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<h4 style="FTEL: 21.45pt"><a ftel="_Toc507815041"><span lang="EN-US">5. </span></a><span style="FONT-FAMILY: 黑体">嵌套的</span> <span lang="EN-US">try</span><span style="FONT-FAMILY: 黑体">块</span></h4>
<p class="MsoNormal"><a ftel="nested"><span style="FONT-FAMILY: 宋体">异常的一个特性是</span><span lang="EN-US">try</span></a><span style="FONT-FAMILY: 宋体">块可以嵌套，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">try </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">&nbsp;&nbsp; // Point A</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">&nbsp;&nbsp; try</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">&nbsp;&nbsp; {</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; // Point B</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">&nbsp;&nbsp; }</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">&nbsp;&nbsp; catch</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">&nbsp;&nbsp; {</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; // Point C</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">&nbsp;&nbsp; }</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">&nbsp;&nbsp; finally</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">&nbsp;&nbsp; {</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; // clean up</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">&nbsp;&nbsp; } </span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">&nbsp;&nbsp; // Point D</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">catch</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">&nbsp;&nbsp; // error handling</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">finally</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">&nbsp;&nbsp; // clean up</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" style="LINE-HEIGHT: 17pt"><span style="FONT-FAMILY: 宋体">在上面的代码中，每个</span><span lang="EN-US">try</span><span style="FONT-FAMILY: 宋体">块都只有一个</span><span lang="EN-US">catch</span><span style="FONT-FAMILY: 宋体">块，但可以把多个</span><span lang="EN-US">catch</span><span style="FONT-FAMILY: 宋体">块连接在一起。下面详细讨论嵌套的</span><span lang="EN-US">try</span><span style="FONT-FAMILY: 宋体">块如何工作。</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 17pt"><a ftel="catch3"><span style="FONT-FAMILY: 宋体">如果抛出的异常在外层的</span><span lang="EN-US">try</span></a><span style="FONT-FAMILY: 宋体">块中，且在内层</span><span lang="EN-US">try</span><span style="FONT-FAMILY: 宋体">块的外部</span><span lang="EN-US">(</span><span style="FONT-FAMILY: 宋体">标记为</span><span lang="EN-US">A</span><span style="FONT-FAMILY: 宋体">或</span><span lang="EN-US">D</span><span style="FONT-FAMILY: 宋体">的代码块</span><span lang="EN-US">)</span><span style="FONT-FAMILY: 宋体">，这种情况就与前面介绍的情况没有任何区别：异常由外层的</span><span lang="EN-US">catch</span><span style="FONT-FAMILY: 宋体">块捕获，并执行外层的</span><span lang="EN-US">finally</span><span style="FONT-FAMILY: 宋体">块，或者执行</span><span lang="EN-US">finally</span><span style="FONT-FAMILY: 宋体">块，由</span><span lang="EN-US">.NET</span><span style="FONT-FAMILY: 宋体">运行库处理异常。</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 17pt"><span style="FONT-FAMILY: 宋体">如果异常是在内层</span><span lang="EN-US">try</span><span style="FONT-FAMILY: 宋体">块</span><span lang="EN-US">(</span><span style="FONT-FAMILY: 宋体">代码块</span><span lang="EN-US">B)</span><span style="FONT-FAMILY: 宋体">中抛出的，且有一个合适的内层</span><span lang="EN-US">catch</span><span style="FONT-FAMILY: 宋体">块处理该异常，这又是我们熟悉的情况：在内层处理异常，执行内层的</span><span lang="EN-US">finally</span><span style="FONT-FAMILY: 宋体">块，之后继续执行外层的</span><span lang="EN-US">try</span><span style="FONT-FAMILY: 宋体">块</span><span lang="EN-US">(</span><span style="FONT-FAMILY: 宋体">标记为</span><span lang="EN-US">D</span><span style="FONT-FAMILY: 宋体">的代码块</span><span lang="EN-US">)</span><span style="FONT-FAMILY: 宋体">。</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 17pt"><span style="FONT-FAMILY: 宋体">现在假定异常是在代码块的内层</span><span lang="EN-US">try</span><span style="FONT-FAMILY: 宋体">块中抛出的，但内层的</span><span lang="EN-US">catch</span><span style="FONT-FAMILY: 宋体">块中没有合适的处理程序。这时通常就要执行内层的</span><span lang="EN-US">finally</span><span style="FONT-FAMILY: 宋体">块，但</span><span lang="EN-US">.NET</span><span style="FONT-FAMILY: 宋体">运行库只能退出内层的</span><span lang="EN-US">try</span><span style="FONT-FAMILY: 宋体">块，才能搜索到合适的处理程序。下一个要搜索的区域显然是外层的</span><span lang="EN-US">catch</span><span style="FONT-FAMILY: 宋体">块。如果系统在这里找到一个处理程序，就会执行该处理程序，再执行外层的</span><span lang="EN-US">finally</span><span style="FONT-FAMILY: 宋体">块，如果没有找到合适的处理程序，就会继续搜索。在这里，执行的是外层的</span><span lang="EN-US">finally</span><span style="FONT-FAMILY: 宋体">块，因为没有更多的</span><span lang="EN-US">catch</span><span style="FONT-FAMILY: 宋体">块，所以控制权会返回</span><span lang="EN-US">.NET</span><span style="FONT-FAMILY: 宋体">运行库。注意，不会执行不在外层</span><span lang="EN-US">try</span><span style="FONT-FAMILY: 宋体">块中的代码。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">如果异常是在</span><span lang="EN-US">C</span><span style="FONT-FAMILY: 宋体">代码块中抛出的，就更有趣了。如果程序执行到代码块</span><span lang="EN-US">C</span><span style="FONT-FAMILY: 宋体">中，就必须已经处理了由代码块</span><span lang="EN-US">B</span><span style="FONT-FAMILY: 宋体">抛出的异常。在</span><span lang="EN-US">catch</span><span style="FONT-FAMILY: 宋体">块中抛出另一个异常是很正常的。此时，异常的处理就跟它是在外层</span><span lang="EN-US">try</span><span style="FONT-FAMILY: 宋体">块中抛出的一样，程序流会立即退出内层的</span><span lang="EN-US">catch</span><span style="FONT-FAMILY: 宋体">块，执行内层的</span><span lang="EN-US">finally</span><span style="FONT-FAMILY: 宋体">块，在外层的</span><span lang="EN-US">catch</span><span style="FONT-FAMILY: 宋体">中搜索处理程序。同样，如果在内层的</span><span lang="EN-US">finally</span><span style="FONT-FAMILY: 宋体">块中抛出一个异常，搜索会在外层的</span><span lang="EN-US">catch</span><span style="FONT-FAMILY: 宋体">块开始，执行最匹配的处理程序。</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">最好在</span><span lang="EN-US">catch</span><span style="FONT-FAMILY: 楷体_GB2312">和</span><span lang="EN-US">finally</span><span style="FONT-FAMILY: 楷体_GB2312">块中抛出异常。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">尽管本例只有两个</span><span lang="EN-US">try</span><span style="FONT-FAMILY: 宋体">块，但无论嵌套了多少个</span><span lang="EN-US">try</span><span style="FONT-FAMILY: 宋体">块，规则都是一样的。在每个块中，</span><span lang="EN-US">.NET</span><span style="FONT-FAMILY: 宋体">运行库顺序执行</span><span lang="EN-US">try</span><span style="FONT-FAMILY: 宋体">块，查找合适的处理程序，在每个步骤中，当退出</span><span lang="EN-US">catch</span><span style="FONT-FAMILY: 宋体">块后，就会执行对应</span><span lang="EN-US">finally</span><span style="FONT-FAMILY: 宋体">块中的清理代码，但不执行</span><span lang="EN-US">finally</span><span style="FONT-FAMILY: 宋体">块外部的代码，直到找到合适的</span><span lang="EN-US">catch</span><span style="FONT-FAMILY: 宋体">处理程序，并执行为止。</span></p>
<p class="MsoNormal"><a ftel="finally3"><span style="FONT-FAMILY: 宋体">前面说明了嵌套</span><span lang="EN-US">try</span></a><span style="FONT-FAMILY: 宋体">块的工作方式。下一个问题显然是为什么要这么做？这有两个原因：</span></p>
<p class="1" style="MARGIN-LEFT: 37.55pt; FTEL: -16.1pt"><span lang="EN-US">●<span style="FONT: 7pt 'Times New Roman'">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; </span></span><span style="FONT-FAMILY: 宋体">修改所抛出的异常的类型。</span></p>
<p class="1" style="MARGIN-LEFT: 37.55pt; FTEL: -16.1pt"><span lang="EN-US">●<span style="FONT: 7pt 'Times New Roman'">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; </span></span><span style="FONT-FAMILY: 宋体">在代码的不同地方处理不同类型的异常。</span></p>
<p class="MsoNormal"><span lang="EN-US">(1) </span><span style="FONT-FAMILY: 宋体">修改异常的类型</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">当最初抛出的异常不足以说明问题时，修改异常的类型就非常重要了。通常的情况是抛出的异常</span><span lang="EN-US">(</span><span style="FONT-FAMILY: 宋体">可能由</span><span lang="EN-US">.NET</span><span style="FONT-FAMILY: 宋体">运行库抛出</span><span lang="EN-US">)</span><span style="FONT-FAMILY: 宋体">是一种相当低级的异常，说明发生溢出</span><span lang="EN-US">(OverflowException)</span><span style="FONT-FAMILY: 宋体">或传递给方法的参数不正确</span><span lang="EN-US">(</span><span style="FONT-FAMILY: 宋体">派生于</span><span lang="EN-US">ArgumentException</span><span style="FONT-FAMILY: 宋体">的一个类</span><span lang="EN-US">)</span><span style="FONT-FAMILY: 宋体">。但是，由于抛出异常的环境，我们知道这暴露了一些底层的问题</span><span lang="EN-US">(</span><span style="FONT-FAMILY: 宋体">例如，因为刚才读取的文件包含了不正确的数据，才发生了溢出异常</span><span lang="EN-US">)</span><span style="FONT-FAMILY: 宋体">。此时处理程序对于第一个异常所能做的最佳处理就是抛出另一个异常，以便更准确地说明这个问题，让另一个</span><span lang="EN-US">catch</span><span style="FONT-FAMILY: 宋体">块更恰当地处理它。也可以通过一个由</span><span lang="EN-US">System.Exception</span><span style="FONT-FAMILY: 宋体">执行的属性</span><span lang="EN-US">InnerException</span><span style="FONT-FAMILY: 宋体">来处理最初的异常。</span><span lang="EN-US">InnerException</span><span style="FONT-FAMILY: 宋体">只包含另一个相关异常的引用<span style="LETTER-SPACING: -1pt">&mdash;&mdash;</span></span><span style="LETTER-SPACING: -1pt"> </span><span style="FONT-FAMILY: 宋体">最终的处理程序例程需要这个额外的信息。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">当然，还应指出，异常可能在</span><span lang="EN-US">catch</span><span style="FONT-FAMILY: 宋体">块中抛出。例如，可以从某个配置文件中读取数据，这个文件包含处理错误的详细指令&mdash;&mdash;结果可能是这个文件不存在。</span></p>
<p class="MsoNormal"><span lang="EN-US">(2) </span><span style="FONT-FAMILY: 宋体">在不同的地方处理不同的异常</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">嵌套</span><span lang="EN-US">try</span><span style="FONT-FAMILY: 宋体">块的第二个原因是不同类型的异常可以在代码的不同地方处理。例如，在循环中，可能会发生各种异常。其中一些异常比较严重，需要退出整个循环，而另外一些则不太严重，只需退出这次迭代，进入循环的下一次迭代即可。在循环的内部有一个</span><span lang="EN-US">try</span><span style="FONT-FAMILY: 宋体">块就可以处理不太严重的异常，再在循环外面用一个外层的</span><span lang="EN-US">try</span><span style="FONT-FAMILY: 宋体">块来处理比较严重的错误。在下面的异常示例中，将解释具体的操作情况。</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='100165376.htm'><font color='red'>11.1.2  捕获异常(2)</font></a></h1>
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                        <div style='float:left;width:49%'>·<a href='100165373.htm'>11.1  错误和异常处理</a></div><div style='float:right;width:49%'>·<a href='100165374.htm'>11.1.1  异常类</a></div><div style='float:left;width:49%'>·<a href='100165375.htm'>11.1.2  捕获异常(1)</a></div><div style='float:right;width:49%'>·<a href='100165377.htm'>11.1.3  用户定义的异常类(1)</a></div><div style='float:left;width:49%'>·<a href='100165378.htm'>11.1.3  用户定义的异常类(2)</a></div><div style='float:right;width:49%'>·<a href='100165379.htm'>11.2  小结</a></div></div>
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