Showing a client-side message box at the next page load by use the below technique.

   ' Show a client-side message box at the next page load
   ' Example:
   ' ShowMessageBox(Me, "Order successfully submitted!")
   
   public void ShowMessageBox(System.Web.UI.Page webPage, string message)
   {
       // replace ' with \', otherwise the resulting javascript may raise errors
       message = message.Replace("'", "\\'");

       // create the script and add it to the page's response
       string script = "";
        if ((!webPage.IsStartupScriptRegistered("StartupAlert"))) 
        {
            webPage.RegisterStartupScript("StartupAlert", script);
        }
   }