public class CreditCard
    {
        #region Attributes
        private double limit, rate, balance;
        public double Limit
        {
            get { return this.limit; }
            set { if (value > 0) this.limit = value; }
        }
        public double Rate
        {
            get { return this.rate; }
            set { if (value > 0) this.rate = value; }
        }
        public double Balance
        {
            get { return this.balance; }
            set { this.balance = value; }
        }
        private string ccNumber, name;
        public string CCNumber
        {
            get { return "XXXXXXXXXX" + this.ccNumber.Substring(this.ccNumber.Length - 4, 4); }
            set { this.ccNumber = "XXXXXXXXXX" + value.Substring(value.Length - 4, 4); }
        }
        public string Name
        {
            get { return this.name; }
            set { this.name = value; }
        }
        private Collection<Payment> payments = new Collection<Payment>();
        public Collection<Payment> Payments
        {
            get { return payments as Collection<Payment>; }
            set { payments = value; }
        }
        DateTime lastUpdated;
        public DateTime LastUpdated
        {
            get { return this.lastUpdated; }
            set { this.lastUpdated = value; }
        }
        #endregion
        #region Constructors
        public CreditCard()
            : this("Unknown", "Unknown", 0.0, 0.0, 0.0)
        {
        }
        public CreditCard(string name, string ccNumber, double limit, double rate)
            : this(name, ccNumber, limit, rate, 0)
        {
        }
        public CreditCard(string name, string ccNumber, double limit, double rate, double balance)
        {
            this.name = name;
            this.lastUpdated = DateTime.Now;
            this.ccNumber = ccNumber;
            this.limit = limit;
            this.rate = rate;
            this.balance = balance;
            if (this.rate < 1)
                rate *= 100;
        }
        #endregion
        #region Methods
        public void update()
        {
            balance = 0;
            foreach (Payment p in payments)
            {
                //balance += ((float)p.Amount / 12.0f) * (this.rate / 100) * p.DaysSincePurchase();
            }
        }
        public void addPayment(Payment payment)
        {
            payments.Add(payment);
        }
        public void removePayment(Payment payment)
        {
            payments.Remove(payment);
        }
        public void increaseLimit(double inc)
        {
            if (inc > 0)
                limit += inc;
            else
                limit -= inc;
        }
        public void decreaseLimit(double dec)
        {
            if (dec > 0)
                limit -= dec;
            else
                limit += dec;
        }
        public override string ToString()
        {
            StringBuilder s = new StringBuilder();
            s.AppendLine("Name on Card:  " + name);
            s.AppendLine("CC Number:  " + ccNumber);
            s.AppendLine("Balance:  " + balance);
            s.AppendLine("Limit:  " + limit);
            s.AppendLine("Rate:  " + rate);
            s.AppendLine("Payments: \n--------------");
            foreach(Payment p in payments)
                s.AppendLine(p.ToString());
            s.AppendLine("--------------");
            return s.ToString();
        }
        public void Save()
        {
            try
            {
                XmlSerializer serializer = new XmlSerializer(typeof(CreditCard));
                XmlTextWriter writer = new XmlTextWriter("CC_" + this.ccNumber.Substring(this.ccNumber.Length - 4, 4) + ".xml", null);
                serializer.Serialize(writer, this);
            }
            catch (IOException e) { }
        }
        public static CreditCard Load(string file)
        {
            XmlSerializer serlializer = new XmlSerializer(typeof(CreditCard));
            return (CreditCard)serlializer.Deserialize(File.OpenRead(file));
        }
        #endregion
    }

public class Payment
    {
        #region Attributes
        DateTime purchaseDate;
        public DateTime PurchaseDate
        {
            get { return this.purchaseDate; }
            set { this.purchaseDate = value; }
        }
        double amount;
        public double Amount
        {
            get { return this.amount; }
            set { this.amount = value; }
        }
        string item, description;
        public string Item
        {
            get { return this.item; }
            set { this.item = value; }
        }
        public string Description
        {
            get { return this.description; }
            set { this.description = value; }
        }
        #endregion
        #region Constructors
        public Payment()
            : this(new DateTime(0, 0, 0), 0.0, "Not Set", "No Description")
        {

        }
        public Payment(DateTime purchaseDate, double amount, string item)
            : this(purchaseDate, amount, item, "No Discription")
        {
        }
        public Payment(DateTime purchaseDate, double amount, string item, string description)
        {
            this.purchaseDate = purchaseDate;
            this.amount = amount;
            this.item = item;
            this.description = description;
        }
        #endregion
        #region Methods
        public int DaysSincePurchase()
        {
            return DateTime.Now.Subtract(purchaseDate).Days;
        }
        public override string ToString()
        {
            StringBuilder s = new StringBuilder();
            s.AppendLine("Item:  " + item);
            s.AppendLine("Description:  " + description);
            s.AppendLine("Amount:  " + amount.ToString());
            s.AppendLine("Date of Purchase:  " + purchaseDate.ToString());
            return s.ToString();
        }
        #endregion
    }