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6.1.4 多播委托
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6.1.4 多播委托
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2006-10-13 14:41:00
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<div style='float:left;width:49%'>·<a href='100165337.htm'>6.1.1 在C#中使用委托</a></div><div style='float:right;width:49%'>·<a href='100165338.htm'>6.1.2 简单的委托示例</a></div><div style='float:left;width:49%'>·<a href='100165339.htm'>6.1.3 BubbleSorter示例</a></div><div style='float:right;width:49%'>·<a href='100165341.htm'>6.2 事件</a></div><div style='float:left;width:49%'>·<a href='100165342.htm'>6.2.1 从客户的角度讨论事件</a></div><div style='float:right;width:49%'>·<a href='100165343.htm'>6.2.2 生成事件</a></div></div>
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<div id="text"> <link href="css.css" rel="stylesheet" type="text/css" /><h3 style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 0cm; MARGIN-RIGHT: 0cm; FTEL: 8.15pt"><span lang="EN-US">6.1.4 </span><span style="FONT-FAMILY: 黑体">多播委托</span></h3>
<p class="MsoNormal" style="LINE-HEIGHT: 17pt"><a ftel="multicast"><span style="FONT-FAMILY: 宋体">前面使用的每个委托都只包含一个方法调用。调用委托的次数与调用方法的次数相同。如果要调用多个方法，就需要多次显式调用这个委托。委托也可以包含多个方法。这种委托称为多播委托。如果调用多播委托，就可以按顺序连续调用多个方法。为此，委托的签名就必须返回</span><span lang="EN-US">void(</span></a><span style="FONT-FAMILY: 宋体">否则，返回值应送到何处？</span><span lang="EN-US">)</span><span style="FONT-FAMILY: 宋体">。实际上，如果编译器发现某个委托返回</span><span lang="EN-US">void</span><span style="FONT-FAMILY: 宋体">，就会自动假定这是一个多播委托。下面的代码取自于</span><span lang="EN-US">SimpleDelegate</span><span style="FONT-FAMILY: 宋体">示例，尽管其语法与以前相同，但实际上它实例化了一个多播委托</span><span lang="EN-US">Operations</span><span style="FONT-FAMILY: 宋体">：</span></p>
<p class="2" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US"> delegate void DoubleOp(double value);</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">// delegate double DoubleOp(double value); // can't do this now</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> </span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> class MainEntryPoint</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> {</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> static void Main()</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> {</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> DoubleOp operations = new DoubleOp(MathOperations.MultiplyByTwo);</span></p>
<p class="2" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US"> operations += new DoubleOp(MathOperations.Square);</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">在前面的示例中，要存储对两个方法的引用，所以实例化了一个委托数组。而这里只是在同一个委托中添加两个操作。多播委托可以识别运算符</span><span lang="EN-US">+</span><span style="FONT-FAMILY: 宋体">和</span><span lang="EN-US">+=</span><span style="FONT-FAMILY: 宋体">。还可以扩展上述代码中的最后两行，它们具有相同的效果：</span></p>
<p class="2" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US"> DoubleOp operation1 = new DoubleOp(MathOperations.MultiplyByTwo);</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> DoubleOp operation2 = new DoubleOp(MathOperations.Square);</span></p>
<p class="2" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US"> DoubleOp operations = operation1 + operation2;</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">多播委托还识别运算符–和–</span><span lang="EN-US">=</span><span style="FONT-FAMILY: 宋体">，以从委托中删除方法调用。</span></p>
<p class="a3" style="MARGIN-TOP: 8.15pt; FTEL: 21.45pt"><span style="FONT-FAMILY: 黑体">注意：</span></p>
<p class="a1" style="FTEL: 21.45pt"><span style="FONT-FAMILY: 楷体_GB2312">根据后面的内容，多播委托是一个派生于</span><span lang="EN-US">System.MulticastDelegate</span><span style="FONT-FAMILY: 楷体_GB2312">的类，它派生于基类</span><span lang="EN-US">System.Delegate</span><span style="FONT-FAMILY: 楷体_GB2312">。</span><span lang="EN-US">System.MulticastDelegate</span><span style="FONT-FAMILY: 楷体_GB2312">的其他成员允许把多个方法调用链接在一起，成为一个列表。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体; LETTER-SPACING: -0.3pt">为了说明多播委托的用法，下面把</span><span lang="EN-US" style="LETTER-SPACING: -0.3pt">SimpleDelegate</span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: -0.3pt">示例改写为一个新示例</span><span lang="EN-US" style="LETTER-SPACING: -0.3pt">MulticastDelegate</span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: -0.3pt">。</span><span style="FONT-FAMILY: 宋体">现在需要把委托表示为返回</span><span lang="EN-US">void</span><span style="FONT-FAMILY: 宋体">的方法，就应重写</span><span lang="EN-US">MathOperations</span><span style="FONT-FAMILY: 宋体">类中的方法，让它们显示其结果，而不是返回它们：</span></p>
<p class="2" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US"> class MathOperations</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> {</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> public static void MultiplyByTwo(double value)</span></p>
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