Breaking change: Untyped Deserialize no longer forwards the type to JsonConverter<T>

[bjuris]

Description

When calling JsonSerializer.Deserialize(json, type, option) with a converter the type sent to the converter is hardcoded to the type defined in the converter, and not what was sent to the method.

In this method typeToConvert will always be T in JsonConverter<T>.

public override BlockData Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)

System.Text.Json .NET 6 implementation:
bool success = TryRead(ref reader, jsonPropertyInfo.RuntimePropertyType!, options, ref state, out T? value);
https://github.com/dotnet/runtime/blob/v6.0.10/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonConverterOfT.ReadCore.cs#L62

System.Text.Json .NET 7 implementation:
bool success = TryRead(ref reader, TypeToConvert, options, ref state, out T? value);
https://github.com/dotnet/runtime/blob/main/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonConverterOfT.ReadCore.cs#L61

Reproduction Steps

using System.Text.Json;
using System.Text.Json.Serialization;

var o = new JsonSerializerOptions();
o.Converters.Add(new C());
var x = JsonSerializer.Deserialize("{}", typeof(B), o);
Console.WriteLine("Output was: " + x.GetType());

public class A
{ }
public class B : A
{ }

public class C : JsonConverter<A>
{
    public override bool CanConvert(Type typeToConvert)
    {
        return typeof(A).IsAssignableFrom(typeToConvert);
    }
    public override A? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        reader.Read();
        Console.WriteLine("Converter sees: " + typeToConvert);
        return (A)Activator.CreateInstance(typeToConvert);
    }

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

Output on .NET 6:

Converter sees: B
Output was: B

Output on .NET 7:

Converter sees: A
Output was: A

Expected behavior

Type sent to Deserialized should be sent to converter.

Actual behavior

Type sent to Deserialized is not sent to converter.

Regression?

This works in .NET 6, but not .NET 7.

Known Workarounds

No response

Configuration

Windows x64, 6.0.400 vs 7.0.100-rc.2.22477.23

Other information

No response

Tagging subscribers to this area: @dotnet/area-system-text-json, @gregsdennis
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Issue Details

---

Description

When calling JsonSerializer.Deserialize(json, type, option) with a converter the type sent to the converter is hardcoded to the type defined in the converter, and not what was sent to the method.

In this method typeToConvert will always be T in JsonConverter<T>.

public override BlockData Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)

System.Text.Json .NET 6 implementation:
bool success = TryRead(ref reader, jsonPropertyInfo.RuntimePropertyType!, options, ref state, out T? value);
https://github.com/dotnet/runtime/blob/v6.0.10/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonConverterOfT.ReadCore.cs#L62

System.Text.Json .NET 7 implementation:
bool success = TryRead(ref reader, TypeToConvert, options, ref state, out T? value);
https://github.com/dotnet/runtime/blob/main/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonConverterOfT.ReadCore.cs#L61

Reproduction Steps

using System.Text.Json;
using System.Text.Json.Serialization;

var o = new JsonSerializerOptions();
o.Converters.Add(new C());
var x = JsonSerializer.Deserialize("{}", typeof(B), o);
Console.WriteLine("Output was: " + x.GetType());

public class A
{ }
public class B : A
{ }

public class C : JsonConverter<A>
{
    public override bool CanConvert(Type typeToConvert)
    {
        return typeof(A).IsAssignableFrom(typeToConvert);
    }
    public override A? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        reader.Read();
        Console.WriteLine("Converter sees: " + typeToConvert);
        return (A)Activator.CreateInstance(typeToConvert);
    }

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

Output on .NET 6:

Converter sees: B
Output was: B

Output on .NET 7:

Converter sees: A
Output was: A

Expected behavior

Type sent to Deserialized should be sent to converter.

Actual behavior

Type sent to Deserialized is not sent to converter.

Regression?

This works in .NET 6, but not .NET 7.

Known Workarounds

No response

Configuration

Windows x64, 6.0.400 vs 7.0.100-rc.2.22477.23

Other information

No response

Author:	bjuris
Assignees:	-
Labels:	area-System.Text.Json
Milestone:	-

[eiriktsarpalis]

Thanks for reporting. This is a clear regression from .NET 6 that we should try to service in .NET 7.

[eiriktsarpalis]

In the meantime, it should be possible to work around the issue using a converter factory:

public class C : JsonConverterFactory
{
    public override bool CanConvert(Type typeToConvert)
    {
        return typeof(A).IsAssignableFrom(typeToConvert);
    }

    public override JsonConverter? CreateConverter(Type typeToConvert, JsonSerializerOptions options)
    {
        Type converterType = typeof(C<>).MakeGenericType(typeToConvert);
        return (JsonConverter)Activator.CreateInstance(converterType)!;
    }
}

public class C<T> : JsonConverter<T> where T : A
{
    public override T? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        reader.Read();
        Console.WriteLine("Converter sees: " + typeToConvert);
        return (T)Activator.CreateInstance(typeToConvert)!;
    }

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