using System;
using System.Collections.Generic;
using System.Text;
namespace ConsoleApplication3
{
    public abstract class Book
    {
        //抽象方法，不含主体，抽象方法所在类必须为抽象类，派生类必须实现该方法
        public abstract void Introduce();
    }
    public class JavaBook : Book
    {
        //实现抽象方法，必须实现，注意!必须添加override关键字
        public override void Introduce()
        {
            Console.WriteLine("I'm Java");
        }
    }

    public class test
    {
        public test()
        {
            JavaBook javaBook = new JavaBook();
            javaBook.Introduce();      //将调用JavaBook中Introduce()
            Book book = new JavaBook();
            book.Introduce();      //将调用JavaBook中Introduce()
        }
        public static void Main()
        {
            test t = new test();
        }
    }
}

using System;
using System.Collections.Generic;
using System.Text;
namespace ConsoleApplication2
{
    public abstract class Book
    {
        public virtual void Introduce()
        {
            Console.WriteLine("I'm book");
        }
        public virtual void SayHi()
        {
            Console.WriteLine("Hi, I'm book");
        }
    }

    public class JavaBook : Book
    {
        public override void Introduce()
        {
            Console.WriteLine("I'm Java");
        }
        //注意，此方法没有override父类的方法
        public void SayHi()
        {
            Console.WriteLine("Hi, I'm Java");
        }
    }

    public class test
    {
        public test()
        {
            JavaBook javaBook = new JavaBook();
            Book book = new JavaBook();
            javaBook.Introduce();       //将调用JavaBook中Introduce()
            book.Introduce();       //将调用JavaBook中Introduce()
            javaBook.SayHi();      //将调用JavaBook中SayHi()
            book.SayHi();           //将调用Book中SayHi()
        }
        public static void Main()
        {
            test t = new test();
        }
    }
}