Added AsReadOnly extension methods for IDictionary<T, U> and IList<T>

src/libraries/System.Collections/ref/System.Collections.cs

@@ -47,6 +47,8 @@ public static partial class CollectionExtensions
         public static TValue GetValueOrDefault<TKey, TValue>(this System.Collections.Generic.IReadOnlyDictionary<TKey, TValue> dictionary, TKey key, TValue defaultValue) { throw null; }
         public static bool Remove<TKey, TValue>(this System.Collections.Generic.IDictionary<TKey, TValue> dictionary, TKey key, [System.Diagnostics.CodeAnalysis.MaybeNullWhenAttribute(false)] out TValue value) { throw null; }
         public static bool TryAdd<TKey, TValue>(this System.Collections.Generic.IDictionary<TKey, TValue> dictionary, TKey key, TValue value) { throw null; }
+        public static System.Collections.ObjectModel.ReadOnlyCollection<T> AsReadOnly<T>(this IList<T> list) { throw null; }
+        public static System.Collections.ObjectModel.ReadOnlyDictionary<TKey, TValue> AsReadOnly<TKey, TValue>(this IDictionary<TKey, TValue> dictionary) where TKey : notnull { throw null; }
     }
     public abstract partial class Comparer<T> : System.Collections.Generic.IComparer<T>, System.Collections.IComparer
     {

src/libraries/System.Collections/src/System/Collections/Generic/CollectionExtensions.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Collections.ObjectModel;
 using System.Diagnostics.CodeAnalysis;
 
 namespace System.Collections.Generic
@@ -55,5 +56,32 @@ public static bool Remove<TKey, TValue>(this IDictionary<TKey, TValue> dictionar
             value = default;
             return false;
         }
+
+        /// <summary>
+        /// Returns a read-only <see cref="ReadOnlyCollection{T}"/> wrapper
+        /// for the specified list.
+        /// </summary>
+        /// <typeparam name="T">The type of elements in the collection.</typeparam>
+        /// <param name="list">The list to wrap.</param>
+        /// <returns>An object that acts as a read-only wrapper around the current <see cref="IList{T}"/>.</returns>
+        /// <exception cref="ArgumentNullException"><paramref name="list"/> is null.</exception>
+        public static ReadOnlyCollection<T> AsReadOnly<T>(this IList<T> list)
+        {
+            return new ReadOnlyCollection<T>(list);
+        }
+
+        /// <summary>
+        /// Returns a read-only <see cref="ReadOnlyDictionary{TKey, TValue}"/> wrapper
+        /// for the current dictionary.
+        /// </summary>
+        /// <typeparam name="TKey">The type of keys in the dictionary.</typeparam>
+        /// <typeparam name="TValue">The type of values in the dictionary.</typeparam>
+        /// <param name="dictionary">The dictionary to wrap.</param>
+        /// <returns>An object that acts as a read-only wrapper around the current <see cref="IDictionary{TKey, TValue}"/>.</returns>
+        /// <exception cref="ArgumentNullException"><paramref name="dictionary"/> is null.</exception>
+        public static ReadOnlyDictionary<TKey, TValue> AsReadOnly<TKey, TValue>(this IDictionary<TKey, TValue> dictionary) where TKey : notnull
+        {
+            return new ReadOnlyDictionary<TKey, TValue>(dictionary);
+        }
     }
 }

src/libraries/System.Collections/tests/Generic/CollectionExtensionsTests.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Collections.Generic;
+using System.Collections.ObjectModel;
 using Xunit;
 
 namespace System.Collections.Tests
@@ -104,5 +105,39 @@ public void Remove_KeyDoesntExistInIDictionary_ReturnsFalse()
             Assert.False(dictionary.Remove("key", out var value));
             Assert.Equal(default(string), value);
         }
+
+        [Fact]
+        public void AsReadOnly_TurnsIListIntoReadOnlyCollection()
+        {
+            IList<string> list = new List<string> { "A", "B" };
+            ReadOnlyCollection<string> readOnlyCollection = list.AsReadOnly();
+            Assert.NotNull(readOnlyCollection);
+            CollectionAsserts.Equal(list, readOnlyCollection);
+        }
+
+        [Fact]
+        public void AsReadOnly_TurnsIDictionaryIntoReadOnlyDictionary()
+        {
+            IDictionary<string, string> dictionary = new Dictionary<string, string> { ["key1"] = "value1", ["key2"] = "value2" };
+            ReadOnlyDictionary<string, string> readOnlyDictionary = dictionary.AsReadOnly();
+            Assert.NotNull(readOnlyDictionary);
+            Assert.Equal(dictionary["key1"], readOnlyDictionary["key1"]);
+            Assert.Equal(dictionary["key2"], readOnlyDictionary["key2"]);
+            Assert.Equal(dictionary.Count, readOnlyDictionary.Count);
+        }
+
+        [Fact]
+        public void AsReadOnly_NullIList_ThrowsArgumentNullException()
+        {
+            IList<string> list = null;
+            Assert.Throws<ArgumentNullException>("list", () => list.AsReadOnly());
+        }
+
+        [Fact]
+        public void AsReadOnly_NullIDictionary_ThrowsArgumentNullException()
+        {
+            IDictionary<string, string> dictionary = null;
+            Assert.Throws<ArgumentNullException>("dictionary", () => dictionary.AsReadOnly());
+        }
     }
 }

src/libraries/System.ObjectModel/ref/System.ObjectModel.Forwards.cs

@@ -0,0 +1,4 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+[assembly: System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.Collections.ObjectModel.ReadOnlyDictionary<,>))] 

src/libraries/System.ObjectModel/ref/System.ObjectModel.cs

@@ -43,77 +43,6 @@ protected virtual void OnPropertyChanged(System.ComponentModel.PropertyChangedEv
         protected override void RemoveItem(int index) { }
         protected override void SetItem(int index, T item) { }
     }
-    public partial class ReadOnlyDictionary<TKey, TValue> : System.Collections.Generic.ICollection<System.Collections.Generic.KeyValuePair<TKey, TValue>>, System.Collections.Generic.IDictionary<TKey, TValue>, System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<TKey, TValue>>, System.Collections.Generic.IReadOnlyCollection<System.Collections.Generic.KeyValuePair<TKey, TValue>>, System.Collections.Generic.IReadOnlyDictionary<TKey, TValue>, System.Collections.ICollection, System.Collections.IDictionary, System.Collections.IEnumerable where TKey : notnull
-    {
-        public ReadOnlyDictionary(System.Collections.Generic.IDictionary<TKey, TValue> dictionary) { }
-        public int Count { get { throw null; } }
-        protected System.Collections.Generic.IDictionary<TKey, TValue> Dictionary { get { throw null; } }
-        public TValue this[TKey key] { get { throw null; } }
-        public System.Collections.ObjectModel.ReadOnlyDictionary<TKey, TValue>.KeyCollection Keys { get { throw null; } }
-        bool System.Collections.Generic.ICollection<System.Collections.Generic.KeyValuePair<TKey, TValue>>.IsReadOnly { get { throw null; } }
-        TValue System.Collections.Generic.IDictionary<TKey, TValue>.this[TKey key] { get { throw null; } set { } }
-        System.Collections.Generic.ICollection<TKey> System.Collections.Generic.IDictionary<TKey, TValue>.Keys { get { throw null; } }
-        System.Collections.Generic.ICollection<TValue> System.Collections.Generic.IDictionary<TKey, TValue>.Values { get { throw null; } }
-        System.Collections.Generic.IEnumerable<TKey> System.Collections.Generic.IReadOnlyDictionary<TKey, TValue>.Keys { get { throw null; } }
-        System.Collections.Generic.IEnumerable<TValue> System.Collections.Generic.IReadOnlyDictionary<TKey, TValue>.Values { get { throw null; } }
-        bool System.Collections.ICollection.IsSynchronized { get { throw null; } }
-        object System.Collections.ICollection.SyncRoot { get { throw null; } }
-        bool System.Collections.IDictionary.IsFixedSize { get { throw null; } }
-        bool System.Collections.IDictionary.IsReadOnly { get { throw null; } }
-        object? System.Collections.IDictionary.this[object key] { get { throw null; } set { } }
-        System.Collections.ICollection System.Collections.IDictionary.Keys { get { throw null; } }
-        System.Collections.ICollection System.Collections.IDictionary.Values { get { throw null; } }
-        public System.Collections.ObjectModel.ReadOnlyDictionary<TKey, TValue>.ValueCollection Values { get { throw null; } }
-        public bool ContainsKey(TKey key) { throw null; }
-        public System.Collections.Generic.IEnumerator<System.Collections.Generic.KeyValuePair<TKey, TValue>> GetEnumerator() { throw null; }
-        void System.Collections.Generic.ICollection<System.Collections.Generic.KeyValuePair<TKey, TValue>>.Add(System.Collections.Generic.KeyValuePair<TKey, TValue> item) { }
-        void System.Collections.Generic.ICollection<System.Collections.Generic.KeyValuePair<TKey, TValue>>.Clear() { }
-        bool System.Collections.Generic.ICollection<System.Collections.Generic.KeyValuePair<TKey, TValue>>.Contains(System.Collections.Generic.KeyValuePair<TKey, TValue> item) { throw null; }
-        void System.Collections.Generic.ICollection<System.Collections.Generic.KeyValuePair<TKey, TValue>>.CopyTo(System.Collections.Generic.KeyValuePair<TKey, TValue>[] array, int arrayIndex) { }
-        bool System.Collections.Generic.ICollection<System.Collections.Generic.KeyValuePair<TKey, TValue>>.Remove(System.Collections.Generic.KeyValuePair<TKey, TValue> item) { throw null; }
-        void System.Collections.Generic.IDictionary<TKey, TValue>.Add(TKey key, TValue value) { }
-        bool System.Collections.Generic.IDictionary<TKey, TValue>.Remove(TKey key) { throw null; }
-        void System.Collections.ICollection.CopyTo(System.Array array, int index) { }
-        void System.Collections.IDictionary.Add(object key, object? value) { }
-        void System.Collections.IDictionary.Clear() { }
-        bool System.Collections.IDictionary.Contains(object key) { throw null; }
-        System.Collections.IDictionaryEnumerator System.Collections.IDictionary.GetEnumerator() { throw null; }
-        void System.Collections.IDictionary.Remove(object key) { }
-        System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() { throw null; }
-        public bool TryGetValue(TKey key, [System.Diagnostics.CodeAnalysis.MaybeNullWhenAttribute(false)] out TValue value) { throw null; }
-        public sealed partial class KeyCollection : System.Collections.Generic.ICollection<TKey>, System.Collections.Generic.IEnumerable<TKey>, System.Collections.Generic.IReadOnlyCollection<TKey>, System.Collections.ICollection, System.Collections.IEnumerable
-        {
-            internal KeyCollection() { }
-            public int Count { get { throw null; } }
-            bool System.Collections.Generic.ICollection<TKey>.IsReadOnly { get { throw null; } }
-            bool System.Collections.ICollection.IsSynchronized { get { throw null; } }
-            object System.Collections.ICollection.SyncRoot { get { throw null; } }
-            public void CopyTo(TKey[] array, int arrayIndex) { }
-            public System.Collections.Generic.IEnumerator<TKey> GetEnumerator() { throw null; }
-            void System.Collections.Generic.ICollection<TKey>.Add(TKey item) { }
-            void System.Collections.Generic.ICollection<TKey>.Clear() { }
-            bool System.Collections.Generic.ICollection<TKey>.Contains(TKey item) { throw null; }
-            bool System.Collections.Generic.ICollection<TKey>.Remove(TKey item) { throw null; }
-            void System.Collections.ICollection.CopyTo(System.Array array, int index) { }
-            System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() { throw null; }
-        }
-        public sealed partial class ValueCollection : System.Collections.Generic.ICollection<TValue>, System.Collections.Generic.IEnumerable<TValue>, System.Collections.Generic.IReadOnlyCollection<TValue>, System.Collections.ICollection, System.Collections.IEnumerable
-        {
-            internal ValueCollection() { }
-            public int Count { get { throw null; } }
-            bool System.Collections.Generic.ICollection<TValue>.IsReadOnly { get { throw null; } }
-            bool System.Collections.ICollection.IsSynchronized { get { throw null; } }
-            object System.Collections.ICollection.SyncRoot { get { throw null; } }
-            public void CopyTo(TValue[] array, int arrayIndex) { }
-            public System.Collections.Generic.IEnumerator<TValue> GetEnumerator() { throw null; }
-            void System.Collections.Generic.ICollection<TValue>.Add(TValue item) { }
-            void System.Collections.Generic.ICollection<TValue>.Clear() { }
-            bool System.Collections.Generic.ICollection<TValue>.Contains(TValue item) { throw null; }
-            bool System.Collections.Generic.ICollection<TValue>.Remove(TValue item) { throw null; }
-            void System.Collections.ICollection.CopyTo(System.Array array, int index) { }
-            System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() { throw null; }
-        }
-    }
     public partial class ReadOnlyObservableCollection<T> : System.Collections.ObjectModel.ReadOnlyCollection<T>, System.Collections.Specialized.INotifyCollectionChanged, System.ComponentModel.INotifyPropertyChanged
     {
         public ReadOnlyObservableCollection(System.Collections.ObjectModel.ObservableCollection<T> list) : base (default(System.Collections.Generic.IList<T>)) { }

src/libraries/System.ObjectModel/ref/System.ObjectModel.csproj

@@ -5,6 +5,7 @@
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="System.ObjectModel.cs" />
+    <Compile Include="System.ObjectModel.Forwards.cs" />
   </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\..\System.Runtime\ref\System.Runtime.csproj" />

src/libraries/System.ObjectModel/src/System.ObjectModel.Forwards.cs

@@ -0,0 +1,4 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+[assembly: System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.Collections.ObjectModel.ReadOnlyDictionary<,>))]

src/libraries/System.ObjectModel/src/System.ObjectModel.csproj

@@ -5,14 +5,14 @@
     <Nullable>enable</Nullable>
   </PropertyGroup>
   <ItemGroup>
+    <Compile Include="System.ObjectModel.Forwards.cs" />
     <Compile Include="System\Collections\CollectionHelpers.cs" />
     <Compile Include="System\Collections\Generic\DebugView.cs" />
     <Compile Include="System\Collections\Specialized\INotifyCollectionChanged.cs" />
     <Compile Include="System\Collections\Specialized\NotifyCollectionChangedAction.cs" />
     <Compile Include="System\Collections\Specialized\NotifyCollectionChangedEventArgs.cs" />
     <Compile Include="System\Collections\ObjectModel\KeyedCollection.cs" />
     <Compile Include="System\Collections\ObjectModel\ObservableCollection.cs" />
-    <Compile Include="System\Collections\ObjectModel\ReadOnlyDictionary.cs" />
     <Compile Include="System\Collections\ObjectModel\ReadOnlyObservableCollection.cs" />
     <Compile Include="System\ComponentModel\DataErrorsChangedEventArgs.cs" />
     <Compile Include="System\ComponentModel\INotifyDataErrorInfo.cs" />

src/libraries/System.ObjectModel/src/System/Collections/CollectionHelpers.cs

@@ -34,40 +34,5 @@ internal static void ValidateCopyToArguments(int sourceCount, Array array, int i
                 throw new ArgumentException(SR.Arg_ArrayPlusOffTooSmall);
             }
         }
-
-        internal static void CopyTo<T>(ICollection<T> collection, Array array, int index)
-        {
-            ValidateCopyToArguments(collection.Count, array, index);
-
-            if (collection is ICollection nonGenericCollection)
-            {
-                // Easy out if the ICollection<T> implements the non-generic ICollection
-                nonGenericCollection.CopyTo(array, index);
-            }
-            else if (array is T[] items)
-            {
-                collection.CopyTo(items, index);
-            }
-            else
-            {
-                // We can't cast array of value type to object[], so we don't support widening of primitive types here.
-                if (array is not object?[] objects)
-                {
-                    throw new ArgumentException(SR.Argument_InvalidArrayType, nameof(array));
-                }
-
-                try
-                {
-                    foreach (T item in collection)
-                    {
-                        objects[index++] = item;
-                    }
-                }
-                catch (ArrayTypeMismatchException)
-                {
-                    throw new ArgumentException(SR.Argument_InvalidArrayType, nameof(array));
-                }
-            }
-        }
     }
 }

src/libraries/System.ObjectModel/src/System/Collections/Generic/DebugView.cs

@@ -25,25 +25,4 @@ public T[] Items
             }
         }
     }
-
-    internal sealed class DictionaryDebugView<K, V> where K: notnull
-    {
-        private readonly IDictionary<K, V> _dict;
-
-        public DictionaryDebugView(IDictionary<K, V> dictionary)
-        {
-            _dict = dictionary ?? throw new ArgumentNullException(nameof(dictionary));
-        }
-
-        [DebuggerBrowsable(DebuggerBrowsableState.RootHidden)]
-        public KeyValuePair<K, V>[] Items
-        {
-            get
-            {
-                KeyValuePair<K, V>[] items = new KeyValuePair<K, V>[_dict.Count];
-                _dict.CopyTo(items, 0);
-                return items;
-            }
-        }
-    }
 }

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

@@ -228,7 +228,9 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Collections\KeyValuePairs.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Collections\ListDictionaryInternal.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Collections\ObjectModel\Collection.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Collections\ObjectModel\CollectionHelpers.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Collections\ObjectModel\ReadOnlyCollection.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Collections\ObjectModel\ReadOnlyDictionary.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\ComponentModel\DefaultValueAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\ComponentModel\EditorBrowsableAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\ComponentModel\EditorBrowsableState.cs" />