C# 继承与多态,相关关键字virtual、override、new

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication26
{
    class Program
    {
        static void Main(string[] args)
        {
            animal a = new animal();
            a.name = "animal";
            a.say();
            cat c = new cat();
            c.name = "cat";
            c.say();
            dog d = new dog();
            d.name = "dog";
            d.say();
            sheep s = new sheep();
            s.name = "sheep";
            s.say();
            animal a1 = new cat();
            a1.say();
            animal a2 = new dog();
            a2.say();
            animal a3 = new sheep();
            a3.say();
        }
    }
    class animal
    {
        public string name { get; set;}
        public virtual void say()
        {
            Console.WriteLine("animal say");
        }
    }
    class cat : animal
    {
        public override void say()
        {
            Console.WriteLine("cat say");
        }
    }
    class dog : animal
    {
        public new void say()
        {
            Console.WriteLine("dog say");
        }
    }
    class sheep : animal
    {
    }
}