Question

Is it possible to have the default value for a route parameter be another value from the route?

e.g. something like:

routes.MapRoute(null,
    "{controller}/{action}/{parentid}/{currentid}",
    new { controller = "Home", action = "Display", currentid = "{parentid}" }
);

parentid is compulsory and will always have a value. If no value has been specified for currentid I’d like it to take the value of parentid.

Answer 1

I don’t know if this functionality is supported out of the box but you could always write a custom route to implement it:

public class MyRoute : Route
{
    public MyRoute() :
        base(
            "{controller}/{action}/{parentid}/{currentid}",
            new RouteValueDictionary(new
            {
                controller = "home",
                action = "display",
                currentid = UrlParameter.Optional
            }),
            new RouteValueDictionary(new
            {
                // adjust the constraint if your ids are not numeric
                // but you must have a constraint when a route parameter is
                // non optional unless this is the last parameter
                parentid = @"^([0-9])+$"
            }),
            new MvcRouteHandler()
    )
    {
    }

    public override RouteData GetRouteData(HttpContextBase httpContext)
    {
        var rd = base.GetRouteData(httpContext);
        if (rd == null)
        {
            return null;
        }

        var currentid = rd.Values["currentid"] as string;
        if (!string.IsNullOrEmpty(currentid))
        {
            return rd;
        }

        rd.Values["currentid"] = rd.Values["parentid"];
        return rd;
    }
}

and then register this route:

public static void RegisterRoutes(RouteCollection routes)
{
    routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

    routes.Add("myroute", new MyRoute());
}

Now assuming you have defined the following controller:

public class HomeController : Controller
{
    public ActionResult Display(int parentid, int currentid)
    {
        return Content(string.Format("{0}, {1}", parentid, currentid));
    }
}

you could request it by:

• /home/display/123 -> parentid=123 and currentid=123
• /home/display/123/456 -> parentid=123 and currentid=456