How can I encode a typed class with Kotlinx Serialization?

Question

I'd like to encode a given class of type T: EventData with Kotlinx Serialization encodeToString .

This is my code:

class EventDispatcher<T: EventData>(
    val pubSubTemplate: PubSubTemplate
) {
    /**
     * Dispatch an event to the game engine event manager pipeline
     */
    fun dispatchEvent(event: T, initiator: String) {
        val eventData: String = Json.encodeToString(event)
    }

The compiler tells me:

Cannot use `T` as reified type parameter. Use a class instead

Is there a way to make this still work?

Answer 1

For Json.encodeToString(event) to work, it needs the type information for T . But, this type information is lost at runtime due to the way how generics work in Kotlin/Java.

One way to retain the type information would be by making dispatchEvent an inline function with T as a reified type parameter .

However, this also raises the question how you want to serialize event . You could also use polymorphic serialization of EventData , rather than trying to serialize T . This will include an additional class discriminator in your serialized output (it necessarily has to for polymorphic serialization/deserialization to work).

If you serialize the concrete type T , this class discriminator wouldn't be included, which is questionable; how would whoever will deserialize this know what type it is?

In short, I think you need polymorphic serialization.