策略模式

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

namespace WindowsFormsApplication3
{
    abstract class CashSuper
    {
        public abstract double acceptCash(double money);
    }

    //正常消费
    class CashNomal : CashSuper
    {
        public override double acceptCash(double money)
        {
            return money;
        }
    }

    //打折消费
    class CashRebate : CashSuper
    {
        double rebate = 0;

        public CashRebate(double rebate)
        {
            this.rebate = rebate;
        }

        public override double acceptCash(double money)
        {
            return money * rebate;
        }
    }

    //满300减100
    class CashReturn : CashSuper
    {
        private double moneyCondition = 0.0d;
        private double moneyReturn = 0.0d;

        public CashReturn(double condition, double dReturn)
        {
            moneyCondition = condition;
            moneyReturn = dReturn;
        }

        public override double acceptCash(double money)
        {
            if (money >= moneyCondition)
                return money - (Math.Floor(money / moneyCondition) * moneyReturn);

            return money;
        }
    }

    class CashFactory
    {
        public static CashSuper createCashAccess(string type)
        {
            CashSuper cash = null;

            switch (type)
            {
                case "正常收费":
                    cash = new CashNomal();
                    break;
                case "满300返100":
                    cash = new CashReturn(300, 100);
                    break;
                case "打折":
                    cash = new CashRebate(0.8);
                    break;
                default:
                    break;
            }

            return cash;
        }
    }

    class CashContext
    {
        private CashSuper cs;

        public CashContext(CashSuper csuper)
        {
            this.cs = csuper;
        }

        public double GetResult(double money)
        {
            return cs.acceptCash(money);
        }
    }
}

调用类:

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 WindowsFormsApplication3
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            comboBox1.Items.AddRange(new string[] { "正常收费", "满300返100", "打八折" });
            comboBox1.SelectedIndex = 0;
        }

        private void button1_Click(object sender, EventArgs e)
        {
            CashContext cc = null;

            switch (comboBox1.SelectedItem.ToString())
            {
                case "正常收费":
                    cc = new CashContext(new CashNomal());
                    break;
                case "满300返100":
                    cc = new CashContext(new CashReturn(300, 100));
                    break;
                case "打八折":
                    cc = new CashContext(new CashRebate(0.8));
                    break;
                default:
                    break;
            }

            double totalPrices = 0d;
            totalPrices = cc.GetResult(Convert.ToInt32(textBox1.Text) * Convert.ToDouble(textBox2.Text));
            lblResult.Text = totalPrices.ToString();
        }
    }
}