c# update check

public  class UpdateChecker {

        public static  event EventHandler completeCheck;
        private static  bool isChecking = false;
        private static WebClient wc;
        public static  void doUpdateCheck() {

            if (isChecking) return;
            isChecking = true ;
             wc = new WebClient();
             wc.DownloadStringCompleted += new DownloadStringCompletedEventHandler(wc_DownloadStringCompleted);
             wc.DownloadStringAsync(new Uri("https://files.cnblogs.com/files/wgscd/appupdate.xml"));
        }

        private   static void wc_DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)
        {
            try
            {
                // MessageBox.Show(""+e.Result ,"update",MessageBoxButton.YesNo);
                XmlDocument doc = new XmlDocument();
                doc.LoadXml("" + e.Result);
                string localVar="3.8";//test
                string serverVer = doc.SelectSingleNode("app").Attributes["version"].Value;
                string serverDate = doc.SelectSingleNode("app").Attributes["update_date"].Value;
                string tip = doc.SelectSingleNode("app/tip").InnerText;
                if (localVar.CompareTo (serverVer)<0) {
                    MessageBox.Show("the new version: " + serverVer + "\r\n " + tip + " \r\n update?\r\n", "update", MessageBoxButton.YesNo);
                }
                if (completeCheck != null) { completeCheck(sender, e); }
                isChecking = false;
            }
            catch { }
            wc.DownloadStringCompleted -= new DownloadStringCompletedEventHandler(wc_DownloadStringCompleted);

        }

    }