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<p class="2" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">}</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">注意上述代码段中的闭合花括号。提供它们的原因是为了确保这段代码的作用与前面的</span><span lang="EN-US">foreach</span><span style="FONT-FAMILY: 宋体">循环的作用一样。如果没有使用闭合花括号，这段代码就会有所不同，</span><span lang="EN-US">nextMessage </span><span style="FONT-FAMILY: 宋体">和</span><span lang="EN-US"> enumerator</span><span style="FONT-FAMILY: 宋体">变量在循环结束时应保留在内存中。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">集合的一个重要方面是枚举作为一个单独的对象返回。它不应是与集合相同的对象，原因是多个枚举可以同时应用到同一个集合上。</span></p>
<h4 style="FTEL: 21.45pt"><span lang="EN-US">2. </span><span style="FONT-FAMILY: 黑体">给</span><span lang="EN-US">Vector </span><span style="FONT-FAMILY: 黑体">结构添加集合支持</span></h4>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">下面对第</span><span lang="EN-US">3</span><span style="FONT-FAMILY: 宋体">章的</span><span lang="EN-US">Vector</span><span style="FONT-FAMILY: 宋体">结构进行另一个集合支持扩展。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">在</span><span lang="EN-US">Vector</span><span style="FONT-FAMILY: 宋体">结构中，</span><span lang="EN-US">Vector</span><span style="FONT-FAMILY: 宋体">实例包含</span><span lang="EN-US">3</span><span style="FONT-FAMILY: 宋体">个元素</span><span lang="EN-US">x</span><span style="FONT-FAMILY: 宋体">、</span><span lang="EN-US">y</span><span style="FONT-FAMILY: 宋体">和</span><span lang="EN-US">z</span><span style="FONT-FAMILY: 宋体">，第</span><span lang="EN-US">3</span><span style="FONT-FAMILY: 宋体">章已定义了一个索引符，因此可以把</span><span lang="EN-US">Vector</span><span style="FONT-FAMILY: 宋体">实例当作数组，使用</span><span lang="EN-US">SomeVector[0] </span><span style="FONT-FAMILY: 宋体">访问</span><span lang="EN-US">x</span><span style="FONT-FAMILY: 宋体">元素，使用</span><span lang="EN-US">SomeVector[1]</span><span style="FONT-FAMILY: 宋体">访问</span><span lang="EN-US">y</span><span style="FONT-FAMILY: 宋体">元素，使用</span><span lang="EN-US">SomeVector[2] </span><span style="FONT-FAMILY: 宋体">访问</span><span lang="EN-US">z</span><span style="FONT-FAMILY: 宋体">元素。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">下面把</span><span lang="EN-US">Vector</span><span style="FONT-FAMILY: 宋体">结构扩展为一个新代码示例</span><span lang="EN-US">VectorAsCollection</span><span style="FONT-FAMILY: 宋体">项目，编写如下代码，迭代</span><span lang="EN-US"> Vector</span><span style="FONT-FAMILY: 宋体">中的元素：</span></p>
<p class="2" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">foreach (double component in someVector)</span></p>
<p class="2" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US"> Console.WriteLine("Component is " + component);</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">首先让</span><span lang="EN-US">Vector</span><span style="FONT-FAMILY: 宋体">实现</span><span lang="EN-US">IEnumerable</span><span style="FONT-FAMILY: 宋体">接口，把它标记为一个集合。第一步是修改</span><span lang="EN-US">Vector</span><span style="FONT-FAMILY: 宋体">结构的声明：</span></p>
<p class="2" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><a ftel="IEnumerable2"><span lang="EN-US">struct Vector : IFormattable, IEnumerable</span></a></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">{</span></p>
<p class="a6" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US"> public double x, y, z;</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">注意出现了</span><span lang="EN-US">IFormattable</span><span style="FONT-FAMILY: 宋体">接口，因为我们在前面为它添加了字符串格式说明符。下面需要实现</span><span lang="EN-US">IEnumerable</span><span style="FONT-FAMILY: 宋体">接口。</span></p>
<p class="2" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US"> public IEnumerator GetEnumerator()</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"> return new VectorEnumerator(this);</span></p>
<p class="2" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US"> }</span></p>
<p class="MsoNormal"><span lang="EN-US">GetEnumerator()</span><span style="FONT-FAMILY: 宋体">的实现比较复杂，但它取决于一个新类</span><span lang="EN-US">VectorEnumerator</span><span style="FONT-FAMILY: 宋体">，这个类需要定义。因为</span><span lang="EN-US">VectorEnumerator</span><span style="FONT-FAMILY: 宋体">不是外部代码需要直接看到的类，所以可以把它声明为</span><span lang="EN-US">Vector</span><span style="FONT-FAMILY: 宋体">结构中的私有类，其定义如下所示：</span></p>
<p class="2" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">private class VectorEnumerator : IEnumerator</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"> Vector theVector; // Vector object that this enumerato refers to </span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> int location; // which element of theVector the enumerator is</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> // currently referring to </span></p>
<p class="2" style="MARGIN-TOP: 6pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US"> public VectorEnumerator(Vector theVector)</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"> this.theVector = theVector;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> location =</span><span style="FONT-FAMILY: 宋体">–</span><span lang="EN-US">1;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> }</span></p>
<p class="2" style="MARGIN-TOP: 6pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US"> public bool MoveNext()</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"> ++location;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> return (location > 2) ? false : true;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> }</span></p>
<p class="2" style="MARGIN-TOP: 6pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US"> public object Current</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"> get</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"> if (location < 0 || location > 2)</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> throw new InvalidOperationException(</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> "The enumerator is either before the first element or " +</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> "after the last element of the Vector");</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> return theVector[(uint)location];</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"> }</span></p>
<p class="2" style="MARGIN-TOP: 6pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US"> public void Reset()</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">private class VectorEnumerator : IEnumerator</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 32.25pt"><span lang="EN-US">{</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> location = -1;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> }</span></p>
<p class="2" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">}</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">因为需要一个枚举，所以</span><span lang="EN-US">VectorEnumerator</span><span style="FONT-FAMILY: 宋体">实现</span><span lang="EN-US">IEnumerator</span><span style="FONT-FAMILY: 宋体">接口。它还包含两个成员字段</span><span lang="EN-US">theVector</span><span style="FONT-FAMILY: 宋体">和</span><span lang="EN-US">location</span><span style="FONT-FAMILY: 宋体">。</span><span lang="EN-US">TheVector</span><span style="FONT-FAMILY: 宋体">是与这个枚举相关的</span><span lang="EN-US">Vector(</span><span style="FONT-FAMILY: 宋体">集合</span><span lang="EN-US">)</span><span style="FONT-FAMILY: 宋体">引用，而</span><span lang="EN-US">location</span><span style="FONT-FAMILY: 宋体">是一个</span><span lang="EN-US">int</span><span style="FONT-FAMILY: 宋体">类型的变量，<span style="LETTER-SPACING: 0.1pt">表示枚举在集合中的什么地方引用，换言之，就是</span></span><span lang="EN-US" style="LETTER-SPACING: 0.1pt">Current</span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: 0.1pt">属性是否应获取矢量的</span><span lang="EN-US" style="LETTER-SPACING: 0.1pt">x, y </span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: 0.1pt">或</span><span lang="EN-US" style="LETTER-SPACING: 0.1pt"> z</span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: 0.1pt">元</span><span style="FONT-FAMILY: 宋体">素。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">在本例中，要把</span><span lang="EN-US">location</span><span style="FONT-FAMILY: 宋体">当作一个下标，在内部实现</span><span lang="EN-US">enumerator</span><span style="FONT-FAMILY: 宋体">，把</span><span lang="EN-US">Vector</span><span style="FONT-FAMILY: 宋体">当作一个数组来访问。此时，有效的下标是</span><span lang="EN-US">0</span><span style="FONT-FAMILY: 宋体">、</span><span lang="EN-US">1</span><span style="FONT-FAMILY: 宋体">和</span><span lang="EN-US">2</span><span style="FONT-FAMILY: 宋体">。这里可以使用–</span><span lang="EN-US">1</span><span style="FONT-FAMILY: 宋体">，表示枚举在集合开头的前面，使用</span><span lang="EN-US">3</span><span style="FONT-FAMILY: 宋体">表示到达了集合的末尾。因此，在</span><span lang="EN-US">VectorEnumerator</span><span style="FONT-FAMILY: 宋体">构造函数中把这个字段初始化为–</span><span lang="EN-US">1</span><span style="FONT-FAMILY: 宋体">：</span></p>
<p class="a6" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US"> <span class="codeintext-PRODUCTION"><span style="FONT-FAMILY: 'Times New Roman'">public</span></span> VectorEnumerator(Vector theVector)</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"> this.theVector = theVector;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> location =</span><span style="FONT-FAMILY: 宋体">–</span><span lang="EN-US">1;</span></p>
<p class="a6" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US"> }</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">注意构造函数也带有要枚举的</span><span lang="EN-US">Vector</span><span style="FONT-FAMILY: 宋体">实例的引用<span style="LETTER-SPACING: -1pt">——</span></span><span style="LETTER-SPACING: -1pt"> </span><span style="FONT-FAMILY: 宋体">这是在</span><span lang="EN-US">Vector.GetEnumerator()</span><span style="FONT-FAMILY: 宋体">方法中提供的：</span></p>
<p class="a6" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><a ftel="_Toc507815075"><span lang="EN-US"> <span class="codeintext-PRODUCTION"><span style="FONT-FAMILY: 'Times New Roman'">public</span></span> IEnumerator GetEnumerator()</span></a></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"> <span class="codeintext-PRODUCTION"><span style="FONT-FAMILY: 'Times New Roman'">return new</span></span> VectorEnumerator(this);</span></p>
<p class="a6" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US"> }</span></p></div>
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<h1 id="divCurrentNode2" style="color: #b83507; width: 100%; text-align: left; font-size: 12px; padding-left: 2px">当前章节:<a href='100165360.htm'><font color='red'>9.1.2 集合</font></a></h1>
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<div style='float:left;width:49%'>·<a href='100165344.htm'>6.3 小结</a></div><div style='float:right;width:49%'>·<a href='100165358.htm'>9.1 对象组</a></div><div style='float:left;width:49%'>·<a href='100165359.htm'>9.1.1 数组列表</a></div><div style='float:right;width:49%'>·<a href='100165361.htm'>9.1.3 字典(1)</a></div><div style='float:left;width:49%'>·<a href='100165362.htm'>9.1.3 字典(2)</a></div><div style='float:right;width:49%'>·<a href='100165363.htm'>9.2 小结</a></div></div>
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