private static void AppendObjectCallback(IAsyncResult ar)
 {
     try
     {
         var result = client.EndAppendObject(ar);
         Console.WriteLine("Append object succeeded, next append position:{0}", result.NextAppendPosition);
         Console.WriteLine(ar.AsyncState as string);
     }
     catch (Exception ex)
     {
         Console.WriteLine(ex.Message);
     }
     finally
     {
         _event.Set();
     }
 }