using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Web;

 /// <summary>
 ///Calculator 的摘要说明
 /// </summary>
 public abstract class Calculator
 {
     public abstract double Cal(double a, double b);
 }
 public class Add : Calculator      //派生类Add继承抽象类Calculator
 {
     public override double Cal(double a, double b)//并重写了抽象方法Cal
     {
         ;
         result = a + b;
         return result;
     }
 }
 public class Sub : Calculator
 {
     public override double Cal(double a, double b)
     {
         ;
         result = a - b;
         return result;
     }
 }
 public class Mul : Calculator
 {
     public override double Cal(double a, double b)
     {
         ;
         result = a * b;
         return result;
     }
 }
 public class Div : Calculator
 {
     public override double Cal(double a, double b)
     {
         ;
         result = a / b;
         return result;
     }
 }
 public class Context           //上下文
 {
     private Calculator calculate = null;//实例化一个基类的引用对象
     public Context(Calculator _cal)//_cal为派生类的一个对象
     {
         this.calculate = _cal;    //把派生类的对象赋给基类的引用对象
     }
     public double Cal(double a, double b, String symbol)
     {
         return this.calculate.Cal(a, b);//返回计算结果
     }
 }

 using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Web;
 using System.Web.UI;
 using System.Web.UI.WebControls;

 public partial class index : System.Web.UI.Page
 {
     protected void Page_Load(object sender, EventArgs e)
     {
         //界面加载
     }
     protected void Cal_Click(object sender, EventArgs e)
     {
         string symbol = DropDownList1.SelectedItem.ToString();
         double a = Convert.ToDouble(TextBox1.Text);
         double b = Convert.ToDouble(TextBox2.Text);
         Context contex = null;
         )
         {
             contex = new Context(new Add());    //加法策略
         }
         )
         {
             contex = new Context(new Sub());    //减法策略
         }
         )      //若为乘号
         {
             contex = new Context(new Mul());    //乘法策略
         }
         )      //若为乘号
         {
             contex = new Context(new Div());    //除法策略
         }
         string answer = contex.Cal(a, b, symbol).ToString();  //用answer来存计算出来的答案,此时已经计算出a,b两个数的运算结果。

         string result = TextBox1.Text + DropDownList1.SelectedItem.ToString() + TextBox2.Text;//把运算式子存在result里面
         if (TextBox3.Text == answer)                                  //如果输入答案与计算出的answer相等
         {
             Response.Write("<script>alert('回答正确!')</script>");          //弹出回答正确
             ListBox1.Items.Add(result + "=" + TextBox3.Text.Trim() + "√");//并把运算式子存在listbox里
         }

         else                                                          //如果答错
         {
             Response.Write("<script>alert('答题错误!')</script>");           //弹出答题错误
             ListBox1.Items.Add(result + "=" + TextBox3.Text.Trim() + "×");//同样把运算式子放在listbox
         }
         TextBox1.Text = "";//把文本框清空，进行下一次出题
         TextBox2.Text = "";
         TextBox3.Text = "";
     }
 }