接口的多态化，对重要语句进行注释

Question

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            IPayable[] payers = new IPayable[4];
            payers[0] = new BankAccount(3000);
            payers[1] = new BankAccount(5000);
            payers[2] = new CreditCard(5000);
            payers[3] = new DebitCard((BankAccount)payers[1]);
            foreach(IPayable  payer in payers)
            {
                Receive(payer, 1500);
                Receive(payer, 2000);
            }
        }
        static void Receive(IPayable  payer, decimal money)
        {
            if (payer.Pay(money))
                Console.WriteLine("{0}成功付款{1}元", payer, money);
            else
                Console.WriteLine("{0}付款失败", payer);
        }
    }
}   public interface IPayable
    {
    bool Pay(decimal money);
    }
public class BankAccount : IPayable
{
    protected decimal balance = 0;
    public decimal Balance
    {
        get { return balance; }
        set { balance = value; }
    }
    public BankAccount(decimal balance)
    {
        this.balance = balance;
    }
    public virtual bool Pay(decimal money)
    {
        if (balance >= money)
        {
            balance -= money;
            return true;
        }
        else
            return false;
    }
}
public class CreditCard : BankAccount
{
    private decimal credit;
    public decimal Credit
    {
        get { return credit; }
    }
    public CreditCard(decimal credit)
        : base(0)
    {
        this.credit = credit;
    }
    public override bool Pay(decimal money)
    {
        if (money <= credit + balance)
        {
            balance -= money;
            return true;
        }
        else
            return false;
    }
}
 
    public class DebitCard : IPayable
    {
        private BankAccount account;
        public DebitCard(BankAccount account)
        {
            this.account = account;
        }
        public bool Pay(decimal money)
        {
            return account.Pay(money);
        }
    }

Answer 1

就只是子类继承父类，可以使用父类公共的属性和方法而已。