Unseal StringEnumConverter. Fix #30486 (#54917)

Author: eiriktsarpalis

src/libraries/System.Text.Json/ref/System.Text.Json.cs

@@ -847,12 +847,12 @@ public enum JsonSourceGenerationMode
         Metadata = 1,
         Serialization = 2,
     }
-    public sealed partial class JsonStringEnumConverter : System.Text.Json.Serialization.JsonConverterFactory
+    public partial class JsonStringEnumConverter : System.Text.Json.Serialization.JsonConverterFactory
     {
         public JsonStringEnumConverter() { }
         public JsonStringEnumConverter(System.Text.Json.JsonNamingPolicy? namingPolicy = null, bool allowIntegerValues = true) { }
-        public override bool CanConvert(System.Type typeToConvert) { throw null; }
-        public override System.Text.Json.Serialization.JsonConverter CreateConverter(System.Type typeToConvert, System.Text.Json.JsonSerializerOptions options) { throw null; }
+        public sealed override bool CanConvert(System.Type typeToConvert) { throw null; }
+        public sealed override System.Text.Json.Serialization.JsonConverter CreateConverter(System.Type typeToConvert, System.Text.Json.JsonSerializerOptions options) { throw null; }
     }
     public enum JsonUnknownTypeHandling
     {

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonStringEnumConverter.cs

@@ -11,7 +11,7 @@ namespace System.Text.Json.Serialization
     /// <remarks>
     /// Reading is case insensitive, writing can be customized via a <see cref="JsonNamingPolicy" />.
     /// </remarks>
-    public sealed class JsonStringEnumConverter : JsonConverterFactory
+    public class JsonStringEnumConverter : JsonConverterFactory
     {
         private readonly JsonNamingPolicy? _namingPolicy;
         private readonly EnumConverterOptions _converterOptions;
@@ -45,13 +45,13 @@ public JsonStringEnumConverter(JsonNamingPolicy? namingPolicy = null, bool allow
         }
 
         /// <inheritdoc />
-        public override bool CanConvert(Type typeToConvert)
+        public sealed override bool CanConvert(Type typeToConvert)
         {
             return typeToConvert.IsEnum;
         }
 
         /// <inheritdoc />
-        public override JsonConverter CreateConverter(Type typeToConvert, JsonSerializerOptions options) =>
+        public sealed override JsonConverter CreateConverter(Type typeToConvert, JsonSerializerOptions options) =>
             EnumConverterFactory.Create(typeToConvert, _converterOptions, _namingPolicy, options);
     }
 }

src/libraries/System.Text.Json/tests/System.Text.Json.Tests/Serialization/EnumConverterTests.cs

@@ -135,17 +135,15 @@ public class Week
             [JsonConverter(typeof(JsonStringEnumConverter))]
             public DayOfWeek WorkStart { get; set; }
             public DayOfWeek WorkEnd { get; set; }
-            [LowerCaseEnum]
+            [JsonConverter(typeof(LowerCaseEnumConverter))]
             public DayOfWeek WeekEnd { get; set; }
         }
 
-        [AttributeUsage(AttributeTargets.Property | AttributeTargets.Class, AllowMultiple = false)]
-        private class LowerCaseEnumAttribute : JsonConverterAttribute
+        private class LowerCaseEnumConverter : JsonStringEnumConverter
         {
-            public LowerCaseEnumAttribute() { }
-
-            public override JsonConverter CreateConverter(Type typeToConvert)
-                => new JsonStringEnumConverter(new ToLowerNamingPolicy());
+            public LowerCaseEnumConverter() : base(new ToLowerNamingPolicy())
+            {
+            }
         }
 
         [Fact]