The Constraint Entry 'httpMethod' on the Route Must Have a String Value

Question

I have an asp.net Web API project, and in my WebApiConfig file, I have the following route defined:

config.Routes.MapHttpRoute(
    name: "Web API Get",
    routeTemplate: "api/{controller}",
    defaults: new { action = "Get" },
    constraints: new { httpMethod = new HttpMethodConstraint("GET") }
    );

For integration testing purposes, I want to make a request to an HttpSelfHostServer to verify that we are receiving the proper data back from the api call. I am making the HttpRequestMessage as follows:

var httpMethod = new HttpMethod("GET");
var request = new HttpRequestMessage(httpMethod, "http://localhost:XXXX/api/User/");
var results = _client.SendAsync(request).Result;

I would expect that this would call the Get method on the UserController and then return the results as defined in that method. However, I instead get the following exception:

System.InvalidOperationException: The constraint entry 'httpMethod' on the route with route template 'api/{controller}' must have a string value or be of a type which implements 'IHttpRouteConstraint'

This same url (http://localhost:XXXX/api/User/) works without error when I use it in the browser, so I am pretty sure the issue has to be in the way I am sending the request to the HttpSelfHostServer through the HttpClient. I have tried using the HttpMethod.Get constant instead, but that also threw the same error.

Does anyone have any idea how I could resolve this issue?

Nuli

this might help you emilsblog.lerch.org/2012/05/constraint-for-route-parameter.html – Saddlebag

I don't have access to the WebApi Url object to use the solution he is proposing. I see that the route direction is likely RouteDirection.UrlGeneration and httpMethod is not being found (hence the exception). How can I get the httpMethod with a string value of "GET" into my request without using the Url.Action() method? – Nuli

Okay, I came across the UrlHelper class which seems to do the same thing as the Url object in the controllers. However, now I am getting a new error associated with the configuration object. I'll update my question shortly with more details... – Nuli

Oops, configuration object was null at that point. After fixing that I'm back to where I started with the first exception. – Nuli

Answer 1

Make sure that you are using the proper type for your constraint:

constraints: new { httpMethod = new System.Web.Http.Routing.HttpMethodConstraint(HttpMethod.Get) }

I guess you were using System.Web.Routing.HttpMethodConstraint which is an entirely different class used for ASP.NET MVC routing and which has nothing to do with ASP.NET Web API routing.

Neomaneomah

Wow, yeah that was it. Thank you, I've been beating my head against that one for a while now. – Nuli

Yes, Microsoft created one hell of a mess when they duped every single class for this Web API that was available in MVC but under different assemblies and namespaces. It's a very common cause for mistakes. – Neomaneomah

Great answer, that was really well spotted @DarinDimitrov, thanks. – Gardy

I ended up here because I was looking for the opposite, incase this is useful to anyone else: new System.Web.Routing.HttpMethodConstraint(HttpMethod.Delete.ToString()) – Kowalczyk