Based on your example the Controller = UserProfile and Action=ShowProfile. Now coming to the point, check the routing for this kind of url
Your route should be:
routes.MapRoute(
"Default",
"{controller}/{action}/{customuserName}",
new { controller = "UserProfile", action = "ShowProfile", customuserName = UrlParameter.Optional }
);
public ActionResult ShowProfile(string customuserName)
{
}
I would like tell more about routing, but it will be distracting. Please read about the routing so that you can avoid routing mistakes.