KeyedCollection debug view doesn't work

src/libraries/Common/tests/System/Collections/DebugView.Tests.cs

@@ -47,6 +47,17 @@ private static IEnumerable<object[]> TestDebuggerAttributes_GenericDictionaries(
                     new ("[\"Two\"]", "2"),
                 }
             };
+            CustomKeyedCollection<string, int> collection = new ();
+            collection.GetKeyForItemHandler = value =>  (2 * value).ToString();
+            collection.InsertItem(0, 1);
+            collection.InsertItem(1, 3);
+            yield return new object[] { collection,
+                new KeyValuePair<string, string>[]
+                {
+                    new ("[\"2\"]", "1"),
+                    new ("[\"6\"]", "3"),
+                }
+            };
         }
 
         private static IEnumerable<object[]> TestDebuggerAttributes_NonGenericDictionaries()
@@ -173,7 +184,7 @@ public static IEnumerable<object[]> TestDebuggerAttributes_Inputs()
 
         [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.IsDebuggerTypeProxyAttributeSupported))]
         [MemberData(nameof(TestDebuggerAttributes_InputsPresentedAsDictionary))]
-        public static void TestDebuggerAttributes_Dictionary(IDictionary obj, KeyValuePair<string, string>[] expected)
+        public static void TestDebuggerAttributes_Dictionary(object obj, KeyValuePair<string, string>[] expected)
         {
             DebuggerAttributes.ValidateDebuggerDisplayReferences(obj);
             DebuggerAttributeInfo info = DebuggerAttributes.ValidateDebuggerTypeProxyProperties(obj);
@@ -206,5 +217,32 @@ public static void TestDebuggerAttributes_Null(object obj)
             TargetInvocationException tie = Assert.Throws<TargetInvocationException>(() => Activator.CreateInstance(proxyType, (object)null));
             Assert.IsType<ArgumentNullException>(tie.InnerException);
         }
+
+        private class CustomKeyedCollection<TKey, TValue> : KeyedCollection<TKey, TValue> where TKey : notnull
+        {
+            public CustomKeyedCollection() : base()
+            {
+            }
+
+            public CustomKeyedCollection(IEqualityComparer<TKey> comparer) : base(comparer)
+            {
+            }
+
+            public CustomKeyedCollection(IEqualityComparer<TKey> comparer, int dictionaryCreationThreshold) : base(comparer, dictionaryCreationThreshold)
+            {
+            }
+
+            public Func<TValue, TKey> GetKeyForItemHandler { get; set; }
+
+            protected override TKey GetKeyForItem(TValue item)
+            {
+                return GetKeyForItemHandler(item);
+            }
+
+            public new void InsertItem(int index, TValue item)
+            {
+                base.InsertItem(index, item);
+            }
+        }
     }
 }

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

@@ -8,7 +8,7 @@
 namespace System.Collections.ObjectModel
 {
     [Serializable]
-    [DebuggerTypeProxy(typeof(CollectionDebugView<>))]
+    [DebuggerTypeProxy(typeof(KeyedCollection<,>.KeyedCollectionDebugView))]
     [DebuggerDisplay("Count = {Count}")]
     [System.Runtime.CompilerServices.TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
     public abstract class KeyedCollection<TKey, TItem> : Collection<TItem> where TKey : notnull
@@ -280,5 +280,47 @@ private void RemoveKey(TKey key)
                 keyCount--;
             }
         }
+
+        private sealed class KeyedCollectionDebugView
+        {
+            private readonly KeyedCollection<TKey, TItem> _collection;
+
+            public KeyedCollectionDebugView(KeyedCollection<TKey, TItem> collection)
+            {
+                ArgumentNullException.ThrowIfNull(collection);
+                _collection = collection;
+            }
+
+            [DebuggerBrowsable(DebuggerBrowsableState.RootHidden)]
+            public KeyedCollectionDebugViewItem[] Items
+            {
+                get
+                {
+                    var items = new KeyedCollectionDebugViewItem[_collection.Count];
+                    for (int i = 0; i < items.Length; i++)
+                    {
+                        TItem item = _collection[i];
+                        items[i] = new KeyedCollectionDebugViewItem(_collection.GetKeyForItem(item), item);
+                    }
+                    return items;
+                }
+            }
+
+            [DebuggerDisplay("{Value}", Name = "[{Key}]")]
+            internal readonly struct KeyedCollectionDebugViewItem
+            {
+                public KeyedCollectionDebugViewItem(TKey key, TItem value)
+                {
+                    Key = key;
+                    Value = value;
+                }
+
+                [DebuggerBrowsable(DebuggerBrowsableState.Collapsed)]
+                public TKey Key { get; }
+
+                [DebuggerBrowsable(DebuggerBrowsableState.Collapsed)]
+                public TItem Value { get; }
+            }
+        }
     }
 }