I haven't used urlrewriter.net but URL rewriting works by specifying a rewrite rule in server configurations i.e. if you are using IIS7 then URL Rewrite tool is used for the purpose and only server has access to the rule. So I would imagine no matter what you do in C#, you would only ever get the long/original URL and not the rewritten one. The rewrite URL is just a regular expression which replaces old URL with the new one at the server level, so your C# code doesn't quite get to read the new URL as the server just internally re-directs you to the new URL. You can may be try specifying a reverse write rule or just send the new URL as a GET parameter with the original URL (not sure how exactly, but a rough idea).
See if this does anything to help you :
URL Rewriting in ASP.NET[
^]
EDIT: something more for you :
URL Rewriting with ASP.NET[
^]
Turns out, if you have a finite number of URLs to rewrite then you can define them in the web.config and then u can read in your C# code using the ConfigurationManager class. Hack around with it.
Cheers...