A route named 'DefaultApi' is already in the route collection

Question

This question may seems duplicate but this is slightly different. In all other question in SO I had noticed that they have multiple routes registered. but in my case I have just one route.

I am creating asp.net webapi (framework 4.5) and have just one route in RegisterRoutes() method -

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

        routes.MapRoute(
            name: "DefaultApi",
            url: "rest/{controller}/{id}",
            defaults: new { id = UrlParameter.Optional }
        );

    }

Then why is it throwing error?

A route named 'DefaultApi' is already in the route collection. Route names must be unique. Parameter name: name

Answer 1

I had a similar issue with adding a route DefaultApi. Though the exact 'additional details' message in my ArgumentException stack trace was:

A route named 'MS_attributerouteWebApi' is already in the route collection.
Route names must be unique.

I ofcourse made sure I was adding the DefaultApi route only once, but in the end noticed that in Global.asax's Application_Start method I was calling the WebApiConfig.Register(..) twice, though in the following - not immediately obvious - way:

WebApiConfig.Register(GlobalConfiguration.Configuration);
GlobalConfiguration.Configure(WebApiConfig.Register);

Another serious case of 'copypasterites'! I simply removed the WebApiConfig.Register(..) line and that fixed my issue.

(I am using WEB API 2.0/.NET 5)

Answer 2

CAUSE: renaming namespaces without removing associated bin and output files.

SOLUTION: manually delete the bin and obj folders from the output directory. (cleaning the solution is not enough, some problematic residual files remain causing this problem.)

... that was my experience anyway.