Question

Using the ASP.NET WebApi 4 RC, is it possible to have an ApiController’s return type be a base class and actually return instances of derived classes? Trying to do this now results in an internal server error (500) when returning xml. Returning json using this method works correctly.

public class Base
{
    public int ID { get; set; }
}
public class Derived : Base
{
    public string Message { get; set; }
}
public class ValuesController : ApiController
{
    public IEnumerable<Base> Get()
    {
        return new Derived[] { 
            new Derived(){ Message="test"},
            new Derived(){ Message="another"}
        };
    }
}

It would seem that the XML serialization is what’s throwing the error but all I can see is the generic 500 error.

Answer 1

Yes, you need to use the knowntype serialization hint:

[System.Runtime.Serialization.KnownType(typeof(Derived))]
public class Base
{
    public int ID { get; set; }
}