Shrink Dictionary asm by moving non-generic enumerator out

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

@@ -129,6 +129,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Collections\Generic\ArraySortHelper.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Collections\Generic\Comparer.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Collections\Generic\Dictionary.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Collections\Generic\DictionaryEnumerator.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Collections\Generic\EqualityComparer.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Collections\Generic\IAsyncEnumerable.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Collections\Generic\IAsyncEnumerator.cs" />

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

@@ -302,10 +302,10 @@ private void CopyTo(KeyValuePair<TKey, TValue>[] array, int index)
         }
 
         public Enumerator GetEnumerator()
-            => new Enumerator(this, Enumerator.KeyValuePair);
+            => new Enumerator(this);
 
         IEnumerator<KeyValuePair<TKey, TValue>> IEnumerable<KeyValuePair<TKey, TValue>>.GetEnumerator()
-            => new Enumerator(this, Enumerator.KeyValuePair);
+            => new Enumerator(this);
 
         public virtual void GetObjectData(SerializationInfo info, StreamingContext context)
         {
@@ -976,7 +976,7 @@ void ICollection.CopyTo(Array array, int index)
         }
 
         IEnumerator IEnumerable.GetEnumerator()
-            => new Enumerator(this, Enumerator.KeyValuePair);
+            => new Enumerator(this);
 
         /// <summary>
         /// Ensures that the dictionary can hold up to 'capacity' entries without any further expansion of its backing storage
@@ -1152,7 +1152,7 @@ bool IDictionary.Contains(object key)
         }
 
         IDictionaryEnumerator IDictionary.GetEnumerator()
-            => new Enumerator(this, Enumerator.DictEntry);
+            => new DictionaryEnumerator(this);
 
         void IDictionary.Remove(object key)
         {
@@ -1173,24 +1173,18 @@ private ref int GetBucket(uint hashCode)
 #endif
         }
 
-        public struct Enumerator : IEnumerator<KeyValuePair<TKey, TValue>>,
-            IDictionaryEnumerator
+        public struct Enumerator : IEnumerator<KeyValuePair<TKey, TValue>>
         {
             private readonly Dictionary<TKey, TValue> _dictionary;
             private readonly int _version;
             private int _index;
             private KeyValuePair<TKey, TValue> _current;
-            private readonly int _getEnumeratorRetType;  // What should Enumerator.Current return?
 
-            internal const int DictEntry = 1;
-            internal const int KeyValuePair = 2;
-
-            internal Enumerator(Dictionary<TKey, TValue> dictionary, int getEnumeratorRetType)
+            internal Enumerator(Dictionary<TKey, TValue> dictionary)
             {
                 _dictionary = dictionary;
                 _version = dictionary._version;
                 _index = 0;
-                _getEnumeratorRetType = getEnumeratorRetType;
                 _current = default;
             }
 
@@ -1234,14 +1228,7 @@ public void Dispose()
                         ThrowHelper.ThrowInvalidOperationException_InvalidOperation_EnumOpCantHappen();
                     }
 
-                    if (_getEnumeratorRetType == DictEntry)
-                    {
-                        return new DictionaryEntry(_current.Key, _current.Value);
-                    }
-                    else
-                    {
-                        return new KeyValuePair<TKey, TValue>(_current.Key, _current.Value);
-                    }
+                    return _current;
                 }
             }
 
@@ -1255,45 +1242,6 @@ void IEnumerator.Reset()
                 _index = 0;
                 _current = default;
             }
-
-            DictionaryEntry IDictionaryEnumerator.Entry
-            {
-                get
-                {
-                    if (_index == 0 || (_index == _dictionary._count + 1))
-                    {
-                        ThrowHelper.ThrowInvalidOperationException_InvalidOperation_EnumOpCantHappen();
-                    }
-
-                    return new DictionaryEntry(_current.Key, _current.Value);
-                }
-            }
-
-            object IDictionaryEnumerator.Key
-            {
-                get
-                {
-                    if (_index == 0 || (_index == _dictionary._count + 1))
-                    {
-                        ThrowHelper.ThrowInvalidOperationException_InvalidOperation_EnumOpCantHappen();
-                    }
-
-                    return _current.Key;
-                }
-            }
-
-            object? IDictionaryEnumerator.Value
-            {
-                get
-                {
-                    if (_index == 0 || (_index == _dictionary._count + 1))
-                    {
-                        ThrowHelper.ThrowInvalidOperationException_InvalidOperation_EnumOpCantHappen();
-                    }
-
-                    return _current.Value;
-                }
-            }
         }
 
         [DebuggerTypeProxy(typeof(DictionaryKeyCollectionDebugView<,>))]

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

@@ -0,0 +1,41 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace System.Collections.Generic
+{
+    internal interface IKeyValue
+    {
+        public object? Key { get; }
+        public object? Value { get; }
+    }
+
+    internal class DictionaryEnumerator : IDictionaryEnumerator
+    {
+        private readonly IEnumerator _enumerator;
+        private IKeyValue? _current;
+
+        public DictionaryEnumerator(IEnumerable enumerable)
+        {
+            _enumerator = enumerable.GetEnumerator();
+            _current = null;
+        }
+
+        public object Key => _current?.Key!;
+
+        public object? Value => _current?.Value;
+
+        public DictionaryEntry Entry => new DictionaryEntry(_current?.Key!, _current?.Value);
+
+        public object? Current => _current;
+
+        public bool MoveNext()
+        {
+            bool result = _enumerator.MoveNext();
+            _current = (IKeyValue)_enumerator.Current!;
+            return result;
+        }
+
+        public void Reset() => _enumerator.Reset();
+    }
+}

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

@@ -49,7 +49,7 @@ internal static string PairToString(object? key, object? value)
     // and IReadOnlyDictionary<TKey, TValue>.
     [Serializable]
     [System.Runtime.CompilerServices.TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
-    public readonly struct KeyValuePair<TKey, TValue>
+    public readonly struct KeyValuePair<TKey, TValue> : IKeyValue
     {
         [DebuggerBrowsable(DebuggerBrowsableState.Never)]
         private readonly TKey key; // Do not rename (binary serialization)
@@ -66,6 +66,10 @@ public KeyValuePair(TKey key, TValue value)
 
         public TValue Value => value;
 
+        object? IKeyValue.Key => key;
+
+        object? IKeyValue.Value => value;
+
         public override string ToString()
         {
             return KeyValuePair.PairToString(Key, Value);