已有两个接口，IBankAccount接口代表一个银行账户能提供的操作，包括存入、支付、查询余额。ITransferBankAccount接口继承IBankAccount接口，并提供转账功能。
    public interface IBankAccount
    {
        void PayIn(decimal amount);
        bool Withdraw(decimal amount);
        decimal Balance { get; }
    }
    public interface ITransferBankAccount : IBankAccount
    {
        bool TransferTo(IBankAccount destination, decimal amount);
    }
要求写三个类，按以下方式构建继承体系，并重载ToString方法，输出账户主要信息。
1）类SaverAccount继承接口IBankAccount。当支付金额超出余额时应打印出错信息。
2）类TransferBankAccount继承类SaverAccount和接口ITransferBankAccount。
3）类CreditAccount继承IBankAccount，并添加Limit属性，代表透支额度。在支付时，若透支应打印提示信息；若超出透支额度，支付不成功，应打印出错信息。
运行以下测试程序，将输出结果粘贴
IBankAccount sa = new SaverAccount { Balance = 500 };
sa.PayIn(100);
sa.Withdraw(550);
WriteLine(sa);
WriteLine();

ITransferBankAccount tba = new TransferBankAccount { Balance = 700 };
tba.TransferTo(sa, 750);
tba.TransferTo(sa, 200);
WriteLine(tba);
WriteLine();

IBankAccount ca = new CreditAccount { Balance = 100, Limit = 500 };
ca.Withdraw(300);
ca.Withdraw(400);
WriteLine(ca);

解答：
using static System.Console;

namespace ConsoleApp5
{
    public interface IBankAccount
    {
        void PayIn(decimal amount);
        bool Withdraw(decimal amount);
        decimal Balance { get; }
    }
    public interface ITransferBankAccount : IBankAccount
    {
        bool TransferTo(IBankAccount destination, decimal amount);
    }
    public class SaverAccount : IBankAccount
    {
        public void PayIn(decimal amount) => Balance += amount;

        public virtual bool Withdraw(decimal amount)
        {
            if (Balance >= amount)
            {
                Balance -= amount;
                return true;
            }
            WriteLine("支付失败！");
            return false;
        }

        public decimal Balance { get; set; }

        public override string ToString() =>
            $"存款账户：余额 = {Balance,6:C}";
    }
    public class TransferBankAccount : SaverAccount, ITransferBankAccount
    {
        public bool TransferTo(IBankAccount destination, decimal amount)
        {
            bool result = Withdraw(amount);
            if (result)
            {
                destination.PayIn(amount);
                WriteLine("转账成功！");
            }
            return result;
        }
        public override string ToString() =>
            $"转账账户：余额 = {Balance,6:C}";
    }
    public class CreditAccount : SaverAccount
    {
        public decimal Limit { get; set; }
        public override bool Withdraw(decimal amount)
        {
            if (Balance >= amount)
            {
                Balance -= amount;
                return true;
            }
            else if (Balance + Limit > amount)
            {
                Balance -= amount;
                WriteLine("支付成功，已透支！");
                return true;
            }
            else
            {
                WriteLine("超出限额，支付失败！");
                return false;
            }
        }
        public override string ToString() =>
            $"信用卡账户：余额 = {Balance,6:C}, 透支额度 = {Limit,6:C}";
    }

    class Ex4 : Object
    {
        public static void Main()
        {
            IBankAccount sa = new SaverAccount { Balance = 500 };
            sa.PayIn(100);
            sa.Withdraw(550);
            WriteLine(sa);
            WriteLine();

            ITransferBankAccount tba = new TransferBankAccount { Balance = 700 };
            tba.TransferTo(sa, 750);
            tba.TransferTo(sa, 200);
            WriteLine(tba);
            WriteLine();

            IBankAccount ca = new CreditAccount { Balance = 100, Limit = 500 };
            ca.Withdraw(300);
            ca.Withdraw(400);
            WriteLine(ca);
        }
    }
}