Having issue with multiple controllers of the same name in my project

The error message contains the recommended solution: "If this is the case, register this route by calling an overload of the 'MapRoute' method that takes a 'namespaces' parameter."

routes.MapRoute(
     "Default", // Route name
     "{controller}/{action}/{id}", // URL with parameters
     new { controller = "Home", action = "Index", id = UrlParameter.Optional }, // Parameter defaults
     new string[] { "MyCompany.MyProject.WebMvc.Controllers"}
);

This will make http://server/ go to your HomeController's Index action which is, I think, what you want. http://server/company/home will go to the Company area's HomeController's Index action, as defined in the area registration.

This is the asp.net mvc4 approach:

 routes.MapRoute(
            name: "Default",
            url: "{controller}/{action}/{id}",
            defaults: new { controller = "Home", action = "RegisterNow", id = UrlParameter.Optional },
            namespaces: new[] { "YourCompany.Controllers" }
        );

I had renamed the namespaces, so, i only delete de folders bin and obj and rebuild, work again.

If you're using RazorGenerator, just informing the namespaces parameter could be not enough.

I got to solve adding the statement marked below at Global.asax.cs:

    protected void Application_Start()
    {
        AreaRegistration.RegisterAllAreas();
        FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
        RouteConfig.RegisterRoutes(RouteTable.Routes);
        BundleConfig.RegisterBundles(BundleTable.Bundles);
        ControllerBuilder.Current.DefaultNamespaces.Add("MyProject.Controllers"); // This one
    }

Another plausible cause of this issue could be found below:

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

use this

routes.MapRoute(
            "Default",
            "{controller}/{action}/{id}",
            new { controller = "Home", action = "Index", id = UrlParameter.Optional },
            new[] { "ProjectName.Controllers" }
        );

Use only the name of the project:

Public Class RouteConfig
    Public Shared Sub RegisterRoutes(ByVal routes As RouteCollection)
        routes.IgnoreRoute("{resource}.axd/{*pathInfo}")
        routes.MapRoute( _
            name:="Default", _
            url:="{controller}/{action}/{id}", _
            defaults:=New With {.controller = "Home", .action = "Index", .id = UrlParameter.Optional} _
           , namespaces:={"MvcAreas"})  
    End Sub

I had the same issue and found that the older version had created compiled files in the "bin" folder.

Once I deleted these the error disappeared.

As Chris Moschini mention the namespaces parameter may not be enough if you have two areas with same controller name with different namespaces and the default none area route will return 500 server error.

routes.MapRoute(
     "Default", // Route name
     "{controller}/{action}/{id}", // URL with parameters
     new { controller = "Home", action = "Index", id = UrlParameter.Optional }, // Parameter defaults
     new string[] { "MyCompany.MyProject.WebMvc.Controllers"}
);

It "best" to override the default route handler and add this line:

RequestContext.RouteData.DataTokens["UseNamespaceFallback"] = false;

I had this issue after I added a reference to another project that had the same routes and the problem continued after I removed the reference.

Resolved by deleting the .dll file of that added reference from the bin folder and rebuilding.

Like many others, I had this problem after creating a new MVC template project from VS2017 menu, on building the project I would get the op's error message. I then used the answer https://stackoverflow.com/a/15651619/2417292 posted earlier in this thread by cooloverride for mvc4 projects. This still didn't fix my issue so I renamed my Home view folder and HomeController file to be the view folder Company/ and controller file CompanyController. This then worked for me, not a fix per say but a workaround if your not stuck on having the route Home/Index my other issue was I couldn't find the reference causing the error and due to my dev platform being Azure WebApplication vs a full VM with a complete file system and IIS settings to mess with.

For MVC5 below code works for issue same controller name for different areas. You have to specify which area controller have to hit first.

        routes.MapRoute(
            name: "Default",
            url: "{controller}/{action}/{id}",
            defaults: new { controller = "Home", action = "Index", id = UrlParameter.Optional },
             new[] { "ApplicationName.Areas.AreaName.Controllers" }
        ).DataTokens.Add("area", "AreaName");

This issue occurred when I accidentally added

[Route("api/[controllers]s")]

instead of

[RoutePrefix("api/[controllers]s")]

to the top of my ApiController.