I have a URL that also might have a query string part, the query string might be empty or have multiple items.
I want to replace one of the items in the query string
I answered a similar question a while ago. Basically, the best way would be to use the class HttpValueCollection, which the QueryString property actually is, unfortunately it is internal in the .NET framework.
You could use Reflector to grab it (and place it into your Utils class). This way you could manipulate the query string like a NameValueCollection, but with all the url encoding/decoding issues taken care for you.
HttpValueCollection extends NameValueCollection, and has a constructor that takes an encoded query string (ampersands and question marks included), and it overrides a ToString() method to later rebuild the query string from the underlying collection.