/// <summary>
        /// 记录联系人 customer 的预订 bookings 事件
        /// </summary>
        internal void Log_Booked(ICustomer customer, List <IBooking> bookings)
        {
            string rooms = null;

            foreach (IBooking bk in bookings)
            {
                rooms += (bk.Roomtype.ToString() + "("
                          + bk.StartDate.ToShortDateString().ToString() + " "
                          + bk.EndDate.ToShortDateString().ToString() + ");");
            }
            dbCon.CreateLog(customer.Name + " booked " + bookings.Count + " rooms: " + rooms);
        }