How do I route a URL with a querystring in ASP.NET MVC?

When defining routes, you cannot use a / at the beginning of the route:

routes.MapRoute("OpenCase",
    "/ABC/{controller}/{key}/{group}", // Bad. Uses a / at the beginning
    new { controller = "", action = "OpenCase" },
    new { key = @"\d+", group = @"\d+" }
    );

routes.MapRoute("OpenCase",
    "ABC/{controller}/{key}/{group}",  // Good. No / at the beginning
    new { controller = "", action = "OpenCase" },
    new { key = @"\d+", group = @"\d+" }
    );

Try this:

routes.MapRoute("OpenCase",
    "ABC/{controller}/{key}/{group}",
    new { controller = "", action = "OpenCase" },
    new { key = @"\d+", group = @"\d+" }
    );

Then your action should look as follows:

public ActionResult OpenCase(int key, int group)
{
    //do stuff here
}

It looks like you're putting together the stepNo and the "ABC" to get a controller that is ABC1. That's why I replaced that section of the URL with {controller}.

Since you also have a route that defines the 'key', and 'group', the above route will also catch your initial URL and send it to the action.

You cannot include the query string in the route. Try with a route like this:

routes.MapRoute("OpenCase", "ABC/ABC{stepNo}",
   new { controller = "ABC1", action = "OpenCase" });

Then, on your controller add a method like this:

public class ABC1 : Controller
{
    public ActionResult OpenCase(string stepno, string key, string group)
    {
        // do stuff here
        return View();
    }        
}

ASP.NET MVC will automatically map the query string parameters to the parameters in the method in the controller.