虚方法

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

namespace ConsoleApplication1 {

 class Program { 

static void Main(string[] args) {

 List list = new List(); 
list.Add(new Customer()); 
list.Add(new VIPACustomer()); 
list.Add(new VIPBCustomer()); 
list.Add(new VIPCustomer()); 
foreach(Customer c in list) { c.Buy(); 

}
 Console.ReadLine(); } 
} 

class Customer {

 public virtual void Buy() { 
//普通客户使用该方法 
Console.WriteLine("Cutomer"); } 

} 
class VIPCustomer : Customer {

 public override void Buy() { 

Console.WriteLine("VIP Cutomer"); }

 }


 class VIPACustomer : VIPCustomer {

 public override void Buy() {

 Console.WriteLine("VIP A Cutomer");

 } 

} 

class VIPBCustomer : VIPCustomer { 

public override void Buy() { 

Console.WriteLine("VIP B Cutomer"); } 

} 

}