private void btnDeleteReservation_Click(object sender, EventArgs e)
        {
            var customerID = mainForm.GetSelectedCustomerID();
            var dbm        = new DBmanager();

            dbm.DeleteReservation(customerID);
            MessageBox.Show("Reservation is now annulled");
            this.Close();
        }
        private void btnChangeReservation_Click(object sender, EventArgs e)
        {
            var customerID = mainForm.GetSelectedCustomerID();
            var dbm        = new DBmanager();

            dbm.DeleteReservation(customerID);
            using (var subform = new BookARoomForm(mainForm))
            {
                subform.ShowDialog();
            }
            this.Close();
        }