System.Text.Json of T Buggy JsonConverterOfT

[vintzl]

If I try to register many custom converters of T, in ConfigureServices or elsewhere, and I try to use the custom converter with a CanConvert overridden method, I get:

An unhandled exception of type 'System.InvalidOperationException' occurred in System.Text.Json.dll: 'The converter 'Buggy_JsonConverterOfT.CustomJsonConverter`1[System.Collections.Generic.List`1[Buggy_JsonConverterOfT.Customer]]' is not compatible with the type 'Buggy_JsonConverterOfT.Customer'.'

Indeed, even if the CanConvert method return true, the JsonConverter yield false…

Here a simple (drafty) repro:

• https://github.com/vintzl/Buggy_JsonConverterOfT

[ahsonkhan]

What are you trying to do here with the "buggy" converter and what do you expect the behavior to be? It is hard to parse what you are trying to do here. Does JsonConverterFactory work for your use case?

Did you try different registration order of the converters?

https://docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-converters-how-to#sample-factory-pattern-converter
https://docs.microsoft.com/en-us/dotnet/api/system.text.json.serialization.jsonconverterfactory?view=netcore-3.1

[vintzl]

in CanConvert I want to put some logic, also to avoid such issue.

I did try different registration order but I got the same result.

[layomia]

The CanConvert method of your generic custom converter always returns true which means that you're indicating that the first concrete converter you added to JsonSerializerOptions.Converters should be used for all types in the input object graph. This is what causes the exception you are seeing. Specifically, in your repro project, a CustomJsonConverter<List<Customer>> converter which derives from JsonConverter<List<Customer>> cannot be used to convert a Customer type.

Please see the docs on how to follow the factory converter pattern which is ideal for writing generic converters.