ASP.NET Routing where two controller actions have the same number of parameters

Question

We have a route config like so:

  public static void RegisterRoutes(RouteCollection routes) {
     routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

     routes.MapRoute(
         name: "About",
         url: "{controller}/{action}/{aboutId}",
         defaults: new { controller = "Home", action = "About" }
     );

     routes.MapRoute(
         name: "Contact",
         url: "{controller}/{action}/{contactId}",
         defaults: new { controller = "Home", action = "Contact" }
     );

     routes.MapRoute(
         name: "Default",
         url: "{controller}/{action}/{id}",
         defaults: new { controller = "Home", action = "Index", id = UrlParameter.Optional }
     );
  }

You will notice that there are two routes that have one mandatory extra parameter. The routes About and Contact.

In our app we have two urls

www.myapp.com/Home/About/2 which works fine. But when we navigate our browser to www.myapp.com/Home/Contact/5 we get the dreaded routing exception:

The parameters dictionary contains a null entry for parameter 'contactId' of non-nullable type 'System.Int32' for method 'System.Web.Mvc.ActionResult Contact(Int32)' in 'RoutingTest.Controllers.HomeController'. An optional parameter must be a reference type, a nullable type, or be declared as an optional parameter.

If we change the sequence of the routing so that it looks like so:

  public static void RegisterRoutes(RouteCollection routes) {
     routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

     routes.MapRoute(
         name: "Contact",
         url: "{controller}/{action}/{contactId}",
         defaults: new { controller = "Home", action = "Contact" }
     );

     routes.MapRoute(
         name: "About",
         url: "{controller}/{action}/{aboutId}",
         defaults: new { controller = "Home", action = "About" }
     );

     routes.MapRoute(
         name: "Default",
         url: "{controller}/{action}/{id}",
         defaults: new { controller = "Home", action = "Index", id = UrlParameter.Optional }
     );
  }

Then the Contact url works but the About url does not.

The HomeController looks like this:

  public class HomeController : Controller {
      public ActionResult Index() {
         ViewBag.Message = "Modify this template to jump-start your ASP.NET MVC application.";

         return View();
      }

      public ActionResult About(int aboutId) {
         ViewBag.Message = "Your app description page.";

         return View();
      }

      public ActionResult Contact(int contactId) {
         ViewBag.Message = "Your contact page.";

         return View();
      }
   }

What this seems to imply is that two routings cannot have the same number of parameters regardless of the name of the Controller Action. If the two controller actions have a parameter with the same name, then all works fine. I know I can start doing very hacky things to work around this problem such as calling all parameters the same name or giving the actions meaningless parameters to change the number of parameters but I would actually like to know what is happening under the hood.

How do I solve this problem?

I honestly can't see why you need 3 different routes in this case. All you need to do is remove the non-default routes and replace the aboutId and contactId with id. Everything should just work... — Aron
– Aron, Commented Apr 23, 2013 at 2:48
@Aron: this of course is a contrived example to demonstrate the problem. We have multiple controller actions with different parameters. Naming them all the same thing seriously reduces code readability. — Aran Mulholland
– Aran Mulholland, Commented Apr 23, 2013 at 3:46

Aron · Accepted Answer · 2013-04-23 02:56:54Z

5

The root (no pun intended) of your issue ISN'T that routes can't have the same number of params. They can. The issue is that the routing engine will select the first route that matches the incoming request. Your routes are only different by the defaults, and pattern matching-wise they are exactly the same. So in each and every case you should be hitting the Contact route.

It looks like you are trying to have different routes based on the action. Which I can't actually see why you NEED.

You CAN use the following for that effect.

public static void RegisterRoutes(RouteCollection routes) {
    routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

    routes.MapRoute(
        name: "Contact",
        url: "Home/Contact/{contactId}"
    );

    routes.MapRoute(
        name: "About",
        url: "Home/About/{aboutId}"
    );

    routes.MapRoute(
        name: "Default",
        url: "{controller}/{action}/{id}",
        defaults: new { controller = "Home", action = "Index", id = UrlParameter.Optional }
    );
 }

However. I HIGHLY recommend against this approach, as your "default" route would be the Contact route. This means that (under Razor) the @Html.ActionLink() and related methods will be...wrong.

Honestly, it should just work perfectly if you actually just use...

public static void RegisterRoutes(RouteCollection routes) {
    routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

    routes.MapRoute(
        name: "Default",
        url: "{controller}/{action}/{id}",
        defaults: new { controller = "Home", action = "Index", id = UrlParameter.Optional }
    );
 }

answered Apr 23, 2013 at 2:56

Aron

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4 Comments

Aran Mulholland Over a year ago

Well adopting the second solution (removing all the routings except the default one) does not work and ends up with the same exception. In order for this to work you need to give the same name 'id' to all single parameter controller actions. I mentioned this in my question and don't think it's an acceptable solution.

Erik Funkenbusch Over a year ago

@AranMulholland - Routing works based on pattern matching. There is no other option than to change the routes so they have different patterns, or reuse the same route. You need some kind of differentiator, and if you're unwilling to do that, it simply won't work.

Aran Mulholland Over a year ago

@MystereMan But the routes contain the names of the parameters and the default controller actions, does this mean that the default controller actions are ignored? You would think that the routing engine could determine I was intending to call a controller action as the request names that controller and action.

Erik Funkenbusch Over a year ago

@AranMulholland - The names are not used in the route selection process, only the pattern is used. There is no difference between your about and contact pattern, thus the second one will never be selected. The routing engine has no way to know which one you mean unless you give it a differentiator, and the name of the variables is not a differentiator.

Alexei Levenkov · Accepted Answer · 2013-04-23 02:51:54Z

2

Specify controller and/or action in a route explicitly to allow routing to pick correct route:

 routes.MapRoute(
     name: "About",
     url: "{controller}/About/{aboutId}",
     defaults: new { controller = "Home", action = "About" }
 );

You also can add constraints to parameters to distinguish between routes, but it looks like in your case both actions have the same integer parameter.

answered Apr 23, 2013 at 2:51

Alexei Levenkov

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4 Comments

Aron Over a year ago

I highly recommend against this approach as it will mess up your routing on @Url.Action and related helpers.

Alexei Levenkov Over a year ago

@Aron - I'm not exactly sure why Url.Action does not work for you with non-default routes... Link to related article/question would be helpful in addition to your suggestion.

Aron Over a year ago

This is entirely empirical. MVC does not recognize "Default" as some magic name. The "default" route therefore is the first to be registered, in this case the About route. By default the route that @Url.Action will use is the About route. So when rendering for example @Html.ActionLink("Foo","Bar") you would get a link to http://www.myapp.com/Foo/About/?action=Bar. Which would get you to the right place. Just isn't very RESTful.

djack109 Over a year ago

I have a similar problem. I have 3 controllers in the same folder. The system can find the default controller but it cant find the others e.g. /pages/default works fine but /pages/data doesn't

Jeremy Bell · Accepted Answer · 2013-04-24 05:02:15Z

2

Couldn't you just delete your custom routes and just reuse the {id} parameter (turn {contactId} and {aboutId} into just "id" in your action code)?

public class HomeController : Controller {
  public ActionResult Index() {
     ViewBag.Message = "Modify this template to jump-start your ASP.NET MVC application.";

     return View();
  }

  public ActionResult About(int id) {

     int aboutId = id;

     ViewBag.Message = "Your app description page.";

     return View();
  }

  public ActionResult Contact(int id) {

     int contactId = id;

     ViewBag.Message = "Your contact page.";

     return View();
  }

}

edited Apr 24, 2013 at 5:02

answered Apr 24, 2013 at 4:57

Jeremy Bell

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