Add CollectionsMarshal.GetValueRef API

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

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Collections.Generic;
+using Internal.Runtime.CompilerServices;
 
 namespace System.Runtime.InteropServices
 {
@@ -18,6 +19,25 @@ public static class CollectionsMarshal
         public static Span<T> AsSpan<T>(List<T>? list)
             => list is null ? default : new Span<T>(list._items, 0, list._size);
 
+        /// <summary>
+        /// Gets a ref to a <typeparamref name="TValue"/> in the <see cref="Dictionary{TKey, TValue}"/>.
+        /// </summary>
+        /// <param name="dictionary">The dictionary to get the ref to <typeparamref name="TValue"/> from.</param>
+        /// <param name="key">The key used for lookup.</param>
+        /// <remarks>Items should not be added or removed from the <see cref="Dictionary{TKey, TValue}"/> while the ref <typeparamref name="TValue"/> is in use.</remarks>
+        /// <exception cref="KeyNotFoundException">Thrown when <paramref name="key"/> does not exist in the <paramref name="dictionary"/>.</exception>
+        public static ref TValue GetValueRef<TKey, TValue>(Dictionary<TKey, TValue> dictionary, TKey key) where TKey : notnull
+        {
+            ref TValue valueRef = ref dictionary.FindValue(key);
+
+            if (Unsafe.IsNullRef(ref valueRef))
+            {
+                ThrowHelper.ThrowKeyNotFoundException(key);
+            }
+
+            return ref valueRef;
+        }
+
         /// <summary>
         /// Gets either a ref to a <typeparamref name="TValue"/> in the <see cref="Dictionary{TKey, TValue}"/> or a ref null if it does not exist in the <paramref name="dictionary"/>.
         /// </summary>

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

@@ -176,6 +176,7 @@ public CoClassAttribute(System.Type coClass) { }
     public static partial class CollectionsMarshal
     {
         public static System.Span<T> AsSpan<T>(System.Collections.Generic.List<T>? list) { throw null; }
+        public static ref TValue GetValueRef<TKey, TValue>(System.Collections.Generic.Dictionary<TKey, TValue> dictionary, TKey key) where TKey : notnull { throw null; }
         public static ref TValue GetValueRefOrNullRef<TKey, TValue>(System.Collections.Generic.Dictionary<TKey, TValue> dictionary, TKey key) where TKey : notnull { throw null; }
     }
     [System.AttributeUsageAttribute(System.AttributeTargets.Field | System.AttributeTargets.Parameter | System.AttributeTargets.Property | System.AttributeTargets.ReturnValue, Inherited=false)]

src/libraries/System.Runtime.InteropServices/tests/System/Runtime/InteropServices/CollectionsMarshalTests.cs

@@ -144,6 +144,159 @@ public void ListAsSpanLinkBreaksOnResize()
             }
         }
 
+        [Fact]
+        public void GetValueRefValueType()
+        {
+            var dict = new Dictionary<int, Struct>
+            {
+                {  1, default },
+                {  2, default }
+            };
+
+            Assert.Equal(2, dict.Count);
+
+            Assert.Equal(0, dict[1].Value);
+            Assert.Equal(0, dict[1].Property);
+
+            var itemVal = dict[1];
+            itemVal.Value = 1;
+            itemVal.Property = 2;
+
+            // Does not change values in dictionary
+            Assert.Equal(0, dict[1].Value);
+            Assert.Equal(0, dict[1].Property);
+
+            CollectionsMarshal.GetValueRef(dict, 1).Value = 3;
+            CollectionsMarshal.GetValueRef(dict, 1).Property = 4;
+
+            Assert.Equal(3, dict[1].Value);
+            Assert.Equal(4, dict[1].Property);
+
+            ref var itemRef = ref CollectionsMarshal.GetValueRef(dict, 2);
+
+            Assert.Equal(0, itemRef.Value);
+            Assert.Equal(0, itemRef.Property);
+
+            itemRef.Value = 5;
+            itemRef.Property = 6;
+
+            Assert.Equal(5, itemRef.Value);
+            Assert.Equal(6, itemRef.Property);
+            Assert.Equal(dict[2].Value, itemRef.Value);
+            Assert.Equal(dict[2].Property, itemRef.Property);
+
+            itemRef = new() { Value = 7, Property = 8 };
+
+            Assert.Equal(7, itemRef.Value);
+            Assert.Equal(8, itemRef.Property);
+            Assert.Equal(dict[2].Value, itemRef.Value);
+            Assert.Equal(dict[2].Property, itemRef.Property);
+
+            // Check for exception
+
+            Assert.Throws<KeyNotFoundException>(() => CollectionsMarshal.GetValueRef(dict, 3));
+
+            Assert.Equal(2, dict.Count);
+        }
+
+        [Fact]
+        public void GetValueRefClass()
+        {
+            var dict = new Dictionary<int, IntAsObject>
+            {
+                {  1, new() },
+                {  2, new() }
+            };
+
+            Assert.Equal(2, dict.Count);
+
+            Assert.Equal(0, dict[1].Value);
+            Assert.Equal(0, dict[1].Property);
+
+            var itemVal = dict[1];
+            itemVal.Value = 1;
+            itemVal.Property = 2;
+
+            // Does change values in dictionary
+            Assert.Equal(1, dict[1].Value);
+            Assert.Equal(2, dict[1].Property);
+
+            CollectionsMarshal.GetValueRef(dict, 1).Value = 3;
+            CollectionsMarshal.GetValueRef(dict, 1).Property = 4;
+
+            Assert.Equal(3, dict[1].Value);
+            Assert.Equal(4, dict[1].Property);
+
+            ref var itemRef = ref CollectionsMarshal.GetValueRef(dict, 2);
+
+            Assert.Equal(0, itemRef.Value);
+            Assert.Equal(0, itemRef.Property);
+
+            itemRef.Value = 5;
+            itemRef.Property = 6;
+
+            Assert.Equal(5, itemRef.Value);
+            Assert.Equal(6, itemRef.Property);
+            Assert.Equal(dict[2].Value, itemRef.Value);
+            Assert.Equal(dict[2].Property, itemRef.Property);
+
+            itemRef = new() { Value = 7, Property = 8 };
+
+            Assert.Equal(7, itemRef.Value);
+            Assert.Equal(8, itemRef.Property);
+            Assert.Equal(dict[2].Value, itemRef.Value);
+            Assert.Equal(dict[2].Property, itemRef.Property);
+
+            // Check for exception
+
+            Assert.Throws<KeyNotFoundException>(() => CollectionsMarshal.GetValueRef(dict, 3));
+
+            Assert.Equal(2, dict.Count);
+        }
+
+        [Fact]
+        public void GetValueRefLinkBreaksOnResize()
+        {
+            var dict = new Dictionary<int, Struct>
+            {
+                {  1, new() }
+            };
+
+            Assert.Equal(1, dict.Count);
+
+            ref var itemRef = ref CollectionsMarshal.GetValueRef(dict, 1);
+
+            Assert.Equal(0, itemRef.Value);
+            Assert.Equal(0, itemRef.Property);
+
+            itemRef.Value = 1;
+            itemRef.Property = 2;
+
+            Assert.Equal(1, itemRef.Value);
+            Assert.Equal(2, itemRef.Property);
+            Assert.Equal(dict[1].Value, itemRef.Value);
+            Assert.Equal(dict[1].Property, itemRef.Property);
+
+            // Resize
+            dict.EnsureCapacity(100);
+            for (int i = 2; i <= 50; i++)
+            {
+                dict.Add(i, new());
+            }
+
+            itemRef.Value = 3;
+            itemRef.Property = 4;
+
+            Assert.Equal(3, itemRef.Value);
+            Assert.Equal(4, itemRef.Property);
+
+            // Check connection broken
+            Assert.NotEqual(dict[1].Value, itemRef.Value);
+            Assert.NotEqual(dict[1].Property, itemRef.Property);
+
+            Assert.Equal(50, dict.Count);
+        }
+
         [Fact]
         public void GetValueRefOrNullRefValueType()
         {