Short intro: my new year resolution, decided in April, is that I will write weekly blog posts. Short, concise, something I worked on last week or whatever…  so here is the first one!

This was a small problem I had in the office, one co-worker created two apps, of which one is WebAPI on self-hosted OWIN, other one is a win service. Exposing objects from WebAPI and reading them through HttpClient caused some problems when we refactored them, introduced polymorphism in this case.

Issues were:

  • JsonConvert couldn’t de-serialise received string, throwing this exception:
    An unhandled exception of type 'Newtonsoft.Json.JsonSerializationException' occurred in Newtonsoft.Json.dll

    Additional information: Could not create an instance of type JsonTypeNameHandling.Vehicle. Type is an interface or abstract class and cannot be instantiated

  • We wanted to inspect what WebApi is returning, but XML serializer was kicking in
  • We wanted to see full error details

This is common problem with WebApi, since (I guess) default Json settings are not configured to send object type information (recognized by “$type” in Json). Configuration needed for those issues:

  • both sides (server and client) needs Json.net settings to use type names:
    Client side (using HttpClient):
    JsonConvert.DeserializeObject<MyType>(payload, new JsonSerializerSettings { TypeNameHandling = TypeNameHandling.Auto })
    Server side (WebApi):
    configuration.Formatters.JsonFormatter.SerializerSettings.TypeNameHandling = TypeNameHandling.Auto;
  • Serve Json when invoking WebApi with browser:
    configuration.Formatters.JsonFormatter.SupportedMediaTypes.Add(new MediaTypeHeaderValue("text/html"));
  • Show error information:
    configuration.IncludeErrorDetailPolicy = IncludeErrorDetailPolicy.Always;

The whole example, a small app that creates web server (with OWIN and HttpListener) and calls itself:

Solution can be downloaded from Github