Add CollectionsMarshal (#26867)

* Add CollectionsMarshal

* Feedback

* Feedback

src/System.Private.CoreLib/shared/System.Private.CoreLib.Shared.projitems

@@ -644,6 +644,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\ClassInterfaceAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\ClassInterfaceType.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\CoClassAttribute.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\CollectionsMarshal.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\ComDefaultInterfaceAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\ComEventInterfaceAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\InteropServices\ComImportAttribute.cs" />

src/System.Private.CoreLib/shared/System/Collections/Generic/List.cs

@@ -23,8 +23,8 @@ public class List<T> : IList<T>, IList, IReadOnlyList<T>
     {
         private const int DefaultCapacity = 4;
 
-        private T[] _items; // Do not rename (binary serialization)
-        private int _size; // Do not rename (binary serialization)
+        internal T[] _items; // Do not rename (binary serialization)
+        internal int _size; // Do not rename (binary serialization)
         private int _version; // Do not rename (binary serialization)
 
 #pragma warning disable CA1825 // avoid the extra generic instantiation for Array.Empty<T>()

src/System.Private.CoreLib/shared/System/Runtime/InteropServices/CollectionsMarshal.cs

@@ -0,0 +1,21 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.Collections.Generic;
+
+namespace System.Runtime.InteropServices
+{
+    /// <summary>
+    /// An unsafe class that provides a set of methods to access the underlying data representations of collections.
+    /// </summary>
+    public static class CollectionsMarshal
+    {
+        /// <summary>
+        /// Get a <see cref="Span{T}"/> view over a <see cref="List{T}"/>'s data.
+        /// Items should not be added or removed from the <see cref="List{T}"/> while the <see cref="Span{T}"/> is in use.
+        /// </summary>
+        public static Span<T> AsSpan<T>(List<T> list)
+            => new Span<T>(list._items, 0, list._size);
+    }
+}