How to show a confirmation message using client.registerstartupscript in the server side with update panel in ASP.NET with C#?

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C#

ASP.NET

I want to show a confirmation message using client.registerstartupscript in the server side with update panel in asp.net with c# on a button click .

I added a update panel to show a loading gif image using thread.sleep for a longer task. But with it the confirmation message using client.registerstartuoscript server side. Without the update panel it works.

What I have tried:

Code is as below:

#region ShowMessage

      public static void showMessage(string strMessage, Page page)
      {
          try
          {
              Type csType = typeof(String);
              int nKey = 0;
              nKey++;
              string strKey = nKey.ToString();
              string strScript = string.Empty;

              ClientScriptManager clientScript = page.ClientScript;
              if (clientScript.IsStartupScriptRegistered(csType, strKey))
              {
                  nKey++;
                  strKey = nKey.ToString();
              }
              strScript = "<script language = 'javascript'>alert('" + strMessage + "')</script>";
              page.ClientScript.RegisterStartupScript(csType, strKey, strScript);
          }
          catch (Exception ex)
          {
              Common.WriteErrorLog(ex);
          }
      }
      #endregion