CollectionsMarshal.GetValueRef(Dictionary)

src/libraries/System.Private.CoreLib/src/System/Collections/Generic/Dictionary.cs

@@ -352,7 +352,7 @@ public virtual void GetObjectData(SerializationInfo info, StreamingContext conte
             }
         }
 
-        private ref TValue FindValue(TKey key)
+        internal ref TValue FindValue(TKey key)
         {
             if (key == null)
             {

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

@@ -14,7 +14,20 @@ public static class CollectionsMarshal
         /// Get a <see cref="Span{T}"/> view over a <see cref="List{T}"/>'s data.
         /// Items should not be added or removed from the <see cref="List{T}"/> while the <see cref="Span{T}"/> is in use.
         /// </summary>
+        /// <param name="list">The list to get the data view over.</param>
         public static Span<T> AsSpan<T>(List<T>? list)
             => list is null ? default : new Span<T>(list._items, 0, list._size);
+
+        /// <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>
+        /// <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.
+        /// The ref null can be detected using System.Runtime.CompilerServices.Unsafe.IsNullRef
+        /// </remarks>
+        public static ref TValue GetValueRefOrNullRef<TKey, TValue>(Dictionary<TKey, TValue> dictionary, TKey key) where TKey : notnull
+            => ref dictionary.FindValue(key);
     }
 }

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 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)]
     public sealed partial class ComAliasNameAttribute : System.Attribute

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

@@ -4,6 +4,8 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Runtime.CompilerServices;
+
 using Xunit;
 
 namespace System.Runtime.InteropServices.Tests
@@ -142,9 +144,171 @@ public void ListAsSpanLinkBreaksOnResize()
             }
         }
 
+        [Fact]
+        public void GetValueRefOrNullRefValueType()
+        {
+            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.GetValueRefOrNullRef(dict, 1).Value = 3;
+            CollectionsMarshal.GetValueRefOrNullRef(dict, 1).Property = 4;
+
+            Assert.Equal(3, dict[1].Value);
+            Assert.Equal(4, dict[1].Property);
+
+            ref var itemRef = ref CollectionsMarshal.GetValueRefOrNullRef(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 null refs
+
+            Assert.True(Unsafe.IsNullRef(ref CollectionsMarshal.GetValueRefOrNullRef(dict, 3)));
+            Assert.Throws<NullReferenceException>(() => CollectionsMarshal.GetValueRefOrNullRef(dict, 3).Value = 9);
+
+            Assert.Equal(2, dict.Count);
+        }
+
+        [Fact]
+        public void GetValueRefOrNullRefClass()
+        {
+            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.GetValueRefOrNullRef(dict, 1).Value = 3;
+            CollectionsMarshal.GetValueRefOrNullRef(dict, 1).Property = 4;
+
+            Assert.Equal(3, dict[1].Value);
+            Assert.Equal(4, dict[1].Property);
+
+            ref var itemRef = ref CollectionsMarshal.GetValueRefOrNullRef(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 null refs
+
+            Assert.True(Unsafe.IsNullRef(ref CollectionsMarshal.GetValueRefOrNullRef(dict, 3)));
+            Assert.Throws<NullReferenceException>(() => CollectionsMarshal.GetValueRefOrNullRef(dict, 3).Value = 9);
+
+            Assert.Equal(2, dict.Count);
+        }
+
+        [Fact]
+        public void GetValueRefOrNullRefLinkBreaksOnResize()
+        {
+            var dict = new Dictionary<int, Struct>
+            {
+                {  1, new() }
+            };
+
+            Assert.Equal(1, dict.Count);
+
+            ref var itemRef = ref CollectionsMarshal.GetValueRefOrNullRef(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);
+        }
+
+        private struct Struct
+        {
+            public int Value;
+            public int Property { get; set; }
+        }
+
         private class IntAsObject
         {
             public int Value;
+            public int Property { get; set; }
         }
     }
 }