Unit testing ASP.NET Web API controllers that rely on the UrlHelper

UrlHelper is the class you can use in ASP.NET Web API to automatically infer links from the routing table without hardcoding anything. For example, the following code uses the helper to infer the location url for a new resource,

public HttpResponseMessage Post(User model)
{
  var response = Request.CreateResponse(HttpStatusCode.Created, user);
  var link = Url.Link("DefaultApi", new { id = id, controller = "Users" });
  response.Headers.Location = new Uri(link);
  return response;
}

That code uses a previously defined route “DefaultApi”, which you might configure in the HttpConfiguration object (This is the route generated by default when you create a new Web API project).

The problem with UrlHelper is that it requires from some initialization code before you can invoking it from a unit test (for testing the Post method in this example). If you don’t initialize the HttpConfiguration and Request instances associated to the controller from the unit test, it will fail miserably.

After digging into the ASP.NET Web API source code a little bit, I could figure out what the requirements for using the UrlHelper are. It relies on the routing table configuration, and a few properties you need to add to the HttpRequestMessage. The following code illustrates what’s needed,

var controller = new UserController();
controller.Configuration = new HttpConfiguration();
var route = controller.Configuration.Routes.MapHttpRoute(
         name: "DefaultApi",
         routeTemplate: "api/{controller}/{id}",
         defaults: new { id = RouteParameter.Optional }
);
var routeData = new HttpRouteData(route, 
       new HttpRouteValueDictionary 
       { 
          { "id", "1" },
          { "controller", "Users" } 
       }
);
controller.Request = new HttpRequestMessage(HttpMethod.Post, "http://localhost:9091/");
controller.Request.Properties.Add(HttpPropertyKeys.HttpConfigurationKey, controller.Configuration);
controller.Request.Properties.Add(HttpPropertyKeys.HttpRouteDataKey, routeData);

The HttpRouteData instance should be initialized with the route values you will use in the controller method (“id” and “controller” in this example). Once you have correctly setup all those properties, you shouldn’t have any problem to use the UrlHelper. There is no need to mock anything else.

Enjoy!!.