Manipulating Action Method Parameters

asp.net, asp.net mvc, code comments edit

During the MVP summit, an attendee asked me for some help with a common scenario common among those building content management systems. He wanted his site to use human friendly URLs.

  http://example.com/pages/a-page-about-nothing/

instead of

  http://example.com/pages/123/

Notice how the first URL is descriptive whereas the second is not. The first URL contains a URL “slug” while the second one contains the ID for the content, typically associated with the ID in the database.

This is easy enough to set up with routing, but there’s a slight twist. He still wanted the action method which would respond to the first URL to have the integer integer ID as the parameter, not the slug. Let’s look at one possible approach to solving this.

Here’s an example of what the route might look like:

routes.MapRoute(
  "Slug", // Route name
  "pages/{slug}", // URL with parameters
  new { controller = "Home", action = "Content" } // Parameter defaults
);

Notice that the route URL contains one parameter for “slug” and no “id” parameter whatsoever. Here’s an example of the controller action that route should map to.

public ActionResult Content(int id)
{
  // Note the argument is an id, not slug
  return View();
}

Note that the action method does not accept a parameter named “slug” but instead expects an integer “id” parameter.

Fortunately, there’s an easy way to do this. Action filters, classes which derive from ActionFilterAttribute, allow hooking into the point in time after the parameters of action method have been bound, but just before the action method has been invoked. This gives us a fine opportunity to muck around with the parameters.

The following is an example of an action filter which converts a slug to an ID (you can imagine a real one would probably look it up in the database, not in a static dictionary like the sample does).

public class SlugToIdAttribute : ActionFilterAttribute
{
  static IDictionary<string, int> Slugs = new Dictionary<string, int>
  {
    {"this-is-a-slug", 100}, 
    {"another-slug", 101}, 
    {"and-another", 102}
  };

  public override void OnActionExecuting(ActionExecutingContext filterContext)
  {
    var slug = filterContext.RouteData.Values["slug"] as string;
    if(slug != null)
    {
      int id;
      Slugs.TryGetValue(slug, out id);
      filterContext.ActionParameters["id"] = id;
    }
    base.OnActionExecuting(filterContext);
  }
}

The filter overrides the OnActionExecuting method which is called just before the action method is called. The filter than grabs the slug from the route data, and looks up the corresponding id. Now all we need to do is make sure the id is passed into the action method.

Fortunately the filter context passed into this method allows us to peek into the parameters that will get passed into the action method via the ActionParameters property. Not only that, it allows us to change them!

In this case, I’m grabbing the slug from the route data, and looking up the associated id, and adding a parameter named “id” to the action parameters with the correct id value.

All I need to do now is apply this filter to the action method and when the action method is called, this id will be passed into the method.

This works whether the argument to the action method is a simple primitive type as in this example or whether it’s a complex type. I’ve included a sample project that demonstrates changing parameters to action methods via an action filter.

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