URL Rewriting in ASP.NET

What is "URL rewriting"?

Most sites include variables in their URLs that tell the site what information to be shown to the user. It is something like telling the code to load particular details of item number 7 from a shopping site.

For example, the site may look as in the following:

http://www.cshopping.com/showitem.aspx?itemid=7

The problem with the preceding URL is it cannot be remembered. It is even difficult to read on your mobile. Even the search engines like Google cannot get any information about the content of a page from the URL. What if you wanted to convey that itemid = 7 means a laptop with brand as "DELL"? This is not saying what it should say.

So, what we expect from the preceding URL is it should be easily understandood. Will it not be good if the URL is as below?

http://www.cshopping.com/laptops/Dell/

Now looking at the preceding URL you can easily tell that it is related to laptops and the brand is DELL. So, there is a need to rewrite the URL to be somewhat meaningful and easily conveyable.

How to rewrite URL in ASP.NET?

To rewrite URLs in ASP.NET we are going to use HTTP Modules. HTTP Modules are called before and after the HTTP handler executes. HTTP modules help us to intercept, participate in, or modify each individual request. HTTP Modules implement an IHttpModule interface, which is located in the System.Web namespace. Modules are the ones which handles authentication and authorization of the ASP.NET applications. If you want to implement encryption of the URL string or any custom changes to the application, it can be done by writing our HTTP Module.

Straight to the point, let us create a class in your app_Code directory called "URLRewriter.cs" and add the following code:

1. Create a web site using Visual Studio and name it as "URLRewriteTestingApp" and add a class file "URLRewriter.cs".

2. Visual Studio will ask whether to create a directory called "App_Code" and place the class file in the folder. Select "OK" to proceed.

Your solution structure should look something like this:

3. Now open the file "URLRewriter.cs" and add the code for HTTP Module as below.

Basically your class must implement the IHttpModule interface to implement its methods. You will see these 2 methods created for you once you implement the interface.

• Dispose
• Init

The Init method takes HttpApplication as a parameter. So, you need to create an event handler for the first request of the HttpApplication to handle the request.

Here is your code for the Module:

using System;
using System.Web;
/// <summary>
/// Summary description for URLRewriter
/// </summary>
public class URLRewriter : IHttpModule
{
    #region IHttpModule Members
    /// <summary>
    /// Dispose method for the class
    /// If you have any unmanaged resources to be disposed
    /// free them or release them in this method
    /// </summary>
    public void Dispose()
    {
        //not implementing this method
        //for this example
    }
    /// <summary>
    /// Initialization of the http application instance
    /// </summary>
    /// <param name="context"></param>
    public void Init(HttpApplication context)
    {
        context.BeginRequest += new EventHandler(context_BeginRequest);
    }
    /// <summary>
    /// Event handler of instance begin request
    /// </summary>
    /// <param name="sender"></param>
    /// <param name="e"></param>
    void context_BeginRequest(object sender, EventArgs e)
    {
        //Create an instance of the application that has raised the event
        HttpApplication httpApplication = sender as HttpApplication;
        //Safety check for the variable httpApplication if it is not null
         if (httpApplication != null)
         {
             //get the request path - request path is    something you get in
             //the URL
             string requestPath = httpApplication.Context.Request.Path;
             //variable for translation path
             string translationPath = "";
             //if the request path is /urlrewritetestingapp/laptops/dell/
            //it means the site is for DLL
            //else if "/urlrewritetestingapp/laptops/hp/"
            //it means the site is for HP
            //else it is the default path
            switch (requestPath.ToLower())
            {
                case "/urlrewritetestingapp/laptops/dell/":
                    translationPath = "/urlrewritetestingapp/showitem.aspx?itemid=7";
                    break;
                case "/urlrewritetestingapp/laptops/hp/":
                    translationPath = "/urlrewritetestingapp/showitem.aspx?itemid=8";
                    break;
                default:
                    translationPath = "/urlrewritetestingapp/default.aspx";
                    break;
            }
            //use server transfer to transfer the request to the actual translated path
            httpApplication.Context.Server.Transfer(translationPath);
        }
    }
    #endregion
}