声明接口在语法上与声明抽象类完全相同，但不允许提供接口中任何成员的实现方式。 一般情况下,接口只能包含方法、属性、索引器和事件的声明。不能实例化接口 ,它只能包含其成员的 签名。接口既不能有构造函数，接口定义也不允许包含运算符重载。

6.1 定义和实现接口

using System;using System.Collections.Generic;using System.Linq;using System.Text;namespace 定义接口{    public interface IBankAccount    {        void PayIn(decimal amount);        bool Withdraw(decimal amount);        decimal Balance        { get; }    }    class Program    {        static void Main(string[] args)        {            IBankAccount x = new SaveAccount();            IBankAccount y = new SaveAccount();            x.PayIn(200);            x.Withdraw(100);            Console.WriteLine(x.ToString());            y.PayIn(500);            y.Withdraw(600);            y.Withdraw(100);            Console.WriteLine(y.ToString());        }    }}using System;using System.Collections.Generic;using System.Linq;using System.Text;namespace 定义接口{    public class SaveAccount : IBankAccount    {        private decimal balance;        public void PayIn(decimal amount)        {             balance+= amount;        }        public bool Withdraw(decimal amount)        {            if (balance >= amount)            {                balance -= amount;                return true;            }            else            {                Console.WriteLine("Withdraw1 attempt failed");                return false;            }        }        public decimal Balance        {            get            {                return balance;            }        }        public override string ToString()        {            return String.Format("Venus Bank Saver:Balance={0,6:C}", balance);        }    }}using System;using System.Collections.Generic;using System.Linq;using System.Text;namespace 定义接口{    public class GoldAccount : IBankAccount    {        private decimal balance;        public void PayIn(decimal amount)        {            balance += amount;        }        public bool Withdraw(decimal amount)        {            if (balance >= amount)            {                balance -= amount;                return true;            }            else            {                Console.WriteLine("Withdraw1 attempt failed");                return false;            }        }        public decimal Balance        {            get            {                return balance;            }        }        public override string ToString()        {            return String.Format("Venus Bank Saver:Balance={0,6:C}", balance);        }    }}