diff --git a/src/libraries/System.Collections/ref/System.Collections.cs b/src/libraries/System.Collections/ref/System.Collections.cs
index 028b1637eb4825..7a15ca70fb3e51 100644
--- a/src/libraries/System.Collections/ref/System.Collections.cs
+++ b/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
     {
diff --git a/src/libraries/System.Collections/src/System/Collections/Generic/CollectionExtensions.cs b/src/libraries/System.Collections/src/System/Collections/Generic/CollectionExtensions.cs
index 7a5bce9b0a1d4e..4f5c545603dbbe 100644
--- a/src/libraries/System.Collections/src/System/Collections/Generic/CollectionExtensions.cs
+++ b/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);
+        }
     }
 }
diff --git a/src/libraries/System.Collections/tests/Generic/CollectionExtensionsTests.cs b/src/libraries/System.Collections/tests/Generic/CollectionExtensionsTests.cs
index 153a3111907685..213d9d6faca996 100644
--- a/src/libraries/System.Collections/tests/Generic/CollectionExtensionsTests.cs
+++ b/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());
+        }
     }
 }
diff --git a/src/libraries/System.ObjectModel/ref/System.ObjectModel.Forwards.cs b/src/libraries/System.ObjectModel/ref/System.ObjectModel.Forwards.cs
new file mode 100644
index 00000000000000..df45eb9a68a6ee
--- /dev/null
+++ b/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<,>))] 
diff --git a/src/libraries/System.ObjectModel/ref/System.ObjectModel.cs b/src/libraries/System.ObjectModel/ref/System.ObjectModel.cs
index 744576e587d067..21cd3dc855f118 100644
--- a/src/libraries/System.ObjectModel/ref/System.ObjectModel.cs
+++ b/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>)) { }
diff --git a/src/libraries/System.ObjectModel/ref/System.ObjectModel.csproj b/src/libraries/System.ObjectModel/ref/System.ObjectModel.csproj
index 9b427746cd8afa..623c85ec556705 100644
--- a/src/libraries/System.ObjectModel/ref/System.ObjectModel.csproj
+++ b/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" />
diff --git a/src/libraries/System.ObjectModel/src/System.ObjectModel.Forwards.cs b/src/libraries/System.ObjectModel/src/System.ObjectModel.Forwards.cs
new file mode 100644
index 00000000000000..deb07efe4f82cb
--- /dev/null
+++ b/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<,>))]
diff --git a/src/libraries/System.ObjectModel/src/System.ObjectModel.csproj b/src/libraries/System.ObjectModel/src/System.ObjectModel.csproj
index 66c53d0dfb08b4..ecf4f2a77fe9e2 100644
--- a/src/libraries/System.ObjectModel/src/System.ObjectModel.csproj
+++ b/src/libraries/System.ObjectModel/src/System.ObjectModel.csproj
@@ -5,6 +5,7 @@
     <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" />
@@ -12,7 +13,6 @@
     <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" />
diff --git a/src/libraries/System.ObjectModel/src/System/Collections/CollectionHelpers.cs b/src/libraries/System.ObjectModel/src/System/Collections/CollectionHelpers.cs
index 86058208873282..bff227728ecff7 100644
--- a/src/libraries/System.ObjectModel/src/System/Collections/CollectionHelpers.cs
+++ b/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));
-                }
-            }
-        }
     }
 }
diff --git a/src/libraries/System.ObjectModel/src/System/Collections/Generic/DebugView.cs b/src/libraries/System.ObjectModel/src/System/Collections/Generic/DebugView.cs
index 2233d2518fe36c..3ab6a770ca772b 100644
--- a/src/libraries/System.ObjectModel/src/System/Collections/Generic/DebugView.cs
+++ b/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;
-            }
-        }
-    }
 }
diff --git a/src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems b/src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems
index 01fd0776225fa6..9723fc8954385a 100644
--- a/src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems
+++ b/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" />
diff --git a/src/libraries/System.Private.CoreLib/src/System/Collections/ObjectModel/CollectionHelpers.cs b/src/libraries/System.Private.CoreLib/src/System/Collections/ObjectModel/CollectionHelpers.cs
new file mode 100644
index 00000000000000..89af10ba7109f4
--- /dev/null
+++ b/src/libraries/System.Private.CoreLib/src/System/Collections/ObjectModel/CollectionHelpers.cs
@@ -0,0 +1,73 @@
+// 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.Generic;
+
+namespace System.Collections.ObjectModel
+{
+    internal static class CollectionHelpers
+    {
+        internal static void ValidateCopyToArguments(int sourceCount, Array array, int index)
+        {
+            if (array == null)
+            {
+                throw new ArgumentNullException(nameof(array));
+            }
+
+            if (array.Rank != 1)
+            {
+                throw new ArgumentException(SR.Arg_RankMultiDimNotSupported, nameof(array));
+            }
+
+            if (array.GetLowerBound(0) != 0)
+            {
+                throw new ArgumentException(SR.Arg_NonZeroLowerBound, nameof(array));
+            }
+
+            if (index < 0 || index > array.Length)
+            {
+                throw new ArgumentOutOfRangeException(nameof(index), SR.ArgumentOutOfRange_NeedNonNegNum);
+            }
+
+            if (array.Length - index < sourceCount)
+            {
+                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));
+                }
+            }
+        }
+    }
+}
diff --git a/src/libraries/System.ObjectModel/src/System/Collections/ObjectModel/ReadOnlyDictionary.cs b/src/libraries/System.Private.CoreLib/src/System/Collections/ObjectModel/ReadOnlyDictionary.cs
similarity index 98%
rename from src/libraries/System.ObjectModel/src/System/Collections/ObjectModel/ReadOnlyDictionary.cs
rename to src/libraries/System.Private.CoreLib/src/System/Collections/ObjectModel/ReadOnlyDictionary.cs
index 428b0663d263ce..57c6b4604dc1d8 100644
--- a/src/libraries/System.ObjectModel/src/System/Collections/ObjectModel/ReadOnlyDictionary.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Collections/ObjectModel/ReadOnlyDictionary.cs
@@ -8,7 +8,7 @@
 namespace System.Collections.ObjectModel
 {
     [Serializable]
-    [DebuggerTypeProxy(typeof(DictionaryDebugView<,>))]
+    [DebuggerTypeProxy(typeof(IDictionaryDebugView<,>))]
     [DebuggerDisplay("Count = {Count}")]
     [System.Runtime.CompilerServices.TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
     public class ReadOnlyDictionary<TKey, TValue> : IDictionary<TKey, TValue>, IDictionary, IReadOnlyDictionary<TKey, TValue> where TKey : notnull
@@ -248,7 +248,7 @@ public DictionaryEntry Entry
 
         IEnumerable<TValue> IReadOnlyDictionary<TKey, TValue>.Values => Values;
 
-        [DebuggerTypeProxy(typeof(CollectionDebugView<>))]
+        [DebuggerTypeProxy(typeof(ICollectionDebugView<>))]
         [DebuggerDisplay("Count = {Count}")]
         public sealed class KeyCollection : ICollection<TKey>, ICollection, IReadOnlyCollection<TKey>
         {
@@ -302,7 +302,7 @@ void ICollection.CopyTo(Array array, int index)
             object ICollection.SyncRoot => (_collection is ICollection coll) ? coll.SyncRoot : this;
         }
 
-        [DebuggerTypeProxy(typeof(CollectionDebugView<>))]
+        [DebuggerTypeProxy(typeof(ICollectionDebugView<>))]
         [DebuggerDisplay("Count = {Count}")]
         public sealed class ValueCollection : ICollection<TValue>, ICollection, IReadOnlyCollection<TValue>
         {
diff --git a/src/libraries/System.Runtime/ref/System.Runtime.cs b/src/libraries/System.Runtime/ref/System.Runtime.cs
index 7157e359ccd540..04b63cbf2a8070 100644
--- a/src/libraries/System.Runtime/ref/System.Runtime.cs
+++ b/src/libraries/System.Runtime/ref/System.Runtime.cs
@@ -8657,6 +8657,77 @@ void System.Collections.IList.Insert(int index, object? value) { }
         void System.Collections.IList.Remove(object? value) { }
         void System.Collections.IList.RemoveAt(int index) { }
     }
+    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; }
+        }
+    }
 }
 namespace System.ComponentModel
 {
diff --git a/src/libraries/shims/ApiCompatBaseline.PreviousNetCoreApp.txt b/src/libraries/shims/ApiCompatBaseline.PreviousNetCoreApp.txt
index 32beb555e41775..0fdeb001ef9d87 100644
--- a/src/libraries/shims/ApiCompatBaseline.PreviousNetCoreApp.txt
+++ b/src/libraries/shims/ApiCompatBaseline.PreviousNetCoreApp.txt
@@ -226,6 +226,8 @@ CannotRemoveAttribute : Attribute 'System.Runtime.CompilerServices.IsReadOnlyAtt
 CannotRemoveAttribute : Attribute 'System.Runtime.CompilerServices.IsReadOnlyAttribute' exists on 'System.Numerics.Vector<T>.ToString(System.String, System.IFormatProvider)' in the contract but not the implementation.
 CannotRemoveAttribute : Attribute 'System.Runtime.CompilerServices.IsReadOnlyAttribute' exists on 'System.Numerics.Vector<T>.TryCopyTo(System.Span<System.Byte>)' in the contract but not the implementation.
 CannotRemoveAttribute : Attribute 'System.Runtime.CompilerServices.IsReadOnlyAttribute' exists on 'System.Numerics.Vector<T>.TryCopyTo(System.Span<T>)' in the contract but not the implementation.
+Compat issues with assembly System.ObjectModel:
+TypesMustExist : Type 'System.Collections.ObjectModel.ReadOnlyDictionary<TKey, TValue>' does not exist in the implementation but it does exist in the contract.
 Compat issues with assembly System.Reflection.Emit:
 CannotChangeAttribute : Attribute 'System.Diagnostics.CodeAnalysis.DynamicallyAccessedMembersAttribute' on 'System.Reflection.Emit.EnumBuilder.GetMember(System.String, System.Reflection.MemberTypes, System.Reflection.BindingFlags)' changed from '[DynamicallyAccessedMembersAttribute(DynamicallyAccessedMemberTypes.All)]' in the contract to '[DynamicallyAccessedMembersAttribute(DynamicallyAccessedMemberTypes.NonPublicConstructors | DynamicallyAccessedMemberTypes.NonPublicEvents | DynamicallyAccessedMemberTypes.NonPublicFields | DynamicallyAccessedMemberTypes.NonPublicMethods | DynamicallyAccessedMemberTypes.NonPublicNestedTypes | DynamicallyAccessedMemberTypes.NonPublicProperties | DynamicallyAccessedMemberTypes.PublicConstructors | DynamicallyAccessedMemberTypes.PublicEvents | DynamicallyAccessedMemberTypes.PublicFields | DynamicallyAccessedMemberTypes.PublicMethods | DynamicallyAccessedMemberTypes.PublicNestedTypes | DynamicallyAccessedMemberTypes.PublicProperties)]' in the implementation.
 CannotChangeAttribute : Attribute 'System.Diagnostics.CodeAnalysis.DynamicallyAccessedMembersAttribute' on 'System.Reflection.Emit.EnumBuilder.GetMembers(System.Reflection.BindingFlags)' changed from '[DynamicallyAccessedMembersAttribute(DynamicallyAccessedMemberTypes.All)]' in the contract to '[DynamicallyAccessedMembersAttribute(DynamicallyAccessedMemberTypes.NonPublicConstructors | DynamicallyAccessedMemberTypes.NonPublicEvents | DynamicallyAccessedMemberTypes.NonPublicFields | DynamicallyAccessedMemberTypes.NonPublicMethods | DynamicallyAccessedMemberTypes.NonPublicNestedTypes | DynamicallyAccessedMemberTypes.NonPublicProperties | DynamicallyAccessedMemberTypes.PublicConstructors | DynamicallyAccessedMemberTypes.PublicEvents | DynamicallyAccessedMemberTypes.PublicFields | DynamicallyAccessedMemberTypes.PublicMethods | DynamicallyAccessedMemberTypes.PublicNestedTypes | DynamicallyAccessedMemberTypes.PublicProperties)]' in the implementation.
@@ -303,4 +305,4 @@ CannotRemoveAttribute : Attribute 'System.Diagnostics.CodeAnalysis.DynamicallyAc
 CannotChangeAttribute : Attribute 'System.AttributeUsageAttribute' on 'System.Text.Json.Serialization.JsonConverterAttribute' changed from '[AttributeUsageAttribute(AttributeTargets.Class | AttributeTargets.Enum | AttributeTargets.Field | AttributeTargets.Property | AttributeTargets.Struct, AllowMultiple=false)]' in the contract to '[AttributeUsageAttribute(AttributeTargets.Class | AttributeTargets.Enum | AttributeTargets.Field | AttributeTargets.Interface | AttributeTargets.Property | AttributeTargets.Struct, AllowMultiple=false)]' in the implementation.
 Compat issues with assembly System.Threading.Tasks.Extensions:
 CannotChangeAttribute : Attribute 'System.AttributeUsageAttribute' on 'System.Runtime.CompilerServices.AsyncMethodBuilderAttribute' changed from '[AttributeUsageAttribute(AttributeTargets.Class | AttributeTargets.Delegate | AttributeTargets.Enum | AttributeTargets.Interface | AttributeTargets.Struct, Inherited=false, AllowMultiple=false)]' in the contract to '[AttributeUsageAttribute(AttributeTargets.Class | AttributeTargets.Delegate | AttributeTargets.Enum | AttributeTargets.Interface | AttributeTargets.Method | AttributeTargets.Struct, Inherited=false, AllowMultiple=false)]' in the implementation.
-Total Issues: 286
+Total Issues: 287
diff --git a/src/libraries/shims/ApiCompatBaseline.netcoreapp.netstandard.txt b/src/libraries/shims/ApiCompatBaseline.netcoreapp.netstandard.txt
index 1d4a30c30f10ea..40ec257771a8bc 100644
--- a/src/libraries/shims/ApiCompatBaseline.netcoreapp.netstandard.txt
+++ b/src/libraries/shims/ApiCompatBaseline.netcoreapp.netstandard.txt
@@ -630,4 +630,5 @@ CannotSealType : Type 'System.Xml.Schema.XmlSchemaGroupBase' is effectively (has
 MembersMustExist : Member 'protected void System.Xml.Schema.XmlSchemaGroupBase..ctor()' does not exist in the implementation but it does exist in the contract.
 CannotMakeMemberNonVirtual : Member 'public System.Xml.Schema.XmlSchemaObjectCollection System.Xml.Schema.XmlSchemaGroupBase.Items' is non-virtual in the implementation but is virtual in the contract.
 CannotMakeMemberNonVirtual : Member 'public System.Xml.Schema.XmlSchemaObjectCollection System.Xml.Schema.XmlSchemaGroupBase.Items.get()' is non-virtual in the implementation but is virtual in the contract.
-Total Issues: 617
+TypesMustExist : Type 'System.Collections.ObjectModel.ReadOnlyDictionary<TKey, TValue>' does not exist in the implementation but it does exist in the contract.
+Total Issues: 618