public virtual void Pay(string a_cardNumber, string a_cardOwner)
{
if (Status != OrderStatus.Unpaid)
{
throw new InvalidOrderOperationException(string.Format("The order can not be paid because it is {0}.",
Status.ToString().ToLower()));
}
Status = OrderStatus.Paid;
Payment = new Payment {CardOwner = a_cardOwner, CreditCardNumber = a_cardNumber};
}