map url segment to querystring only if not already in querystring

Feb 14, 2009 at 5:21 AM

how can write a url rule that maps url segments to querystring parameters only if the parameter is not provided already.
e.g. http://www.abcd.com/XXXX/?Listid=1211 ---> http://wwww.abcd.com/XXXX/?store=XXXX&ListID=1211
but http://www.abcd.com/XXXX/?store=XXXX&Listid=1211 should not be rewritten. (Note: store=XXXX can appear anywhere in the querystring)