This issue wasted far too much of my time and the solution was so simple.

Problem

When I was trying to call one of my api controller methods it was saying:

Multiple types were found that match the controller named 'X'

Cause

I had made a copy of a project and renamed everything in the project

I built and ran the site, there was an error with the HomeController, saying it found 2 routes, my previous project route and my new project route.

I checked I hadn't left any reference to the previous project in there, all the namespaces were correct.

I cleaned the solution, and rebuilt loads of times.

I even changed to Guid of the project. Still that didn't fix it.

I found a suggestion online which said I should add the namespace to the MapRoute calls in the RouteConfig.cs file.

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

I also found this solution too, to help when I got an issue with OWIN

<add key="owin:appStartup" value="MyNameSpace.MyProjectName.Startup" />

These worked for the normal controllers, but it is not the right solution to the problem.

When I tried to test the rest of the site, I found I had the same issue with my API Controllers, and you can't set the namespaces in WebApiConfig.cs like you can above in the RouteConfig.cs

What was I missing? It's got to be simple. It turns out it was simple.

Solution

Clear the bin folder manually. For some reason Clean Solution didn't delete the dll for the original project from my bin folder, so it was always there causing trouble. Once I located it and delete it and it and the .pdb and .config files for it, the project worked fine.

So I've learned my lesson and I won't do it again. Hopefully you have learned your lesson too and this post has saved you a lot of time.