c# update check

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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);                       }                         }