Removing route values from links/URLs in Asp.net MVC

I didn't really know how to properly title this post to make it easily searchable by those who bump into routing-related issue. Words like arbitrary, ambient, unwanted, unneeded, extra, unrelated etc route values popped into my mind, but I decided to title it as it is now.

One of the main pillars of Asp.net MVC is routing. Many applications can just use default route definition to cover all their scenarios but some applications require it to be a bit more complex. Look at this example:

   1:  routes.MapRoute(

   2:      "CustomerSpecific",

   3:      "Customers/{customerId}/{controller}/{action}/{id}"

   4:      new { controller = "Customers", action = "Index", id = UrlParameter.Optional },

   5:      new { customerId = @"\d+" }

   6:  );

   7:   

   8:  routes.MapRoute(

   9:      "Default",

  10:      "{controller}/{action}/{id}",

  11:      new { controller = "Home", action = "Index", id = UrlParameter.Optional }

  12:  );

Seems fine? Well it does and it works, but you will have problems when you'll be anywhere in http://www.domain.com/Customers/n/... and would like to also generate links to parts of your application that are covered by default route definition (second route). Let me show you why.

Improving Asp.net MVC maintainability and RESTful conformance

I've used Asp.net MVC for a few years now but this issue I've stumbled upon just a few days ago seems something everydayish and I wonder how come I've never bumped into it. It has to do with Asp.net MVC routing and action method selection. First of all think of default route definition that looks like this:

   1:  // default application route

   2:  routes.MapRoute(

   3:      "Default",

   4:      "{controller}/{action}/{id}",

   5:      new { controller = "Home", action = "Index", id = UrlParameter.Optional }

   6:  );

Mind that id route value is optional? Yes optional. So it should be perfectly feasible to have two action methods: one with the id parameter and one without it: public ActionResult Index() { ... } and public ActionResult Index(int id) { ... } But you've probably guessed it? This doesn't work out of the box. You'll get a runtime error stating that there are two matching action methods for the current request. A bit strange? I thought so as well. So let's try and accomplish just that!

Custom Asp.net MVC route class with catch-all segment anywhere in the URL

Asp.net MVC routing does a fine job with routes that have a finite number of segments. We define them with route URL pattern string. The default provided by the Asp.net MVC project template being {controller}/{action}/{id}. Most web applications can do everything using only this single route definition and many developers don't even think beyond this standard. But sometimes this single route just isn't enough or it's just not acceptable.

A real world example

Think of a web site you're building that has hierarchically organised categories. Like Amazon. We have books, music, electronics, etc. And every top category has sub categories. And so on and so forth. Using default route definition we would access a particular (sub)category simply by: www.domain.com/categories/index/647 That's fine, but it's definitely not human friendly. If we'd change our route URL definition to {controller}/{action}/{id}/{name} this would already be much friendlier: www.domain.com/categories/index/647/web-development That's something similar (not the same, because we're still using action names here) to what Stackoverflow does with it's questions.

But now think of this super human readable web address: www.domain.com/books/computer-internet/web-development/latest This one would display latest web development books. As we can see it defines categories in hierarchical order similar to breadcrumbs. All in human readable format. Doing this kind of routing isn't supported out of the box (because we have an action at the end), but I think we could do better. Let's try and create a route that supports this.

Writing a custom IHttpModule that handles Application_OnStart event

If you've been developing Asp.net web applications you've probably come across the IHttpModule interface, that makes it possible to write reusable request-level event handlers. The same thing can of course be done by writing event handlers inside the Global.asax codebehind, but then you wouldn't be able to reuse the same code unless you do a copy/paste.

But Global.asax codebehind has one particular advantage over your custom modules: it can also attach to application-level events like application start event for instance. As per documentation, this is not possible with a custom module. Or so I thought...