1. is 返回bool类型，指示是否可以做这个转换（使用is并没有转换

  2. as 如果转换成功，则返回对象，否则返回null

上代码

    class Program
    {
        static void Main(string[] args)
        {
            Person p = new Student();
            if (p is Teacher)
            {
                ((Teacher)p).TeacherSayHello();
            }
            else
            {
                Console.WriteLine("转换失败");
            }
// 运行结果，转换失败
            Student s = p as Student;//将person转换为student对象
            if (s!=null)
            {
                s.StudentSayHello();
            }
            else
            {
                Console.WriteLine("转换失败");
            }
            //运行为子类的方法
            Console.ReadKey();
        }
    }

<mark>父类</mark>

    class Person
    {
        public void PersonSayHello()
        {
            Console.WriteLine("我是父类！");
        }

    }

<mark>子类①</mark>

    class Teacher : Person
    {
        public  void TeacherSayHello()
        {
            Console.WriteLine("我是老师类！");
        }
    }

<mark>子类②</mark>

    class Student : Person
    {
        public  void StudentSayHello()
        {
            Console.WriteLine("我是学生类！");
        }
    }