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"