MVC Conventional and Attribute routing not working together

Question

I am using conventional routing on an ASP.Net MVC project and would like to enable Attribute routing in parallel. I have created the following but I am getting a 404 on the conventional route when enabling attribute routing

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

    routes.MapMvcAttributeRoutes();

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

Controller

[RoutePrefix("Registration")]
public class RegistrationController : Controller
{    
    [HttpGet]
    [Route("Add/{eventId}")]
    public ActionResult Add(int eventId)
    {
    }
}

Calling

http://localhost/Registration/Add/1

Works, while calling

http://localhost/Registration/Add?eventId=1

No longer works and responds with 404 NotFound

Answer 1

Should work if you make the {eventId} template parameter optional in the route template

[RoutePrefix("Registration")]
public class RegistrationController : Controller {
    //GET Registration/Add/1
    //GET Registration/Add?eventId=1
    [HttpGet]
    [Route("Add/{eventId:int?}")]
    public ActionResult Add(int eventId) {
        //...
    }
}

The reason the two were not working is that the route template Add/{eventId} means that the route will only match if the {eventId} is present, which is why

http://localhost/Registration/Add/1

works.

By making it (eventId) optional eventid? it will allow

http://localhost/Registration/Add

to work as the template parameter is not required. This will now allow query string ?eventId=1 to be used, which the routing table will use to match the int eventId parameter argument on the action.

http://localhost/Registration/Add?eventId=1