title: URL Querystring - Find, replace, add, update values?
body: <p>We inherited some C# code as part of a project from another company which does URL redirects that modifies the existing query string, changing values of items, adding new params, etc as needed.  The issue however is that the code is buggy at best, and ends up duplicating items in the query string instead of updating them properly. The code works on the first pass but on additional calls the duplication issues become apparent.</p>
  
  <p>Ex: MyPage.aspx?startdate=08/22/09&amp;startdate=09/22/09</p>
  
  <p>Instead of duplicating the item it needs to be either updated with the new value if it already exists, or added if not there already.</p>
  
  <p>Is there a C# class or set of functions for handling query strings, allowing a simple means to access and update/add parameters that gets around these issues instead of the blind add approach that seems to be in use now with the code?  This needs to be able to handle multiple parameters that may or may not exists at all times and be added and updated on subsequent calls.</p>
  
  <p>We would sooner use existing logic than recreate something if possible so as to get this resolved quickly in a semi standard way for future maintainability and reuse.</p>