public MonthlyBill(BillingPeriod billingPeriod, string description, PaymentStatus status, decimal amount, CreditCardInformation creditCardInfo)
 {
     BillingPeriod  = billingPeriod;
     PaymentStatus  = status;
     Description    = description;
     Amount         = amount;
     CreditCardInfo = creditCardInfo;
 }
 public void AddMonthlyBill(BillingPeriod billingPeriod, PaymentStatus status, decimal amount, CreditCardInformation creditCardInfo)
 {
     BillingHistory.Add(
         new MonthlyBill(billingPeriod, CurrentSubscriptionName, status, amount, creditCardInfo)
         );
 }