string longurl = "http://somesite.com/news.php?article=1&lang=en";
var uriBuilder = new UriBuilder(longurl);
var query = HttpUtility.ParseQueryString(uriBuilder.Query);
query["action"] = "login1";
query["attempts"] = "11";
uriBuilder.Query = query.ToString();
longurl = uriBuilder.ToString();
// "http://somesite.com:80/news.php?article=1&lang=en&action=login1&attempts=11"