Allow collection expressions for 'System.Collections.ObjectModel' collection types

src/libraries/System.Collections/tests/Generic/ReadOnlySet/ReadOnlySetTests.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Collections.Generic;
+using System.Linq;
 using Xunit;
 
 namespace System.Collections.ObjectModel.Tests
@@ -21,6 +22,24 @@ public void Ctor_SetProperty_Roundtrips()
             Assert.Same(set, new DerivedReadOnlySet<int>(set).Set);
         }
 
+        [Fact]
+        public static void Ctor_CollectionExpressions_Empty()
+        {
+            ReadOnlySet<int> set = [];
+            Assert.IsType<ReadOnlySet<int>>(set);
+            Assert.Empty(set);
+            Assert.Same(set, ReadOnlySet<int>.Empty);
+        }
+
+        [Fact]
+        public static void Ctor_CollectionExpressions()
+        {
+            int[] array = [1, 2, 3, 3, 2, 1];
+            ReadOnlySet<int> set = [.. array];
+            Assert.IsType<ReadOnlySet<int>>(set);
+            Assert.Equal(set.Order(), array.Distinct().Order());
+        }
+
         [Fact]
         public void Empty_EmptyAndIdempotent()
         {

src/libraries/System.Private.CoreLib/src/System/Collections/ObjectModel/ReadOnlyCollection.cs

@@ -8,6 +8,7 @@
 namespace System.Collections.ObjectModel
 {
     [Serializable]
+    [CollectionBuilder(typeof(ReadOnlyCollection), "CreateCollection")]
     [DebuggerTypeProxy(typeof(ICollectionDebugView<>))]
     [DebuggerDisplay("Count = {Count}")]
     [TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
@@ -229,4 +230,34 @@ void IList.RemoveAt(int index)
             ThrowHelper.ThrowNotSupportedException(ExceptionResource.NotSupported_ReadOnlyCollection);
         }
     }
+
+    /// <summary>
+    /// Provides static methods to create read-only collections and sets.
+    /// </summary>
+    public static class ReadOnlyCollection
+    {
+        /// <summary>
+        /// Creates a new <see cref="ReadOnlyCollection{T}"/> from the specified span of values.
+        /// This method (simplifies collection initialization)[/dotnet/csharp/language-reference/operators/collection-expressions]
+        /// to create a new <see cref="ReadOnlyCollection{T}"/> with the specified values.
+        /// </summary>
+        /// <typeparam name="T">The type of elements in the collection.</typeparam>
+        /// <param name="values">The span of values to include in the collection.</param>
+        /// <returns>A new <see cref="ReadOnlyCollection{T}"/> containing the specified values.</returns>
+        [Diagnostics.CodeAnalysis.SuppressMessage("Style", "IDE0301:Simplify collection initialization", Justification = "This method simplifies ReadOnlyCollection<T> initialization, so you cannot simplify collection initialization inside it.")]
+        public static ReadOnlyCollection<T> CreateCollection<T>(params ReadOnlySpan<T> values) =>
+            values.IsEmpty ? ReadOnlyCollection<T>.Empty : new ReadOnlyCollection<T>(values.ToArray());
+
+        /// <summary>
+        /// Creates a new <see cref="ReadOnlySet{T}"/> from the specified span of values.
+        /// This method (simplifies collection initialization)[/dotnet/csharp/language-reference/operators/collection-expressions]
+        /// to create a new <see cref="ReadOnlySet{T}"/> with the specified values.
+        /// </summary>
+        /// <typeparam name="T">The type of elements in the collection.</typeparam>
+        /// <param name="values">The span of values to include in the collection.</param>
+        /// <returns>A new <see cref="ReadOnlySet{T}"/> containing the specified values.</returns>
+        [Diagnostics.CodeAnalysis.SuppressMessage("Style", "IDE0301:Simplify collection initialization", Justification = "This method simplifies ReadOnlySet<T> initialization, so you cannot simplify collection initialization inside it.")]
+        public static ReadOnlySet<T> CreateSet<T>(params ReadOnlySpan<T> values) =>
+            values.IsEmpty ? ReadOnlySet<T>.Empty : new ReadOnlySet<T>((HashSet<T>)[.. values]);
+    }
 }

src/libraries/System.Private.CoreLib/src/System/Collections/ObjectModel/ReadOnlySet.cs

@@ -3,11 +3,13 @@
 
 using System.Collections.Generic;
 using System.Diagnostics;
+using System.Runtime.CompilerServices;
 
 namespace System.Collections.ObjectModel
 {
     /// <summary>Represents a read-only, generic set of values.</summary>
     /// <typeparam name="T">The type of values in the set.</typeparam>
+    [CollectionBuilder(typeof(ReadOnlyCollection), "CreateSet")]
     [DebuggerDisplay("Count = {Count}")]
     public class ReadOnlySet<T> : IReadOnlySet<T>, ISet<T>, ICollection
     {

src/libraries/System.Runtime/ref/System.Runtime.cs

@@ -8366,6 +8366,12 @@ void System.Collections.ICollection.CopyTo(System.Array array, int index) { }
         void System.Collections.IList.Insert(int index, object? value) { }
         void System.Collections.IList.Remove(object? value) { }
     }
+    public static partial class ReadOnlyCollection
+    {
+        public static System.Collections.ObjectModel.ReadOnlyCollection<T> CreateCollection<T>(params System.ReadOnlySpan<T> values) { throw null; }
+        public static System.Collections.ObjectModel.ReadOnlySet<T> CreateSet<T>(params System.ReadOnlySpan<T> values) { throw null; }
+    }
+    [System.Runtime.CompilerServices.CollectionBuilder(typeof(System.Collections.ObjectModel.ReadOnlyCollection), "CreateCollection")]
     public partial class ReadOnlyCollection<T> : System.Collections.Generic.ICollection<T>, System.Collections.Generic.IEnumerable<T>, System.Collections.Generic.IList<T>, System.Collections.Generic.IReadOnlyCollection<T>, System.Collections.Generic.IReadOnlyList<T>, System.Collections.ICollection, System.Collections.IEnumerable, System.Collections.IList
     {
         public ReadOnlyCollection(System.Collections.Generic.IList<T> list) { }
@@ -8471,6 +8477,7 @@ void System.Collections.ICollection.CopyTo(System.Array array, int index) { }
             System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() { throw null; }
         }
     }
+    [System.Runtime.CompilerServices.CollectionBuilder(typeof(System.Collections.ObjectModel.ReadOnlyCollection), "CreateSet")]
     public partial class ReadOnlySet<T> : System.Collections.Generic.ICollection<T>, System.Collections.Generic.IEnumerable<T>, System.Collections.Generic.IReadOnlyCollection<T>, System.Collections.Generic.IReadOnlySet<T>, System.Collections.Generic.ISet<T>, System.Collections.ICollection, System.Collections.IEnumerable
     {
         public ReadOnlySet(System.Collections.Generic.ISet<T> @set) { }

src/libraries/System.Runtime/tests/System.Runtime.Tests/System/Collections/ObjectModel/ReadOnlyCollectionTests.cs

@@ -30,6 +30,23 @@ public static void Ctor_IList()
             Assert.Same(s_intArray, collection.GetItems());
         }
 
+        [Fact]
+        public static void Ctor_CollectionExpressions_Empty()
+        {
+            ReadOnlyCollection<int> c = [];
+            Assert.IsType<ReadOnlyCollection<int>>(c);
+            Assert.Empty(c);
+            Assert.Same(c, ReadOnlyCollection<int>.Empty);
+        }
+
+        [Fact]
+        public static void Ctor_CollectionExpressions()
+        {
+            ReadOnlyCollection<int> c = [.. s_intArray];
+            Assert.IsType<ReadOnlyCollection<int>>(c);
+            Assert.Equal(c, s_intArray);
+        }
+
         [Fact]
         public static void Count()
         {