public void CheckAppIdAvailability(string id, CheckAppIdCallback callback) { var client = new WebClientWithCookies(); var url = "https://appengine.google.com/start/checkappid?app_id=" + HttpUtility.UrlEncode(id); client.DownloadStringAsync(new Uri(url)); client.DownloadStringCompleted += (sender, e) => { if (e.Error != null) { callback(false); } else { callback(!e.Result.Contains("invalid")); } }; }