Emil's Wicked Cool Blog

Wow...what did that mean?  Here I am, just using @Url.Action (and BeginForm and any other method that walks the route table backwards).  My application is RESTful, so most routes have an HttpMethodConstraint so they only match if the method is correct.

I could not wrap my head around this message and ended up using JustDecompile to pull open System.Web.Routing and had a look at the HttpMethodConstraint Match method.  You'll note that the routeDirection parameter tells the object whether to match based on an incoming request (the normal case) or for Url Generation (used for Url.Action, BeginForm and the like).

Pulling open the source, I found this:

    switch (routeDirection1)
    {
        case RouteDirection.IncomingRequest:
        {
            ICollection<string> allowedMethods = this.AllowedMethods;
            if (func == null)
            {
                func = (string method) => string.Equals(method, httpContext.Request.HttpMethod, StringComparison.OrdinalIgnoreCase);
            }
            return allowedMethods.Any<string>(func);
        }
        case RouteDirection.UrlGeneration:
        {
            if (values.TryGetValue(parameterName, out obj))
            {
                string str = obj as string;
                if (str != null)
                {
                    return this.AllowedMethods.Any<string>((string method) => string.Equals(method, str, StringComparison.OrdinalIgnoreCase));
                }
                else
                {
                    object[] url = new object[2];
                    url[0] = parameterName;
                    url[1] = route.Url;
                    throw new InvalidOperationException(string.Format(CultureInfo.CurrentUICulture, RoutingResources.HttpMethodConstraint_ParameterValueMustBeString, url));
                }
            }
            else
            {
                return true;
            }
        }
    }

The RouteDirection.UrlGeneration case block represents what happens when the HttpMethodConstraint is asked to match for a generated Url.  The InvalidOperationException represents the error message we see on the YSOD.

Even this took me a bit to work through, but the bottom line is that it's finding the parameter name from the route and looking for the value of that parameter name from the input provided to it.  Given the following route:

    routes.MapRoute("UpdateByPut",
        "{controller}/{id}",
        new { action = "Update" },
        new { httpMethod = new HttpMethodConstraint("PUT") }
    );

HttpMethodConstraint will find httpMethod (this is the value of parameterName in the code above), and look for a route value of httpMethod, expecting it to be a string.  If it doesn't, it will throw the exception.  So, using the route above with the following code:

  @Url.Action("Show", "MyEntity", new { id = Model.Id })

Won't work.  Instead, you need this:

  @Url.Action("Show", "MyEntity", new { id = Model.Id, httpMethod = "GET" })

Also note that the property name httpMethod in the @Url.Action line above comes from the same property name I used in routes.MapRoute within Global.asax.cs.  This is one of the few times that MVC cares about the property name used on a constraint.  

Once you understand what the error message is trying to tell you it's all pretty clear.  You just need to be a rocket scientist the first time through.