.net Core - Default controller is not loading when Route attribute is used

Question

A new .net core web application project comes with the following route configuration:

    app.UseMvc(routes =>
    {
        routes.MapRoute(
            name: "default",
            template: "{controller=Home}/{action=Index}/{id?}");
    });

If you replace this with app.UseMvc() and add appropriate Route attributes for both HomeController and its actions (Index, About, Contact, Error), it would still work. Since we're not specifying a default route, the default view (Home/Index) will not be rendered if you hit http://localhost:25137/. Hope that understanding is correct!

Now, since I need the default view to be shown when the http://localhost:25137/ is hit, I changed the routing code to app.UseMvcWithDefaultRoute(); which by definition will do the equivalent to the initial snippet. Even then, it was not rendering the default view; but worked when used the complete URL(http://localhost:25137/home/index). That means the routing still works but not the default one!

Then I went back to the controller and removed all the Route attribute from the HomeController and its actions. Then the default routing worked with out any issues.

Is that the expected behavior? What could be the reason behind this behavior?

Solution

From the asp docs:

Actions are either conventionally routed or attribute routed. Placing a route on the controller or the action makes it attribute routed. Actions that define attribute routes cannot be reached through the conventional routes and vice-versa. Any route attribute on the controller makes all actions in the controller attribute routed.

So basically if you use attributes then the routes defined in UseMvc or UseMvcWithDefaultRoute will be ignored. Only the attribute would be used in that case.

You could still use multiple route attributes if you wanted to achieve a similar effect than the default route with optional segments. Again from the same article in the asp docs:

public class MyDemoController : Controller
{
   [Route("")]
   [Route("Home")]
   [Route("Home/Index")]
   public IActionResult MyIndex()
   {
      return View("Index");
   }
   [Route("Home/About")]
   public IActionResult MyAbout()
   {
      return View("About");
   }
   [Route("Home/Contact")]
   public IActionResult MyContact()
   {
      return View("Contact");
   }
}