Add naive implementation for Enum. Add test file.

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Collection/KeyConverter.cs

@@ -82,13 +82,16 @@ internal sealed class EnumKeyConverter<TEnum>: KeyConverter<TEnum> where TEnum :
     {
         public override bool ReadKey(ref Utf8JsonReader reader, out TEnum value)
         {
-            throw new NotImplementedException();
+            string enumValue = reader.GetString()!;
+            value = Enum.Parse<TEnum>(enumValue);
+
+            return true;//?
         }
 
         protected override void WriteKeyAsT(Utf8JsonWriter writer, TEnum value, JsonSerializerOptions options)
         {
-            //string keyName = value.ToString();
-            //writer.WritePropertyName(keyName);
+            string keyName = value.ToString();
+            writer.WritePropertyName(keyName);
         }
     }
 

src/libraries/System.Text.Json/tests/Serialization/DictionaryTests.KeyConverter.cs

@@ -0,0 +1,15 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.Collections.Generic;
+using Xunit;
+
+namespace System.Text.Json.Serialization.Tests
+{
+    public static partial class DictionaryKeyConverterTests
+    {
+        [Fact]
+        public static void TestDictionaryTKeyTValue() { }
+    }
+}

src/libraries/System.Text.Json/tests/System.Text.Json.Tests.csproj

@@ -58,6 +58,7 @@
     <Compile Include="Serialization\CustomConverterTests.Callback.cs" />
     <Compile Include="Serialization\CyclicTests.cs" />
     <Compile Include="Serialization\DictionaryTests.cs" />
+    <Compile Include="Serialization\DictionaryTests.KeyConverter.cs" />
     <Compile Include="Serialization\DictionaryTests.KeyPolicy.cs" />
     <Compile Include="Serialization\EnumConverterTests.cs" />
     <Compile Include="Serialization\EnumTests.cs" />