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

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.

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.

There is no reason to use routing based in querystring in new ASP.NET MVC project. It can be useful for old project that has been converted from classic ASP.NET project and you want to preserve URLs.

One solution can be attribute routing.

Another solution can be in writting custom routing by deriving from RouteBase:

public class MyOldClassicAspRouting : RouteBase
{

  public override RouteData GetRouteData(HttpContextBase httpContext)
  {
    if (httpContext.Request.Headers == null) //for unittest
      return null;

    var queryString = httpContext.Request.QueryString;

    //add your logic here based on querystring
    RouteData routeData = new RouteData(this, new MvcRouteHandler());
    routeData.Values.Add("controller", "...");
    routeData.Values.Add("action", "...");
  }

  public override VirtualPathData GetVirtualPath(RequestContext requestContext, RouteValueDictionary values)
  {
     //Implement your formating Url formating here
     return null;
  }
}

And register your custom routing class

public static void RegisterRoutes(RouteCollection routes)
{
  ...

  routes.Add(new MyOldClassicAspRouting ());
}