diff --git a/PolyShim.Tests/Net100/CollectionExtensionsTests.cs b/PolyShim.Tests/Net100/CollectionExtensionsTests.cs
new file mode 100644
index 00000000..a514313e
--- /dev/null
+++ b/PolyShim.Tests/Net100/CollectionExtensionsTests.cs
@@ -0,0 +1,43 @@
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
+using FluentAssertions;
+using Xunit;
+
+namespace PolyShim.Tests.Net100;
+
+public class CollectionExtensionsTests
+{
+    [Fact]
+    public void AsReadOnly_ISet_Test()
+    {
+        // Arrange
+        var set = (ISet<int>)new HashSet<int> { 1, 2, 3 };
+
+        // Act
+        var readOnly = set.AsReadOnly();
+
+        // Assert
+        readOnly.Should().BeOfType<ReadOnlySet<int>>();
+        readOnly.Count.Should().Be(3);
+        readOnly.Contains(1).Should().BeTrue();
+        readOnly.Contains(4).Should().BeFalse();
+        readOnly.IsSubsetOf(new[] { 1, 2, 3 }).Should().BeTrue();
+        readOnly.IsSupersetOf(new[] { 1 }).Should().BeTrue();
+    }
+
+    [Fact]
+    public void AsReadOnly_ISet_ReflectsChanges_Test()
+    {
+        // Arrange
+        var inner = new HashSet<int> { 1, 2, 3 };
+        var set = (ISet<int>)inner;
+
+        // Act
+        var readOnly = set.AsReadOnly();
+        inner.Add(4);
+
+        // Assert
+        readOnly.Count.Should().Be(4);
+        readOnly.Contains(4).Should().BeTrue();
+    }
+}
diff --git a/PolyShim.Tests/Net70/CollectionExtensionsTests.cs b/PolyShim.Tests/Net70/CollectionExtensionsTests.cs
new file mode 100644
index 00000000..28073652
--- /dev/null
+++ b/PolyShim.Tests/Net70/CollectionExtensionsTests.cs
@@ -0,0 +1,54 @@
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
+using FluentAssertions;
+using Xunit;
+
+namespace PolyShim.Tests.Net70;
+
+public class CollectionExtensionsTests
+{
+    [Fact]
+    public void AsReadOnly_IList_Test()
+    {
+        // Arrange
+        var list = (IList<string>)new List<string> { "a", "b", "c" };
+
+        // Act
+        var readOnly = list.AsReadOnly();
+
+        // Assert
+        readOnly.Should().BeOfType<ReadOnlyCollection<string>>();
+        readOnly.Should().Equal("a", "b", "c");
+    }
+
+    [Fact]
+    public void AsReadOnly_IList_ReflectsChanges_Test()
+    {
+        // Arrange
+        var inner = new List<string> { "a", "b" };
+        var list = (IList<string>)inner;
+
+        // Act
+        var readOnly = list.AsReadOnly();
+        inner.Add("c");
+
+        // Assert
+        readOnly.Should().Equal("a", "b", "c");
+    }
+
+    [Fact]
+    public void AsReadOnly_IDictionary_Test()
+    {
+        // Arrange
+        var dictionary =
+            (IDictionary<string, int>)new Dictionary<string, int> { ["one"] = 1, ["two"] = 2 };
+
+        // Act
+        var readOnly = dictionary.AsReadOnly();
+
+        // Assert
+        readOnly.Should().BeOfType<ReadOnlyDictionary<string, int>>();
+        readOnly["one"].Should().Be(1);
+        readOnly["two"].Should().Be(2);
+    }
+}
diff --git a/PolyShim.Tests/Net80/CollectionExtensionsTests.cs b/PolyShim.Tests/Net80/CollectionExtensionsTests.cs
new file mode 100644
index 00000000..418efa53
--- /dev/null
+++ b/PolyShim.Tests/Net80/CollectionExtensionsTests.cs
@@ -0,0 +1,81 @@
+using System;
+using System.Collections.Generic;
+using FluentAssertions;
+using Xunit;
+
+namespace PolyShim.Tests.Net80;
+
+public class CollectionExtensionsTests
+{
+    [Fact]
+    public void AddRange_Test()
+    {
+        // Arrange
+        var list = new List<int> { 1, 2, 3 };
+        var items = (ReadOnlySpan<int>)new[] { 4, 5, 6 };
+
+        // Act
+        list.AddRange(items);
+
+        // Assert
+        list.Should().Equal(1, 2, 3, 4, 5, 6);
+    }
+
+    [Fact]
+    public void AddRange_Empty_Test()
+    {
+        // Arrange
+        var list = new List<int> { 1, 2, 3 };
+        var items = (ReadOnlySpan<int>)[];
+
+        // Act
+        list.AddRange(items);
+
+        // Assert
+        list.Should().Equal(1, 2, 3);
+    }
+
+    [Fact]
+    public void InsertRange_Test()
+    {
+        // Arrange
+        var list = new List<int> { 1, 2, 6 };
+        var items = (ReadOnlySpan<int>)new[] { 3, 4, 5 };
+
+        // Act
+        list.InsertRange(2, items);
+
+        // Assert
+        list.Should().Equal(1, 2, 3, 4, 5, 6);
+    }
+
+    [Fact]
+    public void CopyTo_Test()
+    {
+        // Arrange
+        var list = new List<int> { 1, 2, 3 };
+        var destination = new int[5];
+
+        // Act
+        list.CopyTo(destination.AsSpan());
+
+        // Assert
+        destination[0].Should().Be(1);
+        destination[1].Should().Be(2);
+        destination[2].Should().Be(3);
+    }
+
+    [Fact]
+    public void CopyTo_TooSmall_Test()
+    {
+        // Arrange
+        var list = new List<int> { 1, 2, 3 };
+        var destination = new int[2];
+
+        // Act
+        var act = () => list.CopyTo(destination.AsSpan());
+
+        // Assert
+        act.Should().Throw<ArgumentException>();
+    }
+}
diff --git a/PolyShim.Tests/NetCore20/CollectionExtensionsTests.cs b/PolyShim.Tests/NetCore20/CollectionExtensionsTests.cs
index 8be5b39b..a68c4770 100644
--- a/PolyShim.Tests/NetCore20/CollectionExtensionsTests.cs
+++ b/PolyShim.Tests/NetCore20/CollectionExtensionsTests.cs
@@ -21,4 +21,69 @@ public void GetValueOrDefault_Test()
         dictionary.GetValueOrDefault("b").Should().Be("B");
         dictionary.GetValueOrDefault("d").Should().BeNull();
     }
+
+    [Fact]
+    public void TryAdd_IDictionary_Test()
+    {
+        // Arrange
+        var dictionary =
+            (IDictionary<string, int>)new Dictionary<string, int> { ["apple"] = 1, ["banana"] = 2 };
+
+        // Act
+        var result = dictionary.TryAdd("cherry", 3);
+
+        // Assert
+        result.Should().BeTrue();
+        dictionary.Should().ContainKey("cherry");
+        dictionary["cherry"].Should().Be(3);
+        dictionary.Should().HaveCount(3);
+    }
+
+    [Fact]
+    public void TryAdd_IDictionary_Exists_Test()
+    {
+        // Arrange
+        var dictionary =
+            (IDictionary<string, int>)new Dictionary<string, int> { ["apple"] = 1, ["banana"] = 2 };
+
+        // Act
+        var result = dictionary.TryAdd("apple", 99);
+
+        // Assert
+        result.Should().BeFalse();
+        dictionary["apple"].Should().Be(1);
+        dictionary.Should().HaveCount(2);
+    }
+
+    [Fact]
+    public void Remove_IDictionary_Test()
+    {
+        // Arrange
+        var dictionary =
+            (IDictionary<string, int>)new Dictionary<string, int> { ["apple"] = 1, ["banana"] = 2 };
+
+        // Act
+        var result = dictionary.Remove("apple", out var value);
+
+        // Assert
+        result.Should().BeTrue();
+        value.Should().Be(1);
+        dictionary.Should().NotContainKey("apple");
+        dictionary.Should().HaveCount(1);
+    }
+
+    [Fact]
+    public void Remove_IDictionary_NonExistent_Test()
+    {
+        // Arrange
+        var dictionary = (IDictionary<string, int>)new Dictionary<string, int> { ["apple"] = 1 };
+
+        // Act
+        var result = dictionary.Remove("cherry", out var value);
+
+        // Assert
+        result.Should().BeFalse();
+        value.Should().Be(default);
+        dictionary.Should().HaveCount(1);
+    }
 }
diff --git a/PolyShim/Net100/CollectionExtensions.cs b/PolyShim/Net100/CollectionExtensions.cs
new file mode 100644
index 00000000..886dda67
--- /dev/null
+++ b/PolyShim/Net100/CollectionExtensions.cs
@@ -0,0 +1,23 @@
+#if (NETCOREAPP && !NET10_0_OR_GREATER) || (NETSTANDARD) || (NETFRAMEWORK)
+#nullable enable
+#pragma warning disable CS0436
+
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
+using System.Diagnostics.CodeAnalysis;
+
+#if !POLYSHIM_INCLUDE_COVERAGE
+[ExcludeFromCodeCoverage]
+#endif
+internal static class MemberPolyfills_Net100_CollectionExtensions
+{
+#if !NETFRAMEWORK || NET45_OR_GREATER
+    // ReadOnlySet<T> is not available in .NET Framework below 4.5
+    extension<T>(ISet<T> set)
+    {
+        // https://learn.microsoft.com/dotnet/api/system.collections.generic.collectionextensions.asreadonly#system-collections-generic-collectionextensions-asreadonly-1(system-collections-generic-iset((-0)))
+        public ReadOnlySet<T> AsReadOnly() => new(set);
+    }
+#endif
+}
+#endif
diff --git a/PolyShim/Net50/IReadOnlySet.cs b/PolyShim/Net50/IReadOnlySet.cs
new file mode 100644
index 00000000..656309de
--- /dev/null
+++ b/PolyShim/Net50/IReadOnlySet.cs
@@ -0,0 +1,27 @@
+#if (NETCOREAPP && !NET5_0_OR_GREATER) || (NETSTANDARD) || (NETFRAMEWORK)
+#nullable enable
+#pragma warning disable CS0436
+
+namespace System.Collections.Generic;
+
+#if !NETFRAMEWORK || NET45_OR_GREATER
+// IReadOnlyCollection<T> (base interface) is not available in .NET Framework below 4.5
+// https://learn.microsoft.com/dotnet/api/system.collections.generic.ireadonlyset-1
+internal interface IReadOnlySet<T> : IReadOnlyCollection<T>
+{
+    bool Contains(T item);
+
+    bool IsProperSubsetOf(IEnumerable<T> other);
+
+    bool IsProperSupersetOf(IEnumerable<T> other);
+
+    bool IsSubsetOf(IEnumerable<T> other);
+
+    bool IsSupersetOf(IEnumerable<T> other);
+
+    bool Overlaps(IEnumerable<T> other);
+
+    bool SetEquals(IEnumerable<T> other);
+}
+#endif
+#endif
diff --git a/PolyShim/Net70/CollectionExtensions.cs b/PolyShim/Net70/CollectionExtensions.cs
new file mode 100644
index 00000000..e38c9cd5
--- /dev/null
+++ b/PolyShim/Net70/CollectionExtensions.cs
@@ -0,0 +1,30 @@
+#if (NETCOREAPP && !NET7_0_OR_GREATER) || (NETFRAMEWORK) || (NETSTANDARD)
+#nullable enable
+#pragma warning disable CS0436
+
+using System.Collections.ObjectModel;
+using System.Diagnostics.CodeAnalysis;
+
+namespace System.Collections.Generic;
+
+#if !POLYSHIM_INCLUDE_COVERAGE
+[ExcludeFromCodeCoverage]
+#endif
+internal static class MemberPolyfills_Net70_CollectionExtensions
+{
+    extension<T>(IList<T> list)
+    {
+        // https://learn.microsoft.com/dotnet/api/system.collections.generic.collectionextensions.asreadonly#system-collections-generic-collectionextensions-asreadonly-1(system-collections-generic-ilist((-0)))
+        public ReadOnlyCollection<T> AsReadOnly() => new(list);
+    }
+
+#if !NETFRAMEWORK || NET45_OR_GREATER
+    extension<TKey, TValue>(IDictionary<TKey, TValue> dictionary)
+        where TKey : notnull
+    {
+        // https://learn.microsoft.com/dotnet/api/system.collections.generic.collectionextensions.asreadonly#system-collections-generic-collectionextensions-asreadonly-2(system-collections-generic-idictionary((-0-1)))
+        public ReadOnlyDictionary<TKey, TValue> AsReadOnly() => new(dictionary);
+    }
+#endif
+}
+#endif
diff --git a/PolyShim/Net80/CollectionExtensions.cs b/PolyShim/Net80/CollectionExtensions.cs
new file mode 100644
index 00000000..60c06fca
--- /dev/null
+++ b/PolyShim/Net80/CollectionExtensions.cs
@@ -0,0 +1,38 @@
+#if (NETCOREAPP && !NET8_0_OR_GREATER) || (NETFRAMEWORK) || (NETSTANDARD)
+#nullable enable
+#pragma warning disable CS0436
+
+using System;
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+
+#if !POLYSHIM_INCLUDE_COVERAGE
+[ExcludeFromCodeCoverage]
+#endif
+internal static class MemberPolyfills_Net80_CollectionExtensions
+{
+    extension<T>(List<T> list)
+    {
+        // https://learn.microsoft.com/dotnet/api/system.collections.generic.collectionextensions.addrange
+        public void AddRange(ReadOnlySpan<T> source)
+        {
+            foreach (var item in source)
+                list.Add(item);
+        }
+
+        // https://learn.microsoft.com/dotnet/api/system.collections.generic.collectionextensions.insertrange
+        public void InsertRange(int index, ReadOnlySpan<T> source) =>
+            list.InsertRange(index, source.ToArray());
+
+        // https://learn.microsoft.com/dotnet/api/system.collections.generic.collectionextensions.copyto
+        public void CopyTo(Span<T> destination)
+        {
+            if (destination.Length < list.Count)
+                throw new ArgumentException("Destination is too short.", nameof(destination));
+
+            for (var i = 0; i < list.Count; i++)
+                destination[i] = list[i];
+        }
+    }
+}
+#endif
diff --git a/PolyShim/Net90/ReadOnlySet.cs b/PolyShim/Net90/ReadOnlySet.cs
new file mode 100644
index 00000000..1caf8c07
--- /dev/null
+++ b/PolyShim/Net90/ReadOnlySet.cs
@@ -0,0 +1,44 @@
+#if (NETCOREAPP && !NET9_0_OR_GREATER) || (NETSTANDARD) || (NETFRAMEWORK)
+#nullable enable
+#pragma warning disable CS0436
+
+using System.Collections;
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+
+namespace System.Collections.ObjectModel;
+
+#if !NETFRAMEWORK || NET45_OR_GREATER
+// IReadOnlySet<T> is not available in .NET Framework below 4.5
+// https://learn.microsoft.com/dotnet/api/system.collections.objectmodel.readonlyset-1
+#if !POLYSHIM_INCLUDE_COVERAGE
+[ExcludeFromCodeCoverage]
+#endif
+internal sealed class ReadOnlySet<T> : IReadOnlySet<T>
+{
+    private readonly ISet<T> _set;
+
+    public ReadOnlySet(ISet<T> set) => _set = set;
+
+    public int Count => _set.Count;
+
+    public bool Contains(T item) => _set.Contains(item);
+
+    public IEnumerator<T> GetEnumerator() => _set.GetEnumerator();
+
+    IEnumerator IEnumerable.GetEnumerator() => ((IEnumerable)_set).GetEnumerator();
+
+    public bool IsProperSubsetOf(IEnumerable<T> other) => _set.IsProperSubsetOf(other);
+
+    public bool IsProperSupersetOf(IEnumerable<T> other) => _set.IsProperSupersetOf(other);
+
+    public bool IsSubsetOf(IEnumerable<T> other) => _set.IsSubsetOf(other);
+
+    public bool IsSupersetOf(IEnumerable<T> other) => _set.IsSupersetOf(other);
+
+    public bool Overlaps(IEnumerable<T> other) => _set.Overlaps(other);
+
+    public bool SetEquals(IEnumerable<T> other) => _set.SetEquals(other);
+}
+#endif
+#endif
diff --git a/PolyShim/NetCore20/CollectionExtensions.cs b/PolyShim/NetCore20/CollectionExtensions.cs
index 7999fb11..0031164f 100644
--- a/PolyShim/NetCore20/CollectionExtensions.cs
+++ b/PolyShim/NetCore20/CollectionExtensions.cs
@@ -25,5 +25,32 @@ IDictionary<TKey, TValue> dictionary
         // https://learn.microsoft.com/dotnet/api/system.collections.generic.collectionextensions.getvalueordefault#system-collections-generic-collectionextensions-getvalueordefault-2(system-collections-generic-ireadonlydictionary((-0-1))-0)
         public TValue? GetValueOrDefault(TKey key) => dictionary.GetValueOrDefault(key, default);
     }
+
+    extension<TKey, TValue>(IDictionary<TKey, TValue> dictionary)
+        where TKey : notnull
+    {
+        // https://learn.microsoft.com/dotnet/api/system.collections.generic.collectionextensions.tryadd
+        public bool TryAdd(TKey key, TValue value)
+        {
+            if (dictionary.ContainsKey(key))
+                return false;
+
+            dictionary.Add(key, value);
+            return true;
+        }
+
+        // https://learn.microsoft.com/dotnet/api/system.collections.generic.collectionextensions.remove
+        public bool Remove(TKey key, out TValue value)
+        {
+            if (dictionary.TryGetValue(key, out value!))
+            {
+                dictionary.Remove(key);
+                return true;
+            }
+
+            value = default!;
+            return false;
+        }
+    }
 }
 #endif