商场经常在节日期间做出不同的促销计划，因此商场的计费机制要不停地变动，因此在设计时应该考虑使用策略模式

策略模式（Strategy），策略模式定义了算法家族，分别封装起来，让它们之间可以相互替换，此模式让算法的变化，不会影响到使用算法的客户。

通过UML图来说事：

其中CashContext算是一个简单工厂吧。现在有些理解简单工厂了，其实并不是一个设计模式，只是多态的使用而已（个人理解，欢迎指正）。

下面看看用vb.net如何实现：（实现的和《大话》上写的不太一样，我觉得这样写更方便一些）

Public Class Form1 Dim result As Double Private Sub Button1_Click(ByVal sender As System.Object,ByVal e As System.EventArgs) Handles btnRun.Click Dim cc As CashContext cc = New CashContext(cmbType.Text) result += cc.GetResult(Double.Parse(txtPrice.Text)) * Double.Parse(txtNumber.Text) lstShow.Items.Add("单价：" + txtPrice.Text + " <a href="/tag/shuliang/" target="_blank" class="keywords">数量</a>：" + txtNumber.Text + " 合计：" + CStr(cc.GetResult(CDbl(txtPrice.Text) * CDbl(txtNumber.Text)))) txtTotal.Text = result.ToString End Sub End Class '从<a href="/tag/gongneng/" target="_blank" class="keywords">功能</a>上来说算得上收费工厂吧。 Public Class CashContext Private cs As CashSuper = Nothing Public Sub New(ByVal type As String) Select Case type Case "正常收费" cs = New CashNormal Case "满300返100" cs = New CashReturn("300","100") Case "打8折" cs = New CashRebate("0.8") Case Else cs = Nothing End Select End Sub Public Function GetResult(ByVal money As Double) As Double Return cs.acceptCash(money) End Function End Class '收费基类 Public MustInherit Class CashSuper Public MustOverride Function acceptCash(ByVal money As Double) As Double End Class '正常收费类 Public Class CashNormal : Inherits CashSuper Public Overrides Function acceptCash(ByVal money As Double) As Double Return money End Function End Class '打折收费的子类 Public Class CashRebate : Inherits CashSuper Private moneyRebate As Double = 1D '通过构造<a href="/tag/hanshu/" target="_blank" class="keywords">函数</a>传递参数 Public Sub New(ByVal moneyRebate As String) Me.moneyRebate = Double.Parse(moneyRebate) End Sub Public Overrides Function acceptCash(ByVal money As Double) As Double Return money * moneyRebate End Function End Class '返利收费子类 Public Class CashReturn : Inherits CashSuper Private moneyCondition As Double = 0D Private moneyReturn As Double = 0D '通过构造<a href="/tag/hanshu/" target="_blank" class="keywords">函数</a>传递消费情况，以及应该返回多少钱 Public Sub New(ByVal moneyCondition As String,ByVal moneyReturn As String) Me.moneyCondition = Double.Parse(moneyCondition) Me.moneyReturn = Double.Parse(moneyReturn) End Sub Public Overrides Function acceptCash(ByVal money As Double) As Double Dim result As Double = money If money >= moneyCondition Then result = money - Math.Floor((money / moneyCondition)) * moneyReturn End If Return result End Function End Class

其实仔细看这个模式会发现其中只是运用了面向对象中的两个机制：

一、多态

二、封装。

只不过这是将多态和封装组合起来使用，就像是搭积木一样。