Prior to Web API 2, the Web API project templates generated code like this:

protected void Application_Start() { // WARNING - Not compatible with attribute routing. WebApiConfig.Register(GlobalConfiguration.Configuration); }

If attribute routing is enabled, this code will throw an exception. If you upgrade an existing Web API project to use attribute routing, make sure to update this configuration code to the following:

protected void Application_Start() { // Pass a delegate to the Configure method. GlobalConfiguration.Configure(WebApiConfig.Register); }