Rewrite URLs on the Fly - 30 Oct 2009

Hot Tip

LANGUAGE: C#

ASP.NET VERSIONS: 1.0 | 1.1

Rewrite URLs on the Fly

Use HttpContext.RewritePath to "fake" URLswithin an application.

By Jeff Prosise

One of the more obscure - but potentially useful - methodsin the portion of the .NET Framework that comprises ASP.NET is the HttpContextclass's RewritePath method. Used internally by ASP.NET to strip session IDsfrom URLs when cookieless session state is enabled, RewritePath can also beused to "fake" URLs within an application. To demonstrate, consider thefollowing ASPX file, which we'll call RewritePath.aspx:

void Page_Load (Object sender, EventArgs e){    string id = Request.QueryString["ID"];    if (id != null&& id != String.Empty) {        switch (id) {          case"1":            Output.Text ="Give me chastity and " +                "continence, but not yet.";            break;          case"2":            Output.Text ="A programmer is a device " +                "forturning coffee into code.";            break;          case"3":            Output.Text ="Blessed is the man who, " +                "having nothing to say, abstains from " +                 "giving wordy evidence of thefact.";            break;        }    }}

If this page is deployed in a virtual directory named Foo,then the following URLs invoke the page and display three different quotations:

http://.../foo/rewritepath.aspx?id=1

http://.../foo/rewritepath.aspx?id=2

http://.../foo/rewritepath.aspx?id=3

So far, so good. Now suppose that you'd like users to beable to display these quotations by typing the following "phantom" URLs:

http://.../foo/quotes/page1.aspx

http://.../foo/quotes/page2.aspx

http://.../foo/quotes/page3.aspx

You can accomplish this little bit of magic - converting aURL of the form .../quotes/page1.aspx into a URL of the form.../rewritepath.aspx?id=1 on the fly - with HttpContext.RewritePath. The trickis to grab each request for /quotes/page1.aspx, /quotes/page2.aspx, and so onas it enters ASP.NET's HTTP pipeline and convert it into a request for/rewritepath.aspx?id=1, /rewritepath.aspx?id=2, and so on. Here's a Global.asaxfile that does just that:

void Application_BeginRequest (Object sender, EventArgs e){    //    // TODO: Convert apath of the form    //.../quotes/page1.aspx into a path of the form    //.../rewritepath.aspx?id=1    //    HttpContext context =HttpContext.Current;    string oldpath =context.Request.Path.ToLower ();      string token ="/quotes/page";    int i =oldpath.IndexOf (token);    int len =token.Length;      if (i != -1) {        int j =oldpath.IndexOf (".aspx");         if (j != -1) {            string id =                oldpath.Substring (i + len, j - (i + len));            string newpath=                oldpath.Replace (token + id + ".aspx",                "/rewritepath.aspx?id=" + id);            context.RewritePath(newpath);        }    }}

Application_BeginRequest fires at the beginning of everyrequest. This implementation extracts the path portion of the URL targeted bythe request (for example, /foo/quotes/page1.aspx), replaces it with a path thatrefers to the real page (for example, /foo/rewrite.aspx?id=1), and callsRewritePath to retarget the request. Try it: Drop RewritePath.aspx into wwwrooton your Web server and try to invoke it by typing this URL into your browser'saddress bar:

http://localhost/quotes/page1.aspx

That should generate a "page not found" error. Now copyGlobal.asax to wwwroot and try again. This time, it should work just fine,thanks to the indirection afforded by Application_BeginRequest.

RewritePath has many applications. You can use it, forexample, to convert query strings into path names so pages that use querystring parameters to drive content can be bookmarked in browsers. You can alsouse it to obfuscate path names within your application for security reasons.Now that you know RewritePath exists, you may find other uses for it, too.

Jeff Prosise isauthor of several books, including Programming Microsoft .NET (Microsoft Press). He also is aco-founder of Wintellect (http://www.wintellect.com/), asoftware consulting and education firm that specializes in .NET. Contact Jeffat   [email protected].