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);
}