How to use a sealed class hierarchy with differing root elements?

[dduits]

How to use a sealed class hierarchy with differing root elements?

I'm writing a HTTP client for a web service that returns a different XML based
on whether records are found or not. I've described a simplified version of my
use-case below.

When records are found the API returns a cars root element with car
elements inside it:

<?xml version="1.0" encoding="UTF-8"?>
<cars context="main">
    <car color="red" />
    <car color="blue" />
    <car color="green" />
</cars>

When no records are found it returns a single car root element instead:

<?xml version="1.0" encoding="UTF-8"?>
<car context="main" />

I've tried the following approach using xmlutil:

@Serializable
sealed class Base {

    abstract val context: String

    @Serializable
    @SerialName("car")
    data class NoResultsFound(override val context: String) : Base()

    @Serializable
    @SerialName("cars")
    data class CarsFound(
        override val context: String,
        @XmlElement(true) val cars: List<Car>
    ) : Base() {

        @Serializable
        @SerialName("car")
        data class Car(val color: String)

    }

}

val xml = XML()

val carsFoundXml = """
    <cars context="main">
        <car color="red" />
        <car color="blue" />
        <car color="green" />
    </cars>""".trimIndent()

val noCarsFoundXml = """<car context="main" />"""

// Works
val carsFound = xml.decodeFromString(Base.CarsFound.serializer(), carsFoundXml)
println(carsFound)

// Works
val noCarsFound = xml.decodeFromString(Base.NoResultsFound.serializer(), noCarsFoundXml)
println(noCarsFound)

// Fails with UnknownXmlFieldException
val carsFoundFromBase = xml.decodeFromString(Base.serializer(), carsFoundXml)
println(carsFoundFromBase)

// Fails with UnknownXmlFieldException
val noCarsFoundFromBase = xml.decodeFromString(Base.serializer(), noCarsFoundXml)
println(noCarsFoundFromBase)

Program output:

CarsFound(context=main, cars=[Car(color=red), Car(color=blue), Car(color=green)])
NoResultsFound(context=main)
Exception in thread "main" nl.adaptivity.xmlutil.serialization.UnknownXmlFieldException: Could not find a field for name Base/context
  candidates: type, value at position Line number = 1
Column number = 22
System Id = null
Public Id = null
Location Uri= null
CharacterOffset = 21

When working with JSON I would use JsonContentPolymorphicSerializer to select
the serializer based on what keys are found in the object. Though for obvious
reasons not an option here. I've went through at the documentation, examples
and issues but I'm unable to figure out how to approach this issue.

Any suggestions how I could approach serializing above XML into a sealed class
using xmlutil?

[pdvrieze]

This is a limitation of the implementation at the moment. Basically it doesn't handle transparently polymorphic root elements. I'll look at a proper solution, but a workaround would be to have a wrapper element that "holds" the actual content. At that point it should work correctly (perhaps with XML { autoPolymorphic = true })

[pdvrieze]

I have added the fix/feature to the dev branch (so wrappers wouldn't be needed anymore).

[pdvrieze]

The new release should have fixed this (with the limitation that the root type's annotations are not accessible - this is a limitation of the plugin)