property => InternalGetCustomAttributes((PropertyInfo)element, type, inherit),
+                MemberTypes.Event => InternalGetCustomAttributes((EventInfo)element, type, inherit),
+                _ => (element.GetCustomAttributes(type, inherit) as Attribute[])!,
+            };
         }
 
         public static Attribute[] GetCustomAttributes(MemberInfo element)
@@ -475,17 +470,12 @@ public static Attribute[] GetCustomAttributes(MemberInfo element, bool inherit)
             if (element == null)
                 throw new ArgumentNullException(nameof(element));
 
-            switch (element.MemberType)
+            return element.MemberType switch
             {
-                case MemberTypes.Property:
-                    return InternalGetCustomAttributes((PropertyInfo)element, typeof(Attribute), inherit);
-
-                case MemberTypes.Event:
-                    return InternalGetCustomAttributes((EventInfo)element, typeof(Attribute), inherit);
-
-                default:
-                    return (element.GetCustomAttributes(typeof(Attribute), inherit) as Attribute[])!;
-            }
+                MemberTypes.Property => InternalGetCustomAttributes((PropertyInfo)element, typeof(Attribute), inherit),
+                MemberTypes.Event => InternalGetCustomAttributes((EventInfo)element, typeof(Attribute), inherit),
+                _ => (element.GetCustomAttributes(typeof(Attribute), inherit) as Attribute[])!,
+            };
         }
 
         public static bool IsDefined(MemberInfo element, Type attributeType)
@@ -505,17 +495,12 @@ public static bool IsDefined(MemberInfo element, Type attributeType, bool inheri
             if (!attributeType.IsSubclassOf(typeof(Attribute)) && attributeType != typeof(Attribute))
                 throw new ArgumentException(SR.Argument_MustHaveAttributeBaseClass);
 
-            switch (element.MemberType)
+            return element.MemberType switch
             {
-                case MemberTypes.Property:
-                    return InternalIsDefined((PropertyInfo)element, attributeType, inherit);
-
-                case MemberTypes.Event:
-                    return InternalIsDefined((EventInfo)element, attributeType, inherit);
-
-                default:
-                    return element.IsDefined(attributeType, inherit);
-            }
+                MemberTypes.Property => InternalIsDefined((PropertyInfo)element, attributeType, inherit),
+                MemberTypes.Event => InternalIsDefined((EventInfo)element, attributeType, inherit),
+                _ => element.IsDefined(attributeType, inherit),
+            };
         }
 
         public static Attribute? GetCustomAttribute(MemberInfo element, Type attributeType)
diff --git a/src/System.Private.CoreLib/src/System/Reflection/CustomAttribute.cs b/src/System.Private.CoreLib/src/System/Reflection/CustomAttribute.cs
index 66ceb1586bcf..24b11bb7ae7b 100644
--- a/src/System.Private.CoreLib/src/System/Reflection/CustomAttribute.cs
+++ b/src/System.Private.CoreLib/src/System/Reflection/CustomAttribute.cs
@@ -505,62 +505,27 @@ public readonly partial struct CustomAttributeTypedArgument
         #region Private Static Methods
         private static Type CustomAttributeEncodingToType(CustomAttributeEncoding encodedType)
         {
-            switch (encodedType)
-            {
-                case (CustomAttributeEncoding.Enum):
-                    return typeof(Enum);
-
-                case (CustomAttributeEncoding.Int32):
-                    return typeof(int);
-
-                case (CustomAttributeEncoding.String):
-                    return typeof(string);
-
-                case (CustomAttributeEncoding.Type):
-                    return typeof(Type);
-
-                case (CustomAttributeEncoding.Array):
-                    return typeof(Array);
-
-                case (CustomAttributeEncoding.Char):
-                    return typeof(char);
-
-                case (CustomAttributeEncoding.Boolean):
-                    return typeof(bool);
-
-                case (CustomAttributeEncoding.SByte):
-                    return typeof(sbyte);
-
-                case (CustomAttributeEncoding.Byte):
-                    return typeof(byte);
-
-                case (CustomAttributeEncoding.Int16):
-                    return typeof(short);
-
-                case (CustomAttributeEncoding.UInt16):
-                    return typeof(ushort);
-
-                case (CustomAttributeEncoding.UInt32):
-                    return typeof(uint);
-
-                case (CustomAttributeEncoding.Int64):
-                    return typeof(long);
-
-                case (CustomAttributeEncoding.UInt64):
-                    return typeof(ulong);
-
-                case (CustomAttributeEncoding.Float):
-                    return typeof(float);
-
-                case (CustomAttributeEncoding.Double):
-                    return typeof(double);
-
-                case (CustomAttributeEncoding.Object):
-                    return typeof(object);
-
-                default:
-                    throw new ArgumentException(SR.Format(SR.Arg_EnumIllegalVal, (int)encodedType), nameof(encodedType));
-            }
+            return encodedType switch
+            {
+                CustomAttributeEncoding.Enum => typeof(Enum),
+                CustomAttributeEncoding.Int32 => typeof(int),
+                CustomAttributeEncoding.String => typeof(string),
+                CustomAttributeEncoding.Type => typeof(Type),
+                CustomAttributeEncoding.Array => typeof(Array),
+                CustomAttributeEncoding.Char => typeof(char),
+                CustomAttributeEncoding.Boolean => typeof(bool),
+                CustomAttributeEncoding.SByte => typeof(sbyte),
+                CustomAttributeEncoding.Byte => typeof(byte),
+                CustomAttributeEncoding.Int16 => typeof(short),
+                CustomAttributeEncoding.UInt16 => typeof(ushort),
+                CustomAttributeEncoding.UInt32 => typeof(uint),
+                CustomAttributeEncoding.Int64 => typeof(long),
+                CustomAttributeEncoding.UInt64 => typeof(ulong),
+                CustomAttributeEncoding.Float => typeof(float),
+                CustomAttributeEncoding.Double => typeof(double),
+                CustomAttributeEncoding.Object => typeof(object),
+                _ => throw new ArgumentException(SR.Format(SR.Arg_EnumIllegalVal, (int)encodedType), nameof(encodedType)),
+            };
         }
 
         private static object EncodedValueToRawValue(long val, CustomAttributeEncoding encodedType)
diff --git a/src/System.Private.CoreLib/src/System/Reflection/MdConstant.cs b/src/System.Private.CoreLib/src/System/Reflection/MdConstant.cs
index 0fa1b385267f..144aa32043f5 100644
--- a/src/System.Private.CoreLib/src/System/Reflection/MdConstant.cs
+++ b/src/System.Private.CoreLib/src/System/Reflection/MdConstant.cs
@@ -111,64 +111,25 @@ internal static class MdConstant
             }
             else
             {
-                switch (corElementType)
+                return corElementType switch
                 {
-                    #region Switch
-
-                    case CorElementType.ELEMENT_TYPE_VOID:
-                        return DBNull.Value;
-
-                    case CorElementType.ELEMENT_TYPE_CHAR:
-                        return *(char*)&buffer;
-
-                    case CorElementType.ELEMENT_TYPE_I1:
-                        return *(sbyte*)&buffer;
-
-                    case CorElementType.ELEMENT_TYPE_U1:
-                        return *(byte*)&buffer;
-
-                    case CorElementType.ELEMENT_TYPE_I2:
-                        return *(short*)&buffer;
-
-                    case CorElementType.ELEMENT_TYPE_U2:
-                        return *(ushort*)&buffer;
-
-                    case CorElementType.ELEMENT_TYPE_I4:
-                        return *(int*)&buffer;
-
-                    case CorElementType.ELEMENT_TYPE_U4:
-                        return *(uint*)&buffer;
-
-                    case CorElementType.ELEMENT_TYPE_I8:
-                        return buffer;
-
-                    case CorElementType.ELEMENT_TYPE_U8:
-                        return (ulong)buffer;
-
-                    case CorElementType.ELEMENT_TYPE_BOOLEAN:
-                        // The boolean value returned from the metadata engine is stored as a
-                        // BOOL, which actually maps to an int. We need to read it out as an int
-                        // to avoid problems on big-endian machines.
-                        return (*(int*)&buffer != 0);
-
-                    case CorElementType.ELEMENT_TYPE_R4:
-                        return *(float*)&buffer;
-
-                    case CorElementType.ELEMENT_TYPE_R8:
-                        return *(double*)&buffer;
-
-                    case CorElementType.ELEMENT_TYPE_STRING:
-                        // A string constant can be empty but never null.
-                        // A nullref constant can only be type CorElementType.ELEMENT_TYPE_CLASS.
-                        return stringVal == null ? string.Empty : stringVal;
-
-                    case CorElementType.ELEMENT_TYPE_CLASS:
-                        return null;
-
-                    default:
-                        throw new FormatException(SR.Arg_BadLiteralFormat);
-                        #endregion
-                }
+                    CorElementType.ELEMENT_TYPE_VOID => DBNull.Value,
+                    CorElementType.ELEMENT_TYPE_CHAR => *(char*)&buffer,
+                    CorElementType.ELEMENT_TYPE_I1 => *(sbyte*)&buffer,
+                    CorElementType.ELEMENT_TYPE_U1 => *(byte*)&buffer,
+                    CorElementType.ELEMENT_TYPE_I2 => *(short*)&buffer,
+                    CorElementType.ELEMENT_TYPE_U2 => *(ushort*)&buffer,
+                    CorElementType.ELEMENT_TYPE_I4 => *(int*)&buffer,
+                    CorElementType.ELEMENT_TYPE_U4 => *(uint*)&buffer,
+                    CorElementType.ELEMENT_TYPE_I8 => buffer,
+                    CorElementType.ELEMENT_TYPE_U8 => (ulong)buffer,
+                    CorElementType.ELEMENT_TYPE_BOOLEAN => (*(int*)&buffer != 0),
+                    CorElementType.ELEMENT_TYPE_R4 => *(float*)&buffer,
+                    CorElementType.ELEMENT_TYPE_R8 => *(double*)&buffer,
+                    CorElementType.ELEMENT_TYPE_STRING => stringVal == null ? string.Empty : stringVal,
+                    CorElementType.ELEMENT_TYPE_CLASS => null,
+                    _ => throw new FormatException(SR.Arg_BadLiteralFormat),
+                };
             }
         }
     }
diff --git a/src/System.Private.CoreLib/src/System/Reflection/MdFieldInfo.cs b/src/System.Private.CoreLib/src/System/Reflection/MdFieldInfo.cs
index f4cee246cc16..8200665eb447 100644
--- a/src/System.Private.CoreLib/src/System/Reflection/MdFieldInfo.cs
+++ b/src/System.Private.CoreLib/src/System/Reflection/MdFieldInfo.cs
@@ -31,12 +31,9 @@ internal MdFieldInfo(
         #region Internal Members
         internal override bool CacheEquals(object? o)
         {
-            MdFieldInfo? m = o as MdFieldInfo;
-
-            if (m is null)
-                return false;
-
-            return m.m_tkField == m_tkField &&
+            return
+                o is MdFieldInfo m &&
+                m.m_tkField == m_tkField &&
                 m_declaringType.GetTypeHandleInternal().GetModuleHandle().Equals(
                     m.m_declaringType.GetTypeHandleInternal().GetModuleHandle());
         }
diff --git a/src/System.Private.CoreLib/src/System/Reflection/RtFieldInfo.cs b/src/System.Private.CoreLib/src/System/Reflection/RtFieldInfo.cs
index 2f45c782076d..cb844cd38474 100644
--- a/src/System.Private.CoreLib/src/System/Reflection/RtFieldInfo.cs
+++ b/src/System.Private.CoreLib/src/System/Reflection/RtFieldInfo.cs
@@ -110,12 +110,7 @@ internal void CheckConsistency(object? target)
 
         internal override bool CacheEquals(object? o)
         {
-            RtFieldInfo? m = o as RtFieldInfo;
-
-            if (m is null)
-                return false;
-
-            return m.m_fieldHandle == m_fieldHandle;
+            return o is RtFieldInfo m && m.m_fieldHandle == m_fieldHandle;
         }
 
         #endregion
diff --git a/src/System.Private.CoreLib/src/System/Reflection/RuntimeConstructorInfo.cs b/src/System.Private.CoreLib/src/System/Reflection/RuntimeConstructorInfo.cs
index 004933af064c..9a8214207517 100644
--- a/src/System.Private.CoreLib/src/System/Reflection/RuntimeConstructorInfo.cs
+++ b/src/System.Private.CoreLib/src/System/Reflection/RuntimeConstructorInfo.cs
@@ -97,12 +97,7 @@ RuntimeMethodHandleInternal IRuntimeMethodInfo.Value
 
         internal override bool CacheEquals(object? o)
         {
-            RuntimeConstructorInfo? m = o as RuntimeConstructorInfo;
-
-            if (m is null)
-                return false;
-
-            return m.m_handle == m_handle;
+            return o is RuntimeConstructorInfo m && m.m_handle == m_handle;
         }
 
         private Signature Signature
diff --git a/src/System.Private.CoreLib/src/System/Reflection/RuntimeEventInfo.cs b/src/System.Private.CoreLib/src/System/Reflection/RuntimeEventInfo.cs
index 20c50fb1f9c3..ad8588310806 100644
--- a/src/System.Private.CoreLib/src/System/Reflection/RuntimeEventInfo.cs
+++ b/src/System.Private.CoreLib/src/System/Reflection/RuntimeEventInfo.cs
@@ -56,12 +56,9 @@ internal RuntimeEventInfo(int tkEvent, RuntimeType declaredType, RuntimeTypeCach
         #region Internal Members
         internal override bool CacheEquals(object? o)
         {
-            RuntimeEventInfo? m = o as RuntimeEventInfo;
-
-            if (m is null)
-                return false;
-
-            return m.m_token == m_token &&
+            return
+                o is RuntimeEventInfo m &&
+                m.m_token == m_token &&
                 RuntimeTypeHandle.GetModule(m_declaringType).Equals(
                     RuntimeTypeHandle.GetModule(m.m_declaringType));
         }
diff --git a/src/System.Private.CoreLib/src/System/Reflection/RuntimeMethodInfo.cs b/src/System.Private.CoreLib/src/System/Reflection/RuntimeMethodInfo.cs
index 465a6db92a11..58707a18589b 100644
--- a/src/System.Private.CoreLib/src/System/Reflection/RuntimeMethodInfo.cs
+++ b/src/System.Private.CoreLib/src/System/Reflection/RuntimeMethodInfo.cs
@@ -127,12 +127,7 @@ private ParameterInfo FetchReturnParameter()
         #region Internal Members
         internal override bool CacheEquals(object? o)
         {
-            RuntimeMethodInfo? m = o as RuntimeMethodInfo;
-
-            if (m is null)
-                return false;
-
-            return m.m_handle == m_handle;
+            return o is RuntimeMethodInfo m && m.m_handle == m_handle;
         }
 
         internal Signature Signature
diff --git a/src/System.Private.CoreLib/src/System/Reflection/RuntimePropertyInfo.cs b/src/System.Private.CoreLib/src/System/Reflection/RuntimePropertyInfo.cs
index 96e2fb370746..0febe692247f 100644
--- a/src/System.Private.CoreLib/src/System/Reflection/RuntimePropertyInfo.cs
+++ b/src/System.Private.CoreLib/src/System/Reflection/RuntimePropertyInfo.cs
@@ -55,12 +55,9 @@ internal RuntimePropertyInfo(
         #region Internal Members
         internal override bool CacheEquals(object? o)
         {
-            RuntimePropertyInfo? m = o as RuntimePropertyInfo;
-
-            if (m is null)
-                return false;
-
-            return m.m_token == m_token &&
+            return
+                o is RuntimePropertyInfo m &&
+                m.m_token == m_token &&
                 RuntimeTypeHandle.GetModule(m_declaringType).Equals(
                     RuntimeTypeHandle.GetModule(m.m_declaringType));
         }
diff --git a/src/System.Private.CoreLib/src/System/RuntimeType.CoreCLR.cs b/src/System.Private.CoreLib/src/System/RuntimeType.CoreCLR.cs
index 5e8b378ebd04..7f09a1ff3ef7 100644
--- a/src/System.Private.CoreLib/src/System/RuntimeType.CoreCLR.cs
+++ b/src/System.Private.CoreLib/src/System/RuntimeType.CoreCLR.cs
@@ -3131,8 +3131,7 @@ public override bool IsSubclassOf(Type type)
         // Reflexive, symmetric, transitive.
         public override bool IsEquivalentTo(Type? other)
         {
-            var otherRtType = other as RuntimeType;
-            if (otherRtType is null)
+            if (!(other is RuntimeType otherRtType))
             {
                 return false;
             }
@@ -4098,8 +4097,7 @@ private object CreateInstanceDefaultCtorSlow(bool publicOnly, bool wrapException
         internal object CreateInstanceDefaultCtor(bool publicOnly, bool skipCheckThis, bool fillCache, bool wrapExceptions)
         {
             // Call the cached
-            ActivatorCache? cacheEntry = GenericCache as ActivatorCache;
-            if (cacheEntry != null)
+            if (GenericCache is ActivatorCache cacheEntry)
             {
                 cacheEntry.EnsureInitialized();
 
diff --git a/src/System.Private.CoreLib/src/System/StubHelpers.cs b/src/System.Private.CoreLib/src/System/StubHelpers.cs
index f7ce565e9bfb..8357d7e4ba5a 100644
--- a/src/System.Private.CoreLib/src/System/StubHelpers.cs
+++ b/src/System.Private.CoreLib/src/System/StubHelpers.cs
@@ -1130,15 +1130,12 @@ internal IntPtr ConvertToNative(object pManagedHome, int dwFlags)
             }
             else
             {
-                string? strValue;
-                StringBuilder? sbValue;
-
-                if ((strValue = pManagedHome as string) != null)
+                if (pManagedHome is string strValue)
                 {
                     // string (LPStr or LPWStr)
                     pNativeHome = ConvertStringToNative(strValue, dwFlags);
                 }
-                else if ((sbValue = pManagedHome as StringBuilder) != null)
+                else if (pManagedHome is StringBuilder sbValue)
                 {
                     // StringBuilder (LPStr or LPWStr)
                     pNativeHome = ConvertStringBuilderToNative(sbValue, dwFlags);
diff --git a/src/System.Private.CoreLib/src/System/Variant.cs b/src/System.Private.CoreLib/src/System/Variant.cs
index 334bdc80e429..87840377670e 100644
--- a/src/System.Private.CoreLib/src/System/Variant.cs
+++ b/src/System.Private.CoreLib/src/System/Variant.cs
@@ -383,100 +383,43 @@ internal int CVType
         // managed variants as an intermediate type.
         internal static void MarshalHelperConvertObjectToVariant(object o, ref Variant v)
         {
-            IConvertible? ic = o as IConvertible;
-
             if (o == null)
             {
                 v = Empty;
             }
-            else if (ic == null)
+            else if (o is IConvertible ic)
+            {
+                IFormatProvider provider = CultureInfo.InvariantCulture;
+                v = ic.GetTypeCode() switch
+                {
+                    TypeCode.Empty => Empty,
+                    TypeCode.Object => new Variant((object)o),
+                    TypeCode.DBNull => DBNull,
+                    TypeCode.Boolean => new Variant(ic.ToBoolean(provider)),
+                    TypeCode.Char => new Variant(ic.ToChar(provider)),
+                    TypeCode.SByte => new Variant(ic.ToSByte(provider)),
+                    TypeCode.Byte => new Variant(ic.ToByte(provider)),
+                    TypeCode.Int16 => new Variant(ic.ToInt16(provider)),
+                    TypeCode.UInt16 => new Variant(ic.ToUInt16(provider)),
+                    TypeCode.Int32 => new Variant(ic.ToInt32(provider)),
+                    TypeCode.UInt32 => new Variant(ic.ToUInt32(provider)),
+                    TypeCode.Int64 => new Variant(ic.ToInt64(provider)),
+                    TypeCode.UInt64 => new Variant(ic.ToUInt64(provider)),
+                    TypeCode.Single => new Variant(ic.ToSingle(provider)),
+                    TypeCode.Double => new Variant(ic.ToDouble(provider)),
+                    TypeCode.Decimal => new Variant(ic.ToDecimal(provider)),
+                    TypeCode.DateTime => new Variant(ic.ToDateTime(provider)),
+                    TypeCode.String => new Variant(ic.ToString(provider)),
+                    _ => throw new NotSupportedException(SR.Format(SR.NotSupported_UnknownTypeCode, ic.GetTypeCode())),
+                };
+            }
+            else
             {
                 // This path should eventually go away. But until
                 // the work is done to have all of our wrapper types implement
                 // IConvertible, this is a cheapo way to get the work done.
                 v = new Variant(o);
             }
-            else
-            {
-                IFormatProvider provider = CultureInfo.InvariantCulture;
-                switch (ic.GetTypeCode())
-                {
-                    case TypeCode.Empty:
-                        v = Empty;
-                        break;
-
-                    case TypeCode.Object:
-                        v = new Variant((object)o);
-                        break;
-
-                    case TypeCode.DBNull:
-                        v = DBNull;
-                        break;
-
-                    case TypeCode.Boolean:
-                        v = new Variant(ic.ToBoolean(provider));
-                        break;
-
-                    case TypeCode.Char:
-                        v = new Variant(ic.ToChar(provider));
-                        break;
-
-                    case TypeCode.SByte:
-                        v = new Variant(ic.ToSByte(provider));
-                        break;
-
-                    case TypeCode.Byte:
-                        v = new Variant(ic.ToByte(provider));
-                        break;
-
-                    case TypeCode.Int16:
-                        v = new Variant(ic.ToInt16(provider));
-                        break;
-
-                    case TypeCode.UInt16:
-                        v = new Variant(ic.ToUInt16(provider));
-                        break;
-
-                    case TypeCode.Int32:
-                        v = new Variant(ic.ToInt32(provider));
-                        break;
-
-                    case TypeCode.UInt32:
-                        v = new Variant(ic.ToUInt32(provider));
-                        break;
-
-                    case TypeCode.Int64:
-                        v = new Variant(ic.ToInt64(provider));
-                        break;
-
-                    case TypeCode.UInt64:
-                        v = new Variant(ic.ToUInt64(provider));
-                        break;
-
-                    case TypeCode.Single:
-                        v = new Variant(ic.ToSingle(provider));
-                        break;
-
-                    case TypeCode.Double:
-                        v = new Variant(ic.ToDouble(provider));
-                        break;
-
-                    case TypeCode.Decimal:
-                        v = new Variant(ic.ToDecimal(provider));
-                        break;
-
-                    case TypeCode.DateTime:
-                        v = new Variant(ic.ToDateTime(provider));
-                        break;
-
-                    case TypeCode.String:
-                        v = new Variant(ic.ToString(provider));
-                        break;
-
-                    default:
-                        throw new NotSupportedException(SR.Format(SR.NotSupported_UnknownTypeCode, ic.GetTypeCode()));
-                }
-            }
         }
 
         // Helper code for marshaling VARIANTS to managed objects (we use
@@ -530,106 +473,34 @@ internal static void MarshalHelperCastVariant(object pValue, int vt, ref Variant
             else
             {
                 IFormatProvider provider = CultureInfo.InvariantCulture;
-                switch (vt)
+                v = vt switch
                 {
-                    case 0: /*VT_EMPTY*/
-                        v = Empty;
-                        break;
-
-                    case 1: /*VT_NULL*/
-                        v = DBNull;
-                        break;
-
-                    case 2: /*VT_I2*/
-                        v = new Variant(iv.ToInt16(provider));
-                        break;
-
-                    case 3: /*VT_I4*/
-                        v = new Variant(iv.ToInt32(provider));
-                        break;
-
-                    case 4: /*VT_R4*/
-                        v = new Variant(iv.ToSingle(provider));
-                        break;
-
-                    case 5: /*VT_R8*/
-                        v = new Variant(iv.ToDouble(provider));
-                        break;
-
-                    case 6: /*VT_CY*/
-                        v = new Variant(new CurrencyWrapper(iv.ToDecimal(provider)));
-                        break;
-
-                    case 7: /*VT_DATE*/
-                        v = new Variant(iv.ToDateTime(provider));
-                        break;
-
-                    case 8: /*VT_BSTR*/
-                        v = new Variant(iv.ToString(provider));
-                        break;
-
-                    case 9: /*VT_DISPATCH*/
-                        v = new Variant(new DispatchWrapper((object)iv));
-                        break;
-
-                    case 10: /*VT_ERROR*/
-                        v = new Variant(new ErrorWrapper(iv.ToInt32(provider)));
-                        break;
-
-                    case 11: /*VT_BOOL*/
-                        v = new Variant(iv.ToBoolean(provider));
-                        break;
-
-                    case 12: /*VT_VARIANT*/
-                        v = new Variant((object)iv);
-                        break;
-
-                    case 13: /*VT_UNKNOWN*/
-                        v = new Variant(new UnknownWrapper((object)iv));
-                        break;
-
-                    case 14: /*VT_DECIMAL*/
-                        v = new Variant(iv.ToDecimal(provider));
-                        break;
-
-                    // case 15: /*unused*/
-                    //  NOT SUPPORTED
-
-                    case 16: /*VT_I1*/
-                        v = new Variant(iv.ToSByte(provider));
-                        break;
-
-                    case 17: /*VT_UI1*/
-                        v = new Variant(iv.ToByte(provider));
-                        break;
-
-                    case 18: /*VT_UI2*/
-                        v = new Variant(iv.ToUInt16(provider));
-                        break;
-
-                    case 19: /*VT_UI4*/
-                        v = new Variant(iv.ToUInt32(provider));
-                        break;
-
-                    case 20: /*VT_I8*/
-                        v = new Variant(iv.ToInt64(provider));
-                        break;
-
-                    case 21: /*VT_UI8*/
-                        v = new Variant(iv.ToUInt64(provider));
-                        break;
-
-                    case 22: /*VT_INT*/
-                        v = new Variant(iv.ToInt32(provider));
-                        break;
-
-                    case 23: /*VT_UINT*/
-                        v = new Variant(iv.ToUInt32(provider));
-                        break;
-
-                    default:
-                        throw new InvalidCastException(SR.InvalidCast_CannotCoerceByRefVariant);
-                }
+                    0 => /*VT_EMPTY*/ Empty,
+                    1 => /*VT_NULL*/ DBNull,
+                    2 => /*VT_I2*/ new Variant(iv.ToInt16(provider)),
+                    3 => /*VT_I4*/ new Variant(iv.ToInt32(provider)),
+                    4 => /*VT_R4*/ new Variant(iv.ToSingle(provider)),
+                    5 => /*VT_R8*/ new Variant(iv.ToDouble(provider)),
+                    6 => /*VT_CY*/ new Variant(new CurrencyWrapper(iv.ToDecimal(provider))),
+                    7 => /*VT_DATE*/ new Variant(iv.ToDateTime(provider)),
+                    8 => /*VT_BSTR*/ new Variant(iv.ToString(provider)),
+                    9 => /*VT_DISPATCH*/ new Variant(new DispatchWrapper((object)iv)),
+                    10 => /*VT_ERROR*/ new Variant(new ErrorWrapper(iv.ToInt32(provider))),
+                    11 => /*VT_BOOL*/ new Variant(iv.ToBoolean(provider)),
+                    12 => /*VT_VARIANT*/ new Variant((object)iv),
+                    13 => /*VT_UNKNOWN*/ new Variant(new UnknownWrapper((object)iv)),
+                    14 => /*VT_DECIMAL*/ new Variant(iv.ToDecimal(provider)),
+                    // 15 => : /*unused*/ NOT SUPPORTED
+                    16 => /*VT_I1*/ new Variant(iv.ToSByte(provider)),
+                    17 => /*VT_UI1*/ new Variant(iv.ToByte(provider)),
+                    18 => /*VT_UI2*/ new Variant(iv.ToUInt16(provider)),
+                    19 => /*VT_UI4*/ new Variant(iv.ToUInt32(provider)),
+                    20 => /*VT_I8*/ new Variant(iv.ToInt64(provider)),
+                    21 => /*VT_UI8*/ new Variant(iv.ToUInt64(provider)),
+                    22 => /*VT_INT*/ new Variant(iv.ToInt32(provider)),
+                    23 => /*VT_UINT*/ new Variant(iv.ToUInt32(provider)),
+                    _ => throw new InvalidCastException(SR.InvalidCast_CannotCoerceByRefVariant),
+                };
             }
         }
     }

From d7bf7fe35aeac7be72c30e433a15972a40602cc3 Mon Sep 17 00:00:00 2001
From: Stephen Toub <stoub@microsoft.com>
Date: Fri, 9 Aug 2019 13:53:24 -0400
Subject: [PATCH 3/6] Contribute to IDE0059 (unnecessary assignment)

---
 .../shared/System/Buffers/ConfigurableArrayPool.cs  |  2 +-
 .../Text/Utf8Formatter/Utf8Formatter.Date.O.cs      |  2 +-
 .../Text/Utf8Formatter/Utf8Formatter.Guid.cs        | 10 +++++-----
 .../Collections/Concurrent/ConcurrentQueue.cs       |  3 +--
 .../shared/System/CurrentSystemTimeZone.cs          |  2 +-
 .../shared/System/DateTime.cs                       |  2 +-
 .../shared/System/Delegate.cs                       |  2 +-
 .../System/Diagnostics/Tracing/EventSource.cs       |  4 +---
 .../System/Globalization/DateTimeFormatInfo.cs      |  3 ++-
 .../shared/System/Globalization/TimeSpanParse.cs    | 13 +++++--------
 .../Resources/ManifestBasedResourceGroveler.cs      |  2 +-
 .../shared/System/SpanHelpers.Byte.cs               |  7 +------
 .../shared/System/String.Manipulation.cs            |  2 --
 .../shared/System/Threading/Tasks/FutureFactory.cs  |  2 +-
 .../shared/System/Threading/Tasks/Task.cs           |  3 ++-
 .../shared/System/Threading/Tasks/TaskScheduler.cs  |  2 +-
 .../shared/System/Threading/ThreadPool.cs           |  5 ++---
 .../WindowsRuntime/WindowsRuntimeMarshal.cs         |  2 +-
 18 files changed, 28 insertions(+), 40 deletions(-)

diff --git a/src/System.Private.CoreLib/shared/System/Buffers/ConfigurableArrayPool.cs b/src/System.Private.CoreLib/shared/System/Buffers/ConfigurableArrayPool.cs
index 054a38fc6028..f70edf110b99 100644
--- a/src/System.Private.CoreLib/shared/System/Buffers/ConfigurableArrayPool.cs
+++ b/src/System.Private.CoreLib/shared/System/Buffers/ConfigurableArrayPool.cs
@@ -74,7 +74,7 @@ public override T[] Rent(int minimumLength)
             }
 
             var log = ArrayPoolEventSource.Log;
-            T[]? buffer = null;
+            T[]? buffer;
 
             int index = Utilities.SelectBucketIndex(minimumLength);
             if (index < _buckets.Length)
diff --git a/src/System.Private.CoreLib/shared/System/Buffers/Text/Utf8Formatter/Utf8Formatter.Date.O.cs b/src/System.Private.CoreLib/shared/System/Buffers/Text/Utf8Formatter/Utf8Formatter.Date.O.cs
index d9b7b181bc44..6c6270b117a1 100644
--- a/src/System.Private.CoreLib/shared/System/Buffers/Text/Utf8Formatter/Utf8Formatter.Date.O.cs
+++ b/src/System.Private.CoreLib/shared/System/Buffers/Text/Utf8Formatter/Utf8Formatter.Date.O.cs
@@ -49,7 +49,7 @@ private static bool TryFormatDateTimeO(DateTime value, TimeSpan offset, Span<byt
             bytesWritten = bytesRequired;
 
             // Hoist most of the bounds checks on buffer.
-            { var unused = destination[MinimumBytesNeeded - 1]; }
+            { _ = destination[MinimumBytesNeeded - 1]; }
 
             FormattingHelpers.WriteFourDecimalDigits((uint)value.Year, destination, 0);
             destination[4] = Utf8Constants.Minus;
diff --git a/src/System.Private.CoreLib/shared/System/Buffers/Text/Utf8Formatter/Utf8Formatter.Guid.cs b/src/System.Private.CoreLib/shared/System/Buffers/Text/Utf8Formatter/Utf8Formatter.Guid.cs
index f2ade2191519..63a9fa248ab0 100644
--- a/src/System.Private.CoreLib/shared/System/Buffers/Text/Utf8Formatter/Utf8Formatter.Guid.cs
+++ b/src/System.Private.CoreLib/shared/System/Buffers/Text/Utf8Formatter/Utf8Formatter.Guid.cs
@@ -117,7 +117,7 @@ public static bool TryFormat(Guid value, Span<byte> destination, out int bytesWr
             // because it may have an observable side effect (throwing).
             // We use 8 instead of 7 so that we also capture the dash if we're asked to insert one.
 
-            { var unused = destination[8]; }
+            { _ = destination[8]; }
             FormattingHelpers.WriteHexByte(guidAsBytes.Byte03, destination, 0, FormattingHelpers.HexCasing.Lowercase);
             FormattingHelpers.WriteHexByte(guidAsBytes.Byte02, destination, 2, FormattingHelpers.HexCasing.Lowercase);
             FormattingHelpers.WriteHexByte(guidAsBytes.Byte01, destination, 4, FormattingHelpers.HexCasing.Lowercase);
@@ -133,7 +133,7 @@ public static bool TryFormat(Guid value, Span<byte> destination, out int bytesWr
                 destination = destination.Slice(8);
             }
 
-            { var unused = destination[4]; }
+            { _ = destination[4]; }
             FormattingHelpers.WriteHexByte(guidAsBytes.Byte05, destination, 0, FormattingHelpers.HexCasing.Lowercase);
             FormattingHelpers.WriteHexByte(guidAsBytes.Byte04, destination, 2, FormattingHelpers.HexCasing.Lowercase);
 
@@ -147,7 +147,7 @@ public static bool TryFormat(Guid value, Span<byte> destination, out int bytesWr
                 destination = destination.Slice(4);
             }
 
-            { var unused = destination[4]; }
+            { _ = destination[4]; }
             FormattingHelpers.WriteHexByte(guidAsBytes.Byte07, destination, 0, FormattingHelpers.HexCasing.Lowercase);
             FormattingHelpers.WriteHexByte(guidAsBytes.Byte06, destination, 2, FormattingHelpers.HexCasing.Lowercase);
 
@@ -161,7 +161,7 @@ public static bool TryFormat(Guid value, Span<byte> destination, out int bytesWr
                 destination = destination.Slice(4);
             }
 
-            { var unused = destination[4]; }
+            { _ = destination[4]; }
             FormattingHelpers.WriteHexByte(guidAsBytes.Byte08, destination, 0, FormattingHelpers.HexCasing.Lowercase);
             FormattingHelpers.WriteHexByte(guidAsBytes.Byte09, destination, 2, FormattingHelpers.HexCasing.Lowercase);
 
@@ -175,7 +175,7 @@ public static bool TryFormat(Guid value, Span<byte> destination, out int bytesWr
                 destination = destination.Slice(4);
             }
 
-            { var unused = destination[11]; } // can't hoist bounds check on the final brace (if exists)
+            { _ = destination[11]; } // can't hoist bounds check on the final brace (if exists)
             FormattingHelpers.WriteHexByte(guidAsBytes.Byte10, destination, 0, FormattingHelpers.HexCasing.Lowercase);
             FormattingHelpers.WriteHexByte(guidAsBytes.Byte11, destination, 2, FormattingHelpers.HexCasing.Lowercase);
             FormattingHelpers.WriteHexByte(guidAsBytes.Byte12, destination, 4, FormattingHelpers.HexCasing.Lowercase);
diff --git a/src/System.Private.CoreLib/shared/System/Collections/Concurrent/ConcurrentQueue.cs b/src/System.Private.CoreLib/shared/System/Collections/Concurrent/ConcurrentQueue.cs
index fd8c9753ebde..39e77904b3b3 100644
--- a/src/System.Private.CoreLib/shared/System/Collections/Concurrent/ConcurrentQueue.cs
+++ b/src/System.Private.CoreLib/shared/System/Collections/Concurrent/ConcurrentQueue.cs
@@ -218,8 +218,7 @@ public bool IsEmpty
                 // IsEmpty == !TryPeek. We use a "resultUsed:false" peek in order to avoid marking
                 // segments as preserved for observation, making IsEmpty a cheaper way than either
                 // TryPeek(out T) or Count == 0 to check whether any elements are in the queue.
-                T ignoredResult;
-                return !TryPeek(out ignoredResult, resultUsed: false);
+                return !TryPeek(out _, resultUsed: false);
             }
         }
 
diff --git a/src/System.Private.CoreLib/shared/System/CurrentSystemTimeZone.cs b/src/System.Private.CoreLib/shared/System/CurrentSystemTimeZone.cs
index 285dcba29be3..8cf6674e0e62 100644
--- a/src/System.Private.CoreLib/shared/System/CurrentSystemTimeZone.cs
+++ b/src/System.Private.CoreLib/shared/System/CurrentSystemTimeZone.cs
@@ -89,7 +89,7 @@ internal long GetUtcOffsetFromUniversalTime(DateTime time, ref bool isAmbiguousL
                 ambiguousEnd = startTime - daylightTime.Delta;
             }
 
-            bool isDst = false;
+            bool isDst;
             if (startTime > endTime)
             {
                 // In southern hemisphere, the daylight saving time starts later in the year, and ends in the beginning of next year.
diff --git a/src/System.Private.CoreLib/shared/System/DateTime.cs b/src/System.Private.CoreLib/shared/System/DateTime.cs
index 6cb1c1ecfc8a..2f705c42b352 100644
--- a/src/System.Private.CoreLib/shared/System/DateTime.cs
+++ b/src/System.Private.CoreLib/shared/System/DateTime.cs
@@ -1110,7 +1110,7 @@ public static DateTime Now
             get
             {
                 DateTime utc = UtcNow;
-                bool isAmbiguousLocalDst = false;
+                bool isAmbiguousLocalDst;
                 long offset = TimeZoneInfo.GetDateTimeNowUtcOffsetFromUtc(utc, out isAmbiguousLocalDst).Ticks;
                 long tick = utc.Ticks + offset;
                 if (tick > DateTime.MaxTicks)
diff --git a/src/System.Private.CoreLib/shared/System/Delegate.cs b/src/System.Private.CoreLib/shared/System/Delegate.cs
index 4e084d15425b..63aa97b8d649 100644
--- a/src/System.Private.CoreLib/shared/System/Delegate.cs
+++ b/src/System.Private.CoreLib/shared/System/Delegate.cs
@@ -82,7 +82,7 @@ public abstract partial class Delegate : ICloneable, ISerializable
 
         public static Delegate? RemoveAll(Delegate? source, Delegate? value)
         {
-            Delegate? newDelegate = null;
+            Delegate? newDelegate;
 
             do
             {
diff --git a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/EventSource.cs b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/EventSource.cs
index 771b5f1ff749..1866d73cf344 100644
--- a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/EventSource.cs
+++ b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/EventSource.cs
@@ -672,9 +672,7 @@ protected EventSource(EventSourceSettings settings, params string[]? traits)
             Guid eventSourceGuid;
             string? eventSourceName;
 
-            EventMetadata[]? eventDescriptors;
-            byte[]? manifest;
-            GetMetadata(out eventSourceGuid, out eventSourceName, out eventDescriptors, out manifest);
+            GetMetadata(out eventSourceGuid, out eventSourceName, out _, out _);
 
             if (eventSourceGuid.Equals(Guid.Empty) || eventSourceName == null)
             {
diff --git a/src/System.Private.CoreLib/shared/System/Globalization/DateTimeFormatInfo.cs b/src/System.Private.CoreLib/shared/System/Globalization/DateTimeFormatInfo.cs
index 07327ff902d8..fb2866613f29 100644
--- a/src/System.Private.CoreLib/shared/System/Globalization/DateTimeFormatInfo.cs
+++ b/src/System.Private.CoreLib/shared/System/Globalization/DateTimeFormatInfo.cs
@@ -2250,8 +2250,9 @@ internal TokenHashValue[] CreateTokenHashTable()
                 // We need to rescan the date words since we're always synthetic
                 DateTimeFormatInfoScanner scanner = new DateTimeFormatInfoScanner();
                 string[]? dateWords = scanner.GetDateWordsOfDTFI(this);
+
                 // Ensure the formatflags is initialized.
-                DateTimeFormatFlags flag = FormatFlags;
+                _ = FormatFlags;
 
                 // For some cultures, the date separator works more like a comma, being allowed before or after any date part.
                 // In these cultures, we do not use normal date separator since we disallow date separator after a date terminal state.
diff --git a/src/System.Private.CoreLib/shared/System/Globalization/TimeSpanParse.cs b/src/System.Private.CoreLib/shared/System/Globalization/TimeSpanParse.cs
index 4f492b755efb..c9e97d6a89b5 100644
--- a/src/System.Private.CoreLib/shared/System/Globalization/TimeSpanParse.cs
+++ b/src/System.Private.CoreLib/shared/System/Globalization/TimeSpanParse.cs
@@ -1120,7 +1120,7 @@ private static bool ProcessTerminal_HM(ref TimeSpanRawInfo raw, TimeSpanStandard
 
             if (match)
             {
-                long ticks = 0;
+                long ticks;
                 var zero = new TimeSpanToken(0);
 
                 if (!TryTimeToTicks(positive, zero, raw._numbers0, raw._numbers1, zero, zero, out ticks))
@@ -1190,7 +1190,7 @@ private static bool ProcessTerminal_D(ref TimeSpanRawInfo raw, TimeSpanStandardS
 
             if (match)
             {
-                long ticks = 0;
+                long ticks;
                 var zero = new TimeSpanToken(0);
 
                 if (!TryTimeToTicks(positive, raw._numbers0, zero, zero, zero, zero, out ticks))
@@ -1261,7 +1261,7 @@ private static bool TryParseByFormat(ReadOnlySpan<char> input, ReadOnlySpan<char
             int leadingZeroes = 0;    // number of leading zeroes in the parsed fraction
             int ff = 0;               // parsed fraction
             int i = 0;                // format string position
-            int tokenLen = 0;         // length of current format token, used to update index 'i'
+            int tokenLen;             // length of current format token, used to update index 'i'
 
             var tokenizer = new TimeSpanTokenizer(input, -1);
 
@@ -1319,8 +1319,7 @@ private static bool TryParseByFormat(ReadOnlySpan<char> input, ReadOnlySpan<char
 
                     case 'd':
                         tokenLen = DateTimeFormat.ParseRepeatPattern(format, i, ch);
-                        int tmp = 0;
-                        if (tokenLen > 8 || seenDD || !ParseExactDigits(ref tokenizer, (tokenLen < 2) ? 1 : tokenLen, (tokenLen < 2) ? 8 : tokenLen, out tmp, out dd))
+                        if (tokenLen > 8 || seenDD || !ParseExactDigits(ref tokenizer, (tokenLen < 2) ? 1 : tokenLen, (tokenLen < 2) ? 8 : tokenLen, out _, out dd))
                         {
                             return result.SetInvalidStringFailure();
                         }
@@ -1417,10 +1416,8 @@ private static bool TryParseByFormat(ReadOnlySpan<char> input, ReadOnlySpan<char
 
         private static bool ParseExactDigits(ref TimeSpanTokenizer tokenizer, int minDigitLength, out int result)
         {
-            result = 0;
-            int zeroes = 0;
             int maxDigitLength = (minDigitLength == 1) ? 2 : minDigitLength;
-            return ParseExactDigits(ref tokenizer, minDigitLength, maxDigitLength, out zeroes, out result);
+            return ParseExactDigits(ref tokenizer, minDigitLength, maxDigitLength, out _, out result);
         }
 
         private static bool ParseExactDigits(ref TimeSpanTokenizer tokenizer, int minDigitLength, int maxDigitLength, out int zeroes, out int result)
diff --git a/src/System.Private.CoreLib/shared/System/Resources/ManifestBasedResourceGroveler.cs b/src/System.Private.CoreLib/shared/System/Resources/ManifestBasedResourceGroveler.cs
index 865d2debb49f..d16546ae8161 100644
--- a/src/System.Private.CoreLib/shared/System/Resources/ManifestBasedResourceGroveler.cs
+++ b/src/System.Private.CoreLib/shared/System/Resources/ManifestBasedResourceGroveler.cs
@@ -58,7 +58,7 @@ public ManifestBasedResourceGroveler(ResourceManager.ResourceManagerMediator med
 
             ResourceSet? rs = null;
             Stream? stream = null;
-            Assembly? satellite = null;
+            Assembly? satellite;
 
             // 1. Fixups for ultimate fallbacks
             CultureInfo lookForCulture = UltimateFallbackFixup(culture);
diff --git a/src/System.Private.CoreLib/shared/System/SpanHelpers.Byte.cs b/src/System.Private.CoreLib/shared/System/SpanHelpers.Byte.cs
index 7b0de7d82a01..c1c50a442eb3 100644
--- a/src/System.Private.CoreLib/shared/System/SpanHelpers.Byte.cs
+++ b/src/System.Private.CoreLib/shared/System/SpanHelpers.Byte.cs
@@ -1449,12 +1449,7 @@ public static unsafe int SequenceCompareTo(ref byte first, int firstLength, ref
                         // So the bit position in 'matches' corresponds to the element offset.
 
                         // 16 elements in Vector128<byte> so we compare to ushort.MaxValue to check if everything matched
-                        if (matches == ushort.MaxValue)
-                        {
-                            // All matched
-                            offset += Vector128<byte>.Count;
-                        }
-                        else
+                        if (matches != ushort.MaxValue)
                         {
                             goto Difference;
                         }
diff --git a/src/System.Private.CoreLib/shared/System/String.Manipulation.cs b/src/System.Private.CoreLib/shared/System/String.Manipulation.cs
index 4ccb37ebdb49..f9cac8c0cdc1 100644
--- a/src/System.Private.CoreLib/shared/System/String.Manipulation.cs
+++ b/src/System.Private.CoreLib/shared/System/String.Manipulation.cs
@@ -1611,8 +1611,6 @@ private void MakeSeparatorList(string?[] separators, ref ValueListBuilder<int> s
         {
             Debug.Assert(separators != null && separators.Length > 0, "separators != null && separators.Length > 0");
 
-            int sepCount = separators.Length;
-
             for (int i = 0; i < Length; i++)
             {
                 for (int j = 0; j < separators.Length; j++)
diff --git a/src/System.Private.CoreLib/shared/System/Threading/Tasks/FutureFactory.cs b/src/System.Private.CoreLib/shared/System/Threading/Tasks/FutureFactory.cs
index 8ad7a5a85685..d0ae44272321 100644
--- a/src/System.Private.CoreLib/shared/System/Threading/Tasks/FutureFactory.cs
+++ b/src/System.Private.CoreLib/shared/System/Threading/Tasks/FutureFactory.cs
@@ -1288,7 +1288,7 @@ internal void Complete(
                 Debug.Assert(!IsCompleted, "The task should not have been completed yet.");
 
                 // Run the end method and complete the task
-                bool successfullySet = false;
+                bool successfullySet;
                 try
                 {
                     var result = endMethod(thisRef, asyncResult);
diff --git a/src/System.Private.CoreLib/shared/System/Threading/Tasks/Task.cs b/src/System.Private.CoreLib/shared/System/Threading/Tasks/Task.cs
index c1a5ac3ed9fb..7257d9a8b696 100644
--- a/src/System.Private.CoreLib/shared/System/Threading/Tasks/Task.cs
+++ b/src/System.Private.CoreLib/shared/System/Threading/Tasks/Task.cs
@@ -1168,7 +1168,8 @@ internal static Task InternalStartNew(
         /// </summary>
         internal static int NewId()
         {
-            int newId = 0;
+            int newId;
+
             // We need to repeat if Interlocked.Increment wraps around and returns 0.
             // Otherwise next time this task's Id is queried it will get a new value
             do
diff --git a/src/System.Private.CoreLib/shared/System/Threading/Tasks/TaskScheduler.cs b/src/System.Private.CoreLib/shared/System/Threading/Tasks/TaskScheduler.cs
index 75b8cab3ec32..0ad671c4fef0 100644
--- a/src/System.Private.CoreLib/shared/System/Threading/Tasks/TaskScheduler.cs
+++ b/src/System.Private.CoreLib/shared/System/Threading/Tasks/TaskScheduler.cs
@@ -377,7 +377,7 @@ public int Id
             {
                 if (m_taskSchedulerId == 0)
                 {
-                    int newId = 0;
+                    int newId;
 
                     // We need to repeat if Interlocked.Increment wraps around and returns 0.
                     // Otherwise next time this scheduler's Id is queried it will get a new value
diff --git a/src/System.Private.CoreLib/shared/System/Threading/ThreadPool.cs b/src/System.Private.CoreLib/shared/System/Threading/ThreadPool.cs
index 6925af04265f..fae558441156 100644
--- a/src/System.Private.CoreLib/shared/System/Threading/ThreadPool.cs
+++ b/src/System.Private.CoreLib/shared/System/Threading/ThreadPool.cs
@@ -582,7 +582,6 @@ internal static bool Dispatch()
             // false later, but only if we're absolutely certain that the queue is empty.
             //
             bool needAnotherThread = true;
-            object? outerWorkItem = null;
             try
             {
                 //
@@ -604,7 +603,7 @@ internal static bool Dispatch()
                 {
                     bool missedSteal = false;
                     // Use operate on workItem local to try block so it can be enregistered
-                    object? workItem = outerWorkItem = workQueue.Dequeue(tl, ref missedSteal);
+                    object? workItem = workQueue.Dequeue(tl, ref missedSteal);
 
                     if (workItem == null)
                     {
@@ -673,7 +672,7 @@ internal static bool Dispatch()
                     currentThread.ResetThreadPoolThread();
 
                     // Release refs
-                    outerWorkItem = workItem = null;
+                    workItem = null;
 
                     // Return to clean ExecutionContext and SynchronizationContext
                     ExecutionContext.ResetThreadPoolThread(currentThread);
diff --git a/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/WindowsRuntimeMarshal.cs b/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/WindowsRuntimeMarshal.cs
index 9a009c86e62a..e3a987e88e68 100644
--- a/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/WindowsRuntimeMarshal.cs
+++ b/src/System.Private.CoreLib/src/System/Runtime/InteropServices/WindowsRuntime/WindowsRuntimeMarshal.cs
@@ -1061,7 +1061,7 @@ internal static unsafe string HStringToString(IntPtr hstring)
 
         internal static Exception GetExceptionForHR(int hresult, Exception? innerException, string? messageResource)
         {
-            Exception? e = null;
+            Exception? e;
             if (innerException != null)
             {
                 string? message = innerException.Message;

From 9076a2d24853f1ab1c3f0edd0794dd47bb2be112 Mon Sep 17 00:00:00 2001
From: Stephen Toub <stoub@microsoft.com>
Date: Fri, 9 Aug 2019 14:59:12 -0400
Subject: [PATCH 4/6] Contribute to IDE0060 (unused argument)

---
 .../System/Diagnostics/Tracing/EventSource.cs  |  9 ++++-----
 .../TraceLogging/TraceLoggingEventSource.cs    |  2 +-
 .../System/Globalization/DateTimeFormat.cs     | 18 ++----------------
 .../shared/System/Number.Formatting.cs         | 10 +++++-----
 4 files changed, 12 insertions(+), 27 deletions(-)

diff --git a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/EventSource.cs b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/EventSource.cs
index 1866d73cf344..47ee18f7c865 100644
--- a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/EventSource.cs
+++ b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/EventSource.cs
@@ -2149,10 +2149,10 @@ internal unsafe void WriteToAllListeners(int eventId, uint* osThreadId, DateTime
             eventCallbackArgs.Message = m_eventData[eventId].Message;
             eventCallbackArgs.Payload = new ReadOnlyCollection<object?>(args);
 
-            DispatchToAllListeners(eventId, childActivityID, eventCallbackArgs);
+            DispatchToAllListeners(eventId, eventCallbackArgs);
         }
 
-        private unsafe void DispatchToAllListeners(int eventId, Guid* childActivityID, EventWrittenEventArgs eventCallbackArgs)
+        private unsafe void DispatchToAllListeners(int eventId, EventWrittenEventArgs eventCallbackArgs)
         {
             Exception? lastThrownException = null;
             for (EventDispatcher? dispatcher = m_Dispatchers; dispatcher != null; dispatcher = dispatcher.m_Next)
@@ -5897,7 +5897,7 @@ private string CreateManifestString()
             List<CultureInfo>? cultures = null;
             if (resources != null && (flags & EventManifestOptions.AllCultures) != 0)
             {
-                cultures = GetSupportedCultures(resources);
+                cultures = GetSupportedCultures();
             }
             else
             {
@@ -5984,9 +5984,8 @@ private void WriteMessageAttrib(StringBuilder stringBuilder, string elementName,
         /// we enumerate through all the "known" cultures and attempt to load a corresponding satellite
         /// assembly
         /// </summary>
-        /// <param name="resources"></param>
         /// <returns></returns>
-        private static List<CultureInfo> GetSupportedCultures(ResourceManager resources)
+        private static List<CultureInfo> GetSupportedCultures()
         {
             var cultures = new List<CultureInfo>();
 
diff --git a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/TraceLogging/TraceLoggingEventSource.cs b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/TraceLogging/TraceLoggingEventSource.cs
index 68c758e17ee1..57f2637f207e 100644
--- a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/TraceLogging/TraceLoggingEventSource.cs
+++ b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/TraceLogging/TraceLoggingEventSource.cs
@@ -743,7 +743,7 @@ private unsafe void WriteToAllListeners(string? eventName, ref EventDescriptor e
                 eventCallbackArgs.PayloadNames = new ReadOnlyCollection<string>((IList<string>)payload.Keys);
             }
 
-            DispatchToAllListeners(-1, pActivityId, eventCallbackArgs);
+            DispatchToAllListeners(-1, eventCallbackArgs);
         }
 
 #if (!ES_BUILD_PCL && !ES_BUILD_PN)
diff --git a/src/System.Private.CoreLib/shared/System/Globalization/DateTimeFormat.cs b/src/System.Private.CoreLib/shared/System/Globalization/DateTimeFormat.cs
index 6f6d10ab94a7..55d40398eca5 100644
--- a/src/System.Private.CoreLib/shared/System/Globalization/DateTimeFormat.cs
+++ b/src/System.Private.CoreLib/shared/System/Globalization/DateTimeFormat.cs
@@ -689,7 +689,7 @@ private static StringBuilder FormatCustomized(
                         break;
                     case 'z':
                         tokenLen = ParseRepeatPattern(format, i, ch);
-                        FormatCustomizedTimeZone(dateTime, offset, format, tokenLen, bTimeOnly, result);
+                        FormatCustomizedTimeZone(dateTime, offset, tokenLen, bTimeOnly, result);
                         break;
                     case 'K':
                         tokenLen = 1;
@@ -777,7 +777,7 @@ private static StringBuilder FormatCustomized(
 
 
         // output the 'z' famliy of formats, which output a the offset from UTC, e.g. "-07:30"
-        private static void FormatCustomizedTimeZone(DateTime dateTime, TimeSpan offset, ReadOnlySpan<char> format, int tokenLen, bool timeOnly, StringBuilder result)
+        private static void FormatCustomizedTimeZone(DateTime dateTime, TimeSpan offset, int tokenLen, bool timeOnly, StringBuilder result)
         {
             // See if the instance already has an offset
             bool dateTimeFormat = (offset == NullOffset);
@@ -958,10 +958,6 @@ private static string ExpandPredefinedFormat(ReadOnlySpan<char> format, ref Date
                         // Convert to UTC invariants mean this will be in range
                         dateTime = dateTime - offset;
                     }
-                    else if (dateTime.Kind == DateTimeKind.Local)
-                    {
-                        InvalidFormatForLocal(format, dateTime);
-                    }
                     dtfi = DateTimeFormatInfo.InvariantInfo;
                     break;
                 case 's':       // Sortable without Time Zone Info
@@ -973,10 +969,6 @@ private static string ExpandPredefinedFormat(ReadOnlySpan<char> format, ref Date
                         // Convert to UTC invariants mean this will be in range
                         dateTime = dateTime - offset;
                     }
-                    else if (dateTime.Kind == DateTimeKind.Local)
-                    {
-                        InvalidFormatForLocal(format, dateTime);
-                    }
                     dtfi = DateTimeFormatInfo.InvariantInfo;
                     break;
                 case 'U':       // Universal time in culture dependent format.
@@ -1394,11 +1386,5 @@ internal static string[] GetAllDateTimes(DateTime dateTime, DateTimeFormatInfo d
 
             return results.ToArray();
         }
-
-        // This is a placeholder for an MDA to detect when the user is using a
-        // local DateTime with a format that will be interpreted as UTC.
-        internal static void InvalidFormatForLocal(ReadOnlySpan<char> format, DateTime dateTime)
-        {
-        }
     }
 }
diff --git a/src/System.Private.CoreLib/shared/System/Number.Formatting.cs b/src/System.Private.CoreLib/shared/System/Number.Formatting.cs
index 2d3c2228f69b..70e893471342 100644
--- a/src/System.Private.CoreLib/shared/System/Number.Formatting.cs
+++ b/src/System.Private.CoreLib/shared/System/Number.Formatting.cs
@@ -1633,7 +1633,7 @@ internal static unsafe void NumberToString(ref ValueStringBuilder sb, ref Number
                     if (number.IsNegative)
                         sb.Append(info.NegativeSign);
 
-                    FormatFixed(ref sb, ref number, nMaxDigits, info, null, info.NumberDecimalSeparator, null);
+                    FormatFixed(ref sb, ref number, nMaxDigits, null, info.NumberDecimalSeparator, null);
 
                     break;
                 }
@@ -2115,7 +2115,7 @@ private static void FormatCurrency(ref ValueStringBuilder sb, ref NumberBuffer n
                 switch (ch)
                 {
                     case '#':
-                        FormatFixed(ref sb, ref number, nMaxDigits, info, info._currencyGroupSizes, info.CurrencyDecimalSeparator, info.CurrencyGroupSeparator);
+                        FormatFixed(ref sb, ref number, nMaxDigits, info._currencyGroupSizes, info.CurrencyDecimalSeparator, info.CurrencyGroupSeparator);
                         break;
                     case '-':
                         sb.Append(info.NegativeSign);
@@ -2130,7 +2130,7 @@ private static void FormatCurrency(ref ValueStringBuilder sb, ref NumberBuffer n
             }
         }
 
-        private static unsafe void FormatFixed(ref ValueStringBuilder sb, ref NumberBuffer number, int nMaxDigits, NumberFormatInfo? info, int[]? groupDigits, string? sDecimal, string? sGroup)
+        private static unsafe void FormatFixed(ref ValueStringBuilder sb, ref NumberBuffer number, int nMaxDigits, int[]? groupDigits, string? sDecimal, string? sGroup)
         {
             int digPos = number.Scale;
             byte* dig = number.GetDigitsPointer();
@@ -2245,7 +2245,7 @@ private static void FormatNumber(ref ValueStringBuilder sb, ref NumberBuffer num
                 switch (ch)
                 {
                     case '#':
-                        FormatFixed(ref sb, ref number, nMaxDigits, info, info._numberGroupSizes, info.NumberDecimalSeparator, info.NumberGroupSeparator);
+                        FormatFixed(ref sb, ref number, nMaxDigits, info._numberGroupSizes, info.NumberDecimalSeparator, info.NumberGroupSeparator);
                         break;
                     case '-':
                         sb.Append(info.NegativeSign);
@@ -2352,7 +2352,7 @@ private static void FormatPercent(ref ValueStringBuilder sb, ref NumberBuffer nu
                 switch (ch)
                 {
                     case '#':
-                        FormatFixed(ref sb, ref number, nMaxDigits, info, info._percentGroupSizes, info.PercentDecimalSeparator, info.PercentGroupSeparator);
+                        FormatFixed(ref sb, ref number, nMaxDigits, info._percentGroupSizes, info.PercentDecimalSeparator, info.PercentGroupSeparator);
                         break;
                     case '-':
                         sb.Append(info.NegativeSign);

From 2e17ae4d532ca543cd38e1e6e45509fa9a0f35c8 Mon Sep 17 00:00:00 2001
From: Stephen Toub <stoub@microsoft.com>
Date: Fri, 9 Aug 2019 15:15:07 -0400
Subject: [PATCH 5/6] Fix IDE0062 (make local functions static)

---
 .../shared/System/ComponentModel/DefaultValueAttribute.cs       | 2 +-
 .../shared/System/Globalization/ISOWeek.cs                      | 2 +-
 .../shared/System/Globalization/NumberFormatInfo.cs             | 2 +-
 src/System.Private.CoreLib/shared/System/IO/Stream.cs           | 2 +-
 src/System.Private.CoreLib/shared/System/Number.Formatting.cs   | 2 +-
 5 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/src/System.Private.CoreLib/shared/System/ComponentModel/DefaultValueAttribute.cs b/src/System.Private.CoreLib/shared/System/ComponentModel/DefaultValueAttribute.cs
index 423436e34c86..428464c6c0ba 100644
--- a/src/System.Private.CoreLib/shared/System/ComponentModel/DefaultValueAttribute.cs
+++ b/src/System.Private.CoreLib/shared/System/ComponentModel/DefaultValueAttribute.cs
@@ -60,7 +60,7 @@ public DefaultValueAttribute(Type type, string? value)
                 }
 
                 // Looking for ad hoc created TypeDescriptor.ConvertFromInvariantString(Type, string)
-                bool TryConvertFromInvariantString(Type? typeToConvert, string? stringValue, out object? conversionResult)
+                static bool TryConvertFromInvariantString(Type? typeToConvert, string? stringValue, out object? conversionResult)
                 {
                     conversionResult = null;
 
diff --git a/src/System.Private.CoreLib/shared/System/Globalization/ISOWeek.cs b/src/System.Private.CoreLib/shared/System/Globalization/ISOWeek.cs
index 60715713b195..e5b122e45018 100644
--- a/src/System.Private.CoreLib/shared/System/Globalization/ISOWeek.cs
+++ b/src/System.Private.CoreLib/shared/System/Globalization/ISOWeek.cs
@@ -90,7 +90,7 @@ public static int GetWeeksInYear(int year)
                 throw new ArgumentOutOfRangeException(nameof(year), SR.ArgumentOutOfRange_Year);
             }
 
-            int P(int y) => (y + (y / 4) - (y / 100) + (y / 400)) % 7;
+            static int P(int y) => (y + (y / 4) - (y / 100) + (y / 400)) % 7;
 
             if (P(year) == 4 || P(year - 1) == 3)
             {
diff --git a/src/System.Private.CoreLib/shared/System/Globalization/NumberFormatInfo.cs b/src/System.Private.CoreLib/shared/System/Globalization/NumberFormatInfo.cs
index 7af038162fc0..3a508cd459ae 100644
--- a/src/System.Private.CoreLib/shared/System/Globalization/NumberFormatInfo.cs
+++ b/src/System.Private.CoreLib/shared/System/Globalization/NumberFormatInfo.cs
@@ -211,7 +211,7 @@ public static NumberFormatInfo GetInstance(IFormatProvider? formatProvider)
                 CurrentInfo : // Fast path for a null provider
                 GetProviderNonNull(formatProvider);
 
-            NumberFormatInfo GetProviderNonNull(IFormatProvider provider)
+            static NumberFormatInfo GetProviderNonNull(IFormatProvider provider)
             {
                 // Fast path for a regular CultureInfo
                 if (provider is CultureInfo cultureProvider && !cultureProvider._isInherited)
diff --git a/src/System.Private.CoreLib/shared/System/IO/Stream.cs b/src/System.Private.CoreLib/shared/System/IO/Stream.cs
index 6910689ee31b..d1347d9f66c0 100644
--- a/src/System.Private.CoreLib/shared/System/IO/Stream.cs
+++ b/src/System.Private.CoreLib/shared/System/IO/Stream.cs
@@ -387,7 +387,7 @@ public virtual ValueTask<int> ReadAsync(Memory<byte> buffer, CancellationToken c
                 byte[] sharedBuffer = ArrayPool<byte>.Shared.Rent(buffer.Length);
                 return FinishReadAsync(ReadAsync(sharedBuffer, 0, buffer.Length, cancellationToken), sharedBuffer, buffer);
 
-                async ValueTask<int> FinishReadAsync(Task<int> readTask, byte[] localBuffer, Memory<byte> localDestination)
+                static async ValueTask<int> FinishReadAsync(Task<int> readTask, byte[] localBuffer, Memory<byte> localDestination)
                 {
                     try
                     {
diff --git a/src/System.Private.CoreLib/shared/System/Number.Formatting.cs b/src/System.Private.CoreLib/shared/System/Number.Formatting.cs
index 70e893471342..fce06ac2f025 100644
--- a/src/System.Private.CoreLib/shared/System/Number.Formatting.cs
+++ b/src/System.Private.CoreLib/shared/System/Number.Formatting.cs
@@ -2411,7 +2411,7 @@ internal static unsafe void RoundNumber(ref NumberBuffer number, int pos, bool i
             number.DigitsCount = i;
             number.CheckConsistency();
 
-            bool ShouldRoundUp(byte* dig, int i, NumberBufferKind numberKind, bool isCorrectlyRounded)
+            static bool ShouldRoundUp(byte* dig, int i, NumberBufferKind numberKind, bool isCorrectlyRounded)
             {
                 // We only want to round up if the digit is greater than or equal to 5 and we are
                 // not rounding a floating-point number. If we are rounding a floating-point number

From bd92f17a9d535c5ac12c7030a6714a41e9c721f4 Mon Sep 17 00:00:00 2001
From: Stephen Toub <stoub@microsoft.com>
Date: Fri, 9 Aug 2019 15:18:50 -0400
Subject: [PATCH 6/6] Fix IDE1005 (simplify delegate invocation)

---
 .../System/Diagnostics/Tracing/EventSource.cs      | 14 +++-----------
 .../shared/System/IO/Stream.cs                     | 10 ++--------
 .../shared/System/Progress.cs                      |  4 ++--
 3 files changed, 7 insertions(+), 21 deletions(-)

diff --git a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/EventSource.cs b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/EventSource.cs
index 47ee18f7c865..df40519728cd 100644
--- a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/EventSource.cs
+++ b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/EventSource.cs
@@ -2782,9 +2782,7 @@ internal void DoCommand(EventCommandEventArgs commandArgs)
                     }
 
                     this.OnEventCommand(commandArgs);
-                    var eventCommandCallback = this.m_eventCommandExecuted;
-                    if (eventCommandCallback != null)
-                        eventCommandCallback(this, commandArgs);
+                    this.m_eventCommandExecuted?.Invoke(this, commandArgs);
 
                     if (!commandArgs.enable)
                     {
@@ -2835,9 +2833,7 @@ internal void DoCommand(EventCommandEventArgs commandArgs)
                     // Debug.Assert(matchAnyKeyword == EventKeywords.None);
 
                     this.OnEventCommand(commandArgs);
-                    var eventCommandCallback = m_eventCommandExecuted;
-                    if (eventCommandCallback != null)
-                        eventCommandCallback(this, commandArgs);
+                    m_eventCommandExecuted?.Invoke(this, commandArgs);
                 }
             }
             catch (Exception e)
@@ -4255,11 +4251,7 @@ protected internal virtual void OnEventSourceCreated(EventSource eventSource)
         /// <param name="eventData"></param>
         protected internal virtual void OnEventWritten(EventWrittenEventArgs eventData)
         {
-            EventHandler<EventWrittenEventArgs>? callBack = this.EventWritten;
-            if (callBack != null)
-            {
-                callBack(this, eventData);
-            }
+            this.EventWritten?.Invoke(this, eventData);
         }
 
 
diff --git a/src/System.Private.CoreLib/shared/System/IO/Stream.cs b/src/System.Private.CoreLib/shared/System/IO/Stream.cs
index d1347d9f66c0..d02f2503dbb4 100644
--- a/src/System.Private.CoreLib/shared/System/IO/Stream.cs
+++ b/src/System.Private.CoreLib/shared/System/IO/Stream.cs
@@ -829,10 +829,7 @@ internal IAsyncResult BlockingBeginRead(byte[] buffer, int offset, int count, As
                 asyncResult = new SynchronousAsyncResult(ex, state, isWrite: false);
             }
 
-            if (callback != null)
-            {
-                callback(asyncResult);
-            }
+            callback?.Invoke(asyncResult);
 
             return asyncResult;
         }
@@ -861,10 +858,7 @@ internal IAsyncResult BlockingBeginWrite(byte[] buffer, int offset, int count, A
                 asyncResult = new SynchronousAsyncResult(ex, state, isWrite: true);
             }
 
-            if (callback != null)
-            {
-                callback(asyncResult);
-            }
+            callback?.Invoke(asyncResult);
 
             return asyncResult;
         }
diff --git a/src/System.Private.CoreLib/shared/System/Progress.cs b/src/System.Private.CoreLib/shared/System/Progress.cs
index b6348bdb773c..e7535bcb9035 100644
--- a/src/System.Private.CoreLib/shared/System/Progress.cs
+++ b/src/System.Private.CoreLib/shared/System/Progress.cs
@@ -88,8 +88,8 @@ private void InvokeHandlers(object? state)
             Action<T>? handler = _handler;
             EventHandler<T>? changedEvent = ProgressChanged;
 
-            if (handler != null) handler(value);
-            if (changedEvent != null) changedEvent(this, value);
+            handler?.Invoke(value);
+            changedEvent?.Invoke(this, value);
         }
     }