I want to include a serializable type `Foo` in my ...

# serialization

[Sam Stone]

I want to include a serializable type Foo in my serializable API model, but I need to serialize it as an empty string if it is null. How can I do this?

@Serializable
data class Foo(val bar: Int)
@Serializable
data class ApiModel(val foo: Foo?)
//ApiModel(null) -> {"foo":""}
//ApiModel(Foo(1)) -> {"foo":{"bar":1}}

[Emil Kantis]

Create a JsonTransformingSerializer<Foo> which transforms JsonNull to empty string:

object FooOrEmptyStringSerializer : JsonTransformingSerializer<Foo>(Foo.serializer()) {
   override fun transformSerialize(element: JsonElement): JsonElement =
      when (element) {
         JsonNull -> JsonPrimitive("")
         else -> element
      }
}

[Sam Stone]

But JsonTransformingSerializer takes T:Any / not null. I am getting a null pointer exception: parameter specified as non-null is null

[Emil Kantis]

What if you change the generic type to Foo?

[Emil Kantis]

Not allowed?

[Sam Stone]

Correct.

[Emil Kantis]

https://github.com/Kotlin/kotlinx.serialization/issues/1927