Developers using JsonSerializer have support for polymorphic types

[terrajobst]

Work

• Add AttributeTargets.Interface to JsonConverterAttribute #33112
• Allow custom JsonConverters for base-classes #30427
• Support polymorphic serialization through new option #29937

[StingyJack]

System.Text.Json is not going to be usable until it can handle serialization of objects. Even then, if including all of the properties is not made the default behavior I'm still going to have to avoid using it.

Why would an entire product team think that any data loss would be acceptable? Especially by default.

[layomia]

@StingyJack

Extending polymorphic support has always been on the roadmap for System.Text.Json. We do not consider this feature an edge case. We just couldn't get to it in previous releases due to product scoping and time constraints. As this issue indicates, it is being planned for .NET 6.0.

Polymorphic (de)serialization cannot be made the default behavior because that would be a breaking change from previous versions of System.Text.Json. Polymorphic serialization can also lead to accidental information disclosure when a object is treated as a different type than declared to the serializer. Such behavior needs to be opt-in only.

Not having built-in support for polymorphic is not a ship-blocker as many applications do not need this feature. Following up on #45764: System.Text.Json was not designed to be a drop-in replacement for Newtonsoft.Json. You are free to continue using it if it satisfies your use cases. Please see our migration guide for some clarity about the relationship between the two libraries:

System.Text.Json focuses primarily on performance, security, and standards compliance. It has some key differences in default behavior and doesn't aim to have feature parity with Newtonsoft.Json. For some scenarios, System.Text.Json has no built-in functionality, but there are recommended workarounds. For other scenarios, workarounds are impractical. If your application depends on a missing feature, consider filing an issue to find out if support for your scenario can be added.

[StingyJack]

@layomia - A Breaking Change means just incrementing the Major Version number I thought. You can say that this is not intended to be a drop in replacement for newtonsoft's package, but being able to serialize an object completely is one of those things that is expected to work in a package like this. What good is the performance going to be to me if its not able to do the primary job? Is getting it to do the complete job going to net worse performance than the existing package?

If you asked 100 random .net developers (that are not already familiar with this shortfall of S.T.J) what Polymorphic Serialization is, you would get maybe one that could correctly answer what it meant, but I would bet $1 that this affects more than 75% of them. The doc page should say "Can serialize base class members? Not supported"