Web API 2 serializer selection

Question

In web api 2 controller handling request with "Accept: application/xml" header field.

public class ConverterController : ApiController
{
    class A {}
    public class B {}
    public class C : Exception {}

    public IHttpActionResult Action()
    {
       var res = Request.CreateResponse(HttpStatusCode.Ok, new A());
       // res.Content.Headers.ContentType.MediaType == "application/json"

       var res = Request.CreateResponse(HttpStatusCode.Ok, new B());
       // res.Content.Headers.ContentType.MediaType == "application/xml"

       var res = Request.CreateResponse(HttpStatusCode.Ok, new C());
       // res.Content.Headers.ContentType.MediaType == "application/json"
    }
}

Why class A and C is serializing to json?

Answer 1

It turns out that when there is an exception in XML serializer web api 2 silently falls back to JSON serializer. Thanks to AarónBC for the hint that must force XML serializer to get serialization exception.

var res = Request.CreateResponse(HttpStatusCode.OK, new C(), Configuration.Formatters.XmlFormatter);