Allow JsonConverterAttribute usage on interfaces.

Fix #33112

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

@@ -763,7 +763,7 @@ public abstract partial class JsonConverter
         internal JsonConverter() { }
         public abstract bool CanConvert(System.Type typeToConvert);
     }
-    [System.AttributeUsageAttribute(System.AttributeTargets.Class | System.AttributeTargets.Enum | System.AttributeTargets.Field | System.AttributeTargets.Property | System.AttributeTargets.Struct, AllowMultiple=false)]
+    [System.AttributeUsageAttribute(System.AttributeTargets.Class | System.AttributeTargets.Interface | System.AttributeTargets.Enum | System.AttributeTargets.Field | System.AttributeTargets.Property | System.AttributeTargets.Struct, AllowMultiple=false)]
     public partial class JsonConverterAttribute : System.Text.Json.Serialization.JsonAttribute
     {
         protected JsonConverterAttribute() { }

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Attributes/JsonConverterAttribute.cs

@@ -15,7 +15,7 @@ namespace System.Text.Json.Serialization
     /// <see cref="JsonSerializerOptions.Converters"/> or there is another <see cref="JsonConverterAttribute"/> on a property or field
     /// of the same type.
     /// </remarks>
-    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Enum | AttributeTargets.Property | AttributeTargets.Field, AllowMultiple = false)]
+    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Interface | AttributeTargets.Struct | AttributeTargets.Enum | AttributeTargets.Property | AttributeTargets.Field, AllowMultiple = false)]
     public class JsonConverterAttribute : JsonAttribute
     {
         /// <summary>

src/libraries/System.Text.Json/tests/System.Text.Json.Tests/Serialization/CustomConverterTests/CustomConverterTests.Interface.cs

@@ -0,0 +1,60 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Linq;
+using System.Collections.Generic;
+using Xunit;
+
+namespace System.Text.Json.Serialization.Tests
+{
+    public static partial class CustomConverterTests
+    {
+        [JsonConverter(typeof(MyInterfaceConverter))]
+        private interface IMyInterface
+        {
+            int IntValue { get; set; }
+            string StringValue { get; set; }
+        }
+
+        // A custom converter that writes and reads the string property as a top-level value
+        private class MyInterfaceConverter : JsonConverter<IMyInterface>
+        {
+            public override IMyInterface Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+                => new MyClass
+                {
+                    IntValue = 42,
+                    StringValue = reader.GetString()
+                };
+
+            public override void Write(Utf8JsonWriter writer, IMyInterface value, JsonSerializerOptions options) => writer.WriteStringValue(value.StringValue);
+        }
+
+        private class MyClass : IMyInterface
+        {
+            public int IntValue { get; set; }
+            public string StringValue { get; set; }
+        }
+
+        [Fact]
+        public static void CustomInterfaceConverter_Serialization()
+        {
+            IMyInterface value = new MyClass { IntValue = 11, StringValue = "myString" };
+
+            string expectedJson = "\"myString\"";
+            string actualJson = JsonSerializer.Serialize(value);
+            Assert.Equal(expectedJson, actualJson);
+        }
+
+        [Fact]
+        public static void CustomInterfaceConverter_Deserialization()
+        {
+            string json = "\"myString\"";
+
+            IMyInterface result = JsonSerializer.Deserialize<IMyInterface>(json);
+
+            Assert.IsType<MyClass>(result);
+            Assert.Equal("myString", result.StringValue);
+            Assert.Equal(42, result.IntValue);
+        }
+    }
+}

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

@@ -98,6 +98,7 @@
     <Compile Include="Serialization\CustomConverterTests\CustomConverterTests.Dynamic.Sample.cs" />
     <Compile Include="Serialization\CustomConverterTests\CustomConverterTests.HandleNull.cs" />
     <Compile Include="Serialization\CustomConverterTests\CustomConverterTests.Int32.cs" />
+    <Compile Include="Serialization\CustomConverterTests\CustomConverterTests.Interface.cs" />
     <Compile Include="Serialization\CustomConverterTests\CustomConverterTests.InvalidCast.cs" />
     <Compile Include="Serialization\CustomConverterTests\CustomConverterTests.List.cs" />
     <Compile Include="Serialization\CustomConverterTests\CustomConverterTests.NullValueType.cs" />