This should work:
string url = "http://example.com/file?a=1&b=2&c=string%20param";
string querystring = url.Substring(url.IndexOf('?'));
System.Collections.Specialized.NameValueCollection parameters =
System.Web.HttpUtility.ParseQueryString(querystring);
According to MSDN. Not the exact collectiontype you are looking for, but nevertheless useful.
Edit: Apparently, if you supply the complete url to ParseQueryString
it will add 'http://example.com/file?a' as the first key of the collection. Since that is probably not what you want, I added the substring to get only the relevant part of the url.