The System.Web.Routing assembly that introduced with .NET 3.5 SP1 brings lots of good stuff to the ASP.NET developers. It was originally part of ASP.NET MVC. There are lots of posts regarding the usage of ASP.NET in Web Form Applications. Check out the posts of Phil Haack and Chris Cavanagh for samples and lots of useful stuffs.

In this post I don’t want to repeat that samples. Here I am going to talk about What URL Routing is and why somebody may want to use it.

When you are using Routing, you can use URLs that do not map to a physical file. Because the URL does not need to map to a physical file, then you can use URLs that have more meaning for the users and search engines.

In ASP.NET, applications that do not use routing or rewriting, an incoming request for a URL typically maps to a physical files on the disk such as “.aspx” file. So a request for /products.aspx?id=xyz maps to file with name “products.aspx” which contains markup and logic for handling the request. Then the webpage uses the query string or posted values to determine what kind of data to display, or which action to be taken. This is still a neat example. Consider below URL be a link to a beautiful painting:

http://sample-art.com/artgallery/default.asp?sid=9DF4BC0580DF11D3ACB60090271E26A8&command=freelist

In MSDN, you can find below definition for ASP.NET Routing:

In ASP.NET routing, you define URL patterns that contain placeholders for values that are used when you handle URL requests. At run time, the pieces of the URL that follow the application name are parsed into discrete values, based on a URL pattern that you have defined.

What a nice definition!!! Let’s write it in a way that dummies like me also could understand it. In routing, you will define some URL patterns. ASP.NET will check these patterns to see which one matches with requested URL. The first matched pattern will be chosen. When you define the pattern, you will also specify to which physical page it should be mapped. Let’s make it clear with an example.

Consider you have a URL like below:

http://yourdomain.com/post/2009/08/24/introduction-to-url-routing.aspx

In URL Routing, you will work with whatever that comes after the domain name. So in our example, we only can handle whatever that comes after http://yourdomain.com/ and you cannot handle the sub domain with URL Routing. To handle this URL, you can use below pattern:

post/{year}/{month}/{day}/{slug}.aspx

And then you can ask from ASP.NET to map this URL to ~/post.aspx. That’s it! It is so clean and very easy! You might ask: what will happen to the values in the pattern? How we can obtain the year, month, day, and slug value? These values will be stored on a structure called Route. Route class has a property called Values. And you can access each part of the URL as below:

int year = Convert.ToInt32(Route.Values["year"]);

As you can see, it is a very clean and straight forward solution. That’s why this method is very interesting. But somebody might say, with URL Rewriting you can do the same thing. Also it is easier because you will pass all the parsed values to the query string and you do not need a new class like Route.

The main difference is, URL Routing working in both directions!! You can give the URL to get the mapped file and parsed values, or you can give the values and get the URL! In URL Rewriting, typically, there is no API to create URL that is based on your pattern. If you have a post with below specifications, how you will know what is URL to that post?

Slug: Introduction-to-url-routing
Date: 2009/08/24

In URL Routing, you can pass above values and you will get your generated URL.

Conclusion

URL routing is a nice and clean alternative for URL rewriter and it fixes the URL issue once forever. URL Routing is based to patterns that you define, so it is easier to manage the mapping process and also to alter them later.

If you are using IIS 7, you can enjoy from extension-less URL with minimum overhead. There are some security issues about URL Routing that you should consider when you are using it. Articles that I mentioned at the beginning of this post cover all those issues.