private void SendEnumRecentUser()
 {
     Task.Run(() =>
     {
         Dictionary <string, int> result = new Dictionary <string, int>();
         foreach (var record in recorder.GetHistoryList())
         {
             result[record.userName] = (record.id);
         }
         StringBuilder sb = new StringBuilder();
         foreach (var pair in result)
         {
             sb.AppendFormat("{0}->{1} || ", pair.Value, pair.Key);
         }
         SendResponseMessage(sb.ToString());
     });
 }