Note: This describes the behavior of ASP.NET MVC 2 as of the release candidate. It’s possible things might change for the RTM.
When using areas in ASP.NET MVC 2, a common problem you might encounter is this exception message.
The controller name 'Home' is ambiguous between the following types:
AreasDemoWeb.Controllers.HomeController
AreasDemoWeb.Areas.Blogs.Controllers.HomeController
This message is telling you that the controller factory found two types that match the route data for the current request. Typically this happens when you have a controller of the same name in an area and in the main project.
For example, in the screenshot below, notice that we have a HomeController in the main Controllers folder as well as in the Blogs area.
If you make a request for the area such as /Blogs/Home, you’ll find that everything works hunky-dory. However, if you make a request for the root HomeController, such as /Home, you’ll get the ambiguous controller exception.
Why is that?
When you register routes for an area, they get a namespace associated with each route. That ensures that only controllers within the namespace associated with that area can fulfill the request. Thus the request that matches an area will have that namespace and the namespace is used to disambiguate controllers.
But by default, the routes in the main application don’t have a namespace associated with them. That means the controller factory will scan all types looking for a match, and in this case finding two types which match the controller name “Home”.
The Fix
There are two very simple workarounds. The simplest falls in the “If it hurts, stop doing that” camp which is to simply avoid naming two controllers the same name.
For many situations, this is not a satisfactory answer. The other workaround, as you might guess from my explanation of why this happens, is to give the route in the main application a specific namespace. Here’s an example of the default route in Global.asax.cs which has the fix.
public static void RegisterRoutes(RouteCollection routes)
{
routes.IgnoreRoute("{resource}.axd/{*pathInfo}");
routes.MapRoute(
"Default", // Route name
"{controller}/{action}/{id}", // URL
new { controller = "Home", action = "Index", id = "" }, // Defaults
new[]{"AreasDemoWeb.Controllers"} // Namespaces
);
}
In the code above, I added a fourth parameter which is an array of namespaces. The controllers for my project live in a namespace called AreasDemoWeb.Controllers.
Follow Up
In a follow-up post, I’ll walk through more details about areas and how namespaces play into routing and controller lookup. For now, I hope this gets you unstuck if you’ve run into this problem before.