1.概念:

是面向对象七大原则中最重要的原则.

任何父类出现的地方,子类都可以替代.

父类容器可以装子类对象.子类容器不能装父类.

2.用法

    class GameObject
    {

    }
    class Player : GameObject
    {
        public void PlayerFunc()
        {

        }
    }
    class Boos : GameObject
    {

    }
    class Program
    {
        static void Main(string[] args)
        {
            //里氏替换,父类容器可以装子类
            GameObject player = new Player();
            //此时player不能调用Player类的成员,它已经是GameObject类了
        }
    }

3.is 和 as

is判断一个对象是否为指定对象,返回值bool.

static void Main(string[] args)
{
    //里氏替换,父类容器可以装子类
    GameObject player = new Player();
    //此时player不能调用Player类的成员,它已经是GameObject类了
    if(player is Player)
    {
        //判断player是不是Player类
    }
}

as将一个对象转换为指定类对象.

若成功,返回指定类型对象;失败返回空.

static void Main(string[] args)
{
    //里氏替换,父类容器可以装子类
    GameObject player = new Player();
    //此时player不能调用Player类的成员,即使它本质还是Player类
    if(player is Player)
    {
        //判断player是不是Player类
        //此处会进入函数,player还是Player
    }
    Player p = player as Player;
    //以下会返回空
    //Boos b = player as Boos;//两者没有关联
}

is,as通常搭配使用

//已知player本质还是Player,但却不能用Player的成员
Player p2 = player as Player;//类型转换
p2.PlayerFunc();//可以了

参考子类<唐老狮C#教程>