How do I set a “Default Action” for my Controller that will be called when no other action matches

routes.MapRoute("Default", "{controller}/{action}/{id}",
    new { controller = "Home", action = "Index", id = "" });

protected override void HandleUnknownAction(string actionName) {
  //your code here.
}

routes.MapRoute("default-action", "{controller}/{actionName}/{id}", new {action="DefaultAction"});

public ActionResult DefaultAction(string actionName, string id) {
  //handle default action
}