Ambient values in mvc2.net routing

I have the same "not clearing out" value problem...

I've stepped into source code and I don't understand the reason for being of segment commented as : // Add all current values that aren't in the URL at all

@ System\Web\Routing\ParsedRoute.cs, public BoundUrl Bind(RouteValueDictionary currentValues, RouteValueDictionary values, RouteValueDictionary defaultValues, RouteValueDictionary constraints) method from line 91 to line 100

While the clearing process is correctly handled in method preceding steps, this code "reinjects" the undesired parameter into acceptedValues dictionary!?

My routing is defined this way:

routes.MapRoute(
    "Planning",
    "Plans/{plan}/{controller}/{action}/{identifier}",
    new { controller = "General", action = "Planning", identifier = UrlParameter.Optional },
    new { plan = @"^\d+$" }
);

// default application route
routes.MapRoute(
    "Default",
    "{controller}/{action}/{identifier}",
    new {
        controller = "General",
        action = "Summary",
        identifier = UrlParameter.Optional,
        plan = string.Empty // mind this default !!!
    }
);

This is very similar to what you're using. But mind my default route where I define defaults. Even though my default route doesn't define plan route value I still set it to string.Empty. So whenever I use Html.ActionLink() or Url.Action() and I want plan to be removed from the URL I call it the usual way:

Url.Action("Action", "Controller", new { plan = string.Empty });

And plan is not included in the URL query string any more. Try it out yourself it may work as well.

Muhammad, I suggest something like this : (written 5 mn ago, not tested in production)

public static class MyHtmlHelperExtensions {

    public static MvcHtmlString FixActionLink(this HtmlHelper htmlHelper, string linkText, string actionName, string controllerName, object routeValues, object htmlAttributes) {
        var linkRvd = new RouteValueDictionary(routeValues);
        var contextRvd = htmlHelper.ViewContext.RouteData.Values;
        var contextRemovedRvd = new RouteValueDictionary();

        // remove clearing route values from current context
        foreach (var rv in linkRvd) {
            if (string.IsNullOrEmpty((string)rv.Value) && contextRvd.ContainsKey(rv.Key)) {
                contextRemovedRvd.Add(rv.Key, contextRvd[rv.Key]);
                contextRvd.Remove(rv.Key);
            }
        }

        // call ActionLink with modified context
        var htmlString = htmlHelper.ActionLink(linkText, actionName, controllerName, routeValues, htmlAttributes);

        // restore context
        foreach (var rv in contextRemovedRvd) {
            contextRvd.Add(rv.Key, rv.Value);
        }

        return htmlString;
    }
}

This is such a frustrating problem and I would venture to say that it is even a bug in ASP.Net MVC. Luckily it's an easy fix using ActionFilters. If you are using MVC3 then I would just put this as a global attribute to clear out ambient values. I made this attribute discriminatory, but you can change it to clear all attributes.

The assumption here is that by the time the Result is executing (your view most likely), you have already explicitly specified all your ActionLinks and Form Actions. Thus this will execute before they (the links) are evaluated, giving you a new foundation to generate them.

public class ClearAmbientRouteValuesAttribute : ActionFilterAttribute 
{
    private readonly string[] _keys;

    public ClearAmbientRouteValuesAttribute(params string [] keys)
    {
        if (keys == null)
            _keys = new string[0];

        _keys = keys;
    }

    public override void OnResultExecuting(ResultExecutingContext filterContext)
    {
        foreach (var key in _keys) {
            // Why are you sticking around!!!
            filterContext.RequestContext.RouteData.Values.Remove(key);        
        }
    }
}

// Inside your Global.asax
public static void RegisterGlobalFilters(GlobalFilterCollection filters)
{
    filters.Add(new HandleErrorAttribute());
    filters.Add(new ClearAmbientRouteValuesAttribute("format"));
}

Hope this helps someone, cause it sure helped me. Thanks for asking this question.

In this particular scenario I have two recommendations:

1. Use named routes. The first parameter to the MapRoute method is a name. To generate links use Html.RouteLink() (and other similar APIs). This way you'll always choose the exact route that you want and never have to wonder what gets chosen.

2. If you still want to use Html.ActionLink() then explicitly set docid="" to clear out its value.

Here's how I solved my problem, it may take a little adapting to get it to work, but I felt like I could get what I needed and just use routing more or less normally:

Excerpted from Apress Pro ASP.Net.MVC 3 Framework:

• A value must be available for every segment variable defined in the URL pattern. To find values for each segment variable, the routing system looks first at the values we have provided (using the properties of anonymous type), then the variable values for the current request, and finally at the default values defined in the route. (We return to the second source of these values later in this chapter.)

• None of the values we provided for the segment variables may disagree with the default-only variables defined in the route. These are variables for which default values have been provided, but which do not occur in the URL pattern. For example, in this route definition, myVar is a default-only variable:

  routes.MapRoute("MyRoute", "{controller}/{action}", new { myVar = "true" });

  For this route to be a match, we must take care to not supply a value for myVar or to make sure that the value we do supply matches the default value.

• The values for all of the segment variables must satisfy the route constraints. See the “Constraining Routes” section earlier in the chapter for examples of different kinds of constraints.

Basically I used the rule about a route not matching if it doesn't define a segment, but has a default variable used to give me a little more control over whether a route was chosen for outbound routing or not.

Here's my fixed routes, notice how I specify a value for category that would never be valid and don't specify a segment for category. This means that route will be skipped if I have a category, but will use it if I only have a page:

routes.MapRoute(null, "receptionists/faq/{page}", new { controller = "Receptionist", action = "Faq", page = 1, category = (Object)null }, new { page = @"^\d+$" });

routes.MapRoute(null, "receptionists/faq/{category}/{page}", new { controller = "Receptionist", action = "Faq", page = 1 }, new { category = @"^\D+$", page = @"^\d+$" });

For Category Links

@Html.ActionLink("All", "Faq", new { page = 1 })

@foreach (var category in Model.Categories)
{
@Html.ActionLink(category.DisplayName, "faq", new { category = category.DisplayName.ToLower(), page = 1 })
}

For Page Links

@for (var p = 1; p <= Model.TotalPages; p++)
{
@Html.ActionLink(p.ToString(), "Faq", new { page = p, category = Model.CurrentCategory})
}