Description

Fixes dotnet/runtime#670, where there was a regression between 3.0 and 3.1 where serializing nested concurrent dictionaries went from supported to unsupported. The issue also exists in master/5.0. dotnet/runtime#784 fixes it in master, and this PR ports the fix to 3.0.

In addition to the fix described above, this PR also also has a deserialization fix for an InvalidCastException thrown when a dictionary element type has a converter that returns a type different from the declared type, e.g. when you're deserializing Dictionary<string, MyClass>, where

private interface IClass { }

private class MyClass : IClass { }

private class MyFactory : JsonConverterFactory
{
    public override bool CanConvert(Type typeToConvert)
    {
        return typeToConvert == typeof(IClass) || typeToConvert == typeof(MyClass);
    }

    public override JsonConverter CreateConverter(Type typeToConvert, JsonSerializerOptions options)
    {
        return new MyStuffConverter();
    }
}

private class MyStuffConverter : JsonConverter<IClass>
{
    public override IClass Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        return new MyClass();
    }

    public override void Write(Utf8JsonWriter writer, IClass value, JsonSerializerOptions options)
    {
        writer.WriteNumberValue(1);
    }
}

In this case, the read method of the converter for typeof(MyClass) returns a type IClass (not the declared type, MyClass) which causes an InvalidCastException later in the deserialization flow.

Customer Impact

The ability to serialize nested concurrent dictionaries is restored: dotnet/runtime#670.

The deserialization scenario is an edge case, but the fix defends against leaking exceptions were it to occur.

Regression

No. The serialization change's tests are wide-ranging and covers various permutations of nested dictionaries, so a regression is unlikely.

The deserialization change adds more support without rescinding support. A regression is unlikely.

Risk

Low, per the regression section above.