购物（工厂模式）

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace 购物_工厂模式_
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        //3.开始写事件了
        private void button1_Click(object sender, EventArgs e)
        {
            try
            {
                double t1 = Convert.ToDouble(textBox1.Text);
                double t2 = Convert.ToDouble(textBox2.Text);

                label7.Text = (t1 * t2).ToString();
            }
            catch
            {
                MessageBox.Show("请输入数量或单价");
            }

            MyClass mc = GongChang.Createclass(comboBox1.SelectedItem.ToString());
            label4.Text = mc.at(Convert.ToDouble(label7.Text)).ToString();
        }
    }
    //2.建一个工厂类：
    //用这个类 调用其余类中的函数
    public class GongChang
    {
        public static MyClass Createclass(string c)
        {
            switch (c)
            {
                case "正常收费":
                    return new MyClass1();
                case "打8折":
                    return new MyClass2(0.8);
                case "满300送50":
                    return new MyClass3(300, 50);
                default:
                    return null;
            }
        }
    }

    //1.首先三种付款方式的类
    //建立个抽象类，使用抽象方法传入没有付款方式时的价钱
    //以后重写抽象方法，更改价钱（三种付款方式）
    public abstract class MyClass
    {
        public abstract double at(double money);
    }

    //三种方式:
    //正常收费
    public class MyClass1 : MyClass
    {
        public override double at(double money)
        {
            return money;
        }
    }

    //打折
    public class MyClass2 : MyClass
    {
        private double dazhe = 1;
        public MyClass2(double dze)
        {
            dazhe = dze;
        }

        public override double at(double money)
        {
            return money * dazhe;
        }


    } //满300免50
    public class MyClass3 : MyClass
    {
        private double x = 0;
        private double y = 0;
        public MyClass3(double _x, double _y)
        {
            x = _x;
            y = _y;
        }

        public override double at(double money)
        {
            if (money > x)
            {
                return money - Math.Floor(money / x) * y;//300的倍数*50
            }
            else
                return money;
        }

    }
}