System.Text.Json source generator does not handle reference types with design time custom converter properly

[DRKV333]

Description

The problem is caused by this part:

runtime/src/libraries/System.Text.Json/gen/JsonSourceGenerator.Emitter.cs

Lines 284 to 285 in 571b4b4

	// Allow nullable handling to forward to the underlying type's converter.
	converter = {JsonMetadataServicesTypeRef}.GetNullableConverter<{typeCompilableName}>(this.{typeFriendlyName});

The source generator always emits this when a JsonConverter attribute is present on the type to serialize, however, the code is only valid for value types.

Here is a minimal setup to reproduce:

public class ReproConverter : JsonConverter<Repro>
{
    public override Repro Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        throw new NotImplementedException();
    }

    public override void Write(Utf8JsonWriter writer, Repro value, JsonSerializerOptions options)
    {
        throw new NotImplementedException();
    }
}

[JsonConverter(typeof(ReproConverter))]
public class Repro
{
}

[JsonSerializable(typeof(Repro))]
public partial class ReproContext : JsonSerializerContext
{
}

The type 'Repro' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method 'JsonMetadataServices.GetNullableConverter<T>(JsonTypeInfo<T>)'

Configuration

The issue is present on the latest build. (System.Text.Json 6.0.0-rc.1.21404.6)

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Issue Details

---

Description

The problem is caused by this part:

runtime/src/libraries/System.Text.Json/gen/JsonSourceGenerator.Emitter.cs

Lines 284 to 285 in 571b4b4

	// Allow nullable handling to forward to the underlying type's converter.
	converter = {JsonMetadataServicesTypeRef}.GetNullableConverter<{typeCompilableName}>(this.{typeFriendlyName});

The source generator always emits this when a JsonConverter attribute is present on the type to serialize, however, the code is only valid for value types.

Here is a minimal setup to reproduce:

    public class ReproConverter : JsonConverter<Repro>
    {
        public override Repro Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
        {
            throw new NotImplementedException();
        }

        public override void Write(Utf8JsonWriter writer, Repro value, JsonSerializerOptions options)
        {
            throw new NotImplementedException();
        }
    }

    [JsonConverter(typeof(ReproConverter))]
    public class Repro
    {

    }

    [JsonSerializable(typeof(Repro))]
    public partial class ReproContext : JsonSerializerContext
    {
    }

The type 'Repro' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method 'JsonMetadataServices.GetNullableConverter<T>(JsonTypeInfo<T>)'

Configuration

The issue is present on the latest build. (System.Text.Json 6.0.0-rc.1.21404.6)

Author:	DRKV333
Assignees:	-
Labels:	area-System.Text.Json, untriaged
Milestone:	-

[steveharter]

Reopened for likely port to https://github.com/dotnet/runtime/tree/release/6.0