private void BtnPickSeat_Click(object sender, RoutedEventArgs e)
        {
            if (Validation() == true)
            {
                flight = CbFlNum.SelectedItem as Flight;
                clas   = (EClass)CbSeatClass.SelectedItem;

                PickSeatWindow s = new PickSeatWindow(flight, clas);
                if (s.ShowDialog() == true)
                {
                    ticket.SeatNum = s.SelectedSeat;
                }
            }
        }
        private void BtnSeat_Click(object sender, RoutedEventArgs e)
        {
            if (cbClass.SelectedIndex > -1)
            {
                clas = (EClass)cbClass.SelectedItem;
                if (clas.Equals(EClass.BUSINESS))
                {
                    txtPrice.Text = (flight.OneWayTicketPrice * (decimal)5.0).ToString();
                }

                PickSeatWindow ps = new PickSeatWindow(flight, clas);
                if (ps.ShowDialog() == true)
                {
                    SelectedSeat = ps.SelectedSeat;
                    txtSeat.Text = SelectedSeat.SeatLabel.ToString();
                }
            }
            else
            {
                MessageBox.Show("You must select a class");
            }
        }