How to debug MVC3/WebAPI routing 404 error?

Question

My WebAPI route is defined as follows:

config.Routes.MapHttpRoute(
  name: "BuildModels",
  routeTemplate: "api/build/models/{productLine}/{language}",
  defaults: new { controller = "Build", action = "GetModels" }
);

Controller:

public class BuildController : ApiController
{
    public async Task<HttpResponseMessage> GetModels(string productLine, string language)
    {
        // Omitted for brevity, I don't feel it's pertinent
        // but if you disagree I will fill this in.  
        // Nothing has changed in here and it worked before.
        return await ...;
    }
}

/api/build/models/x/en

[HttpException]: The controller for path '/api/build/models/x/en' was not found or does not implement IController.
   at System.Web.Mvc.DefaultControllerFactory.GetControllerInstance(RequestContext requestContext, Type controllerType)
   at System.Web.Mvc.DefaultControllerFactory.CreateController(RequestContext requestContext, String controllerName)
   at System.Web.Mvc.MvcHandler.ProcessRequestInit(HttpContextBase httpContext, IController& controller, IControllerFactory& factory)
   at System.Web.Mvc.MvcHandler.BeginProcessRequest(HttpContextBase httpContext, AsyncCallback callback, Object state)
   at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

/api/build/models/x/

{"Message":"The arguments value is not valid : {language}"}

---

All of my routes are working except the ones on this single controller, which were working before. I obviously changed something, just can't figure out what.

Answer 1

Have you tried activating custom errors?

<customErrors mode="On" defaultRedirect="/customerrors/500.aspx">
    <error statusCode="404" redirect="/customerrors/404.html" />
    <error statusCode="403" redirect="/customerrors/403.html" />
</customErrors>