public List <String> selectChatUsers(String userID) { List <String> usersID1 = new List <string>(); List <String> usersID2 = new List <string>(); usersID1 = DatabaseTool.ExecSqlReturnItem(String.Format(selectSendUser, userID), "send_id"); usersID2 = DatabaseTool.ExecSqlReturnItem(String.Format(selectReceiveUser, userID), "receive_id"); List <String> result = usersID1.Union(usersID2).ToList <String>(); return(result); }