C#父类与子类（多态性）

using System;

namespace 父类子类
{
    class Person
    {
        public Person()
        {
            Console.WriteLine("我是父类构造函数");
        }
        public virtual void SayHello()
        {
            Console.WriteLine("父类SayHello");
        }
    }
    class Son : Person
    {
        public Son()
        {
            Console.WriteLine("我是子类构造函数");
        }
        public override void SayHello()
        {
            Console.WriteLine("子类SayHello");
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            Person p1 = new Person();
            p1.SayHello();
            Console.WriteLine("===============================");
            Son s1 = new Son();
            s1.SayHello();
            Console.WriteLine("===============================");
            Person p2 = new Son();
            p2.SayHello();
            Console.WriteLine("===============================");
            Son s2 = new Son();
            Person ps2 = (Person)s2;
            ps2.SayHello();
            Console.WriteLine("===============================");
            Person p3 = new Son();
            Son s3 = (Son)p3;
            s3.SayHello();
            Console.WriteLine("===============================");
            Person p4 = new Person();
            Son s4 = (Son)p4;   //报错！
            s4.SayHello();
        }
    }
}

　运行结果如下: