I am at a loss trying to configure serialization of generics. I have a pretty simple setup:

import kotlinx.serialization.*
import kotlinx.serialization.json.Json

@Serializable
sealed class ApiResponse<T> {
    @Serializable
    @SerialName("result")
    data class Ok<T>(val result: T) : ApiResponse<T>()

    @Serializable
    @SerialName("error")
    data class Error<T>(val error: ErrorResponse) : ApiResponse<T>()
}

@Serializable
data class VersionResponse(val version: String)

@Serializable
data class ErrorResponse(val description: String)

fun main() {
    val data: ApiResponse<VersionResponse> = ApiResponse.Ok(VersionResponse("v42"))
    println(Json.encodeToString(data)) // prints {"type":"result","result":{"version":"v42"}}
}

The last line mentions the result I want to achieve:

ApiResponse

is discriminated using

type

, and the

result

field is just serialized as it is. However, running this with Kotlin 1.7.10 and kotlinx.serialization 1.3.3 (json jvm) produces this error:

Exception in thread "main" kotlinx.serialization.SerializationException: Class 'VersionResponse' is not registered for polymorphic serialization in the scope of 'Any'.
Mark the base class as 'sealed' or register the serializer explicitly.
	at kotlinx.serialization.internal.AbstractPolymorphicSerializerKt.throwSubtypeNotRegistered(AbstractPolymorphicSerializer.kt:102)

Could you please help figuring this out? I tried in many ways to configure polymorphic serialization, but none of them worked.

Try doing this:

val format = Json {
    serializersModule = SerializersModule {
      polymorphic(Any::class) {
        subclass(VersionResponse::class)
      }
    }
  }

  val data: ApiResponse<VersionResponse> = ApiResponse.Ok(VersionResponse("v42"))
  println(format.encodeToString(data)) // prints {"type":"result","result":{"version":"v42"}}

Thank you for the response! It does work with your configuration of the

seralizersModule

. However, what it prints is not exactly what I need:

{
  "type": "result",
  "result": {
    "type": "VersionResponse",
    "version": "v42"
  }
}

As you can see, it adds

type

inside

result

as well.

If its a generic type T, then how is the deserializer supposed to know what type it needs to deserialize without some kind of discriminator?