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<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; uint dollars = (uint)value;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ushort cents = (ushort)((value</span><span style="FONT-FAMILY: 宋体">&ndash;</span><span lang="EN-US">dollars)*100);</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; return new Currency(dollars, cents);</span></p>
<p class="2" style="MARGIN: 0cm 0cm 8.15pt 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; }</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">下面的代码可以成功编译：</span></p>
<p class="2" style="MARGIN: 8.15pt 0cm 0pt 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">&nbsp;&nbsp; float amount = 45.63f;</span></p>
<p class="2" style="MARGIN: 0cm 0cm 8.15pt 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">&nbsp;&nbsp; Currency amount2 = (Currency)amount;</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">但是，下面的代码会抛出一个编译错误，因为试图隐式地使用一个显式的数据类型转换：</span></p>
<p class="2" style="MARGIN: 8.15pt 0cm 0pt 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">&nbsp;&nbsp; float amount = 45.63f;</span></p>
<p class="2" style="MARGIN: 0cm 0cm 8.15pt 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">&nbsp;&nbsp; Currency amount2 = amount;&nbsp;&nbsp; // wrong</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">把数据类型转换声明为显式，就是警告开发人员要小心，因为可能会丢失数据。但这不是我们希望的</span><span lang="EN-US">Currency</span><span style="FONT-FAMILY: 宋体">结构的执行方式。下面编写一个测试程序，运行示例。其中有一个</span><span lang="EN-US">Main()</span><span style="FONT-FAMILY: 宋体">方法，它实例化了一个</span><span lang="EN-US">Currency</span><span style="FONT-FAMILY: 宋体">结构，试图进行几个转换。在这段代码的开头，以两种不同的方式计算</span><span lang="EN-US">balance</span><span style="FONT-FAMILY: 宋体">的值</span><span lang="EN-US">(</span><span style="FONT-FAMILY: 宋体">因为要使用它们来说明后面的内容</span><span lang="EN-US">)</span><span style="FONT-FAMILY: 宋体">：</span></p>
<p class="2" style="MARGIN: 8.15pt 0cm 0pt 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">static void Main()</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">{</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">&nbsp;&nbsp; try</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">&nbsp;&nbsp; {</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Currency balance = new Currency(50,35);</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt; LINE-HEIGHT: 15pt"><span lang="EN-US">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Console.WriteLine(balance);</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt; LINE-HEIGHT: 15pt"><span lang="EN-US">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Console.WriteLine(&quot;balance is &quot; + balance);</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt; LINE-HEIGHT: 15pt"><span lang="EN-US">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Console.WriteLine(&quot;balance is (using ToString()) &quot; +</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt; LINE-HEIGHT: 15pt"><span lang="EN-US">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; balance.ToString());</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt; LINE-HEIGHT: 15pt"><span lang="EN-US">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; float balance2= balance;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt; LINE-HEIGHT: 15pt"><span lang="EN-US">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Console.WriteLine(&quot;After converting to float, = &quot; + balance2);</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt; LINE-HEIGHT: 15pt"><span lang="EN-US">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; balance = (Currency) balance2;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt; LINE-HEIGHT: 15pt"><span lang="EN-US">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Console.WriteLine(&quot;After converting back to Currency, = &quot; + balance);</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt; LINE-HEIGHT: 15pt"><span lang="EN-US">&nbsp;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Console.WriteLine(&quot;Now attempt to convert out of range value of &quot; +</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &quot;</span><span style="FONT-FAMILY: 宋体">&ndash;</span><span lang="EN-US">$100.00 to a Currency:&quot;);</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; checked</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; {</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; balance = (Currency) (</span><span style="FONT-FAMILY: 宋体">&ndash;</span><span lang="EN-US">50.5);</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Console.WriteLine(&quot;Result is &quot; + balance.ToString());</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; }</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">&nbsp;&nbsp; }</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">&nbsp;&nbsp; catch(Exception e)</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">&nbsp;&nbsp; {</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Console.WriteLine(&quot;Exception occurred: &quot; + e.Message);</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">&nbsp;&nbsp; }</span></p>
<p class="2" style="MARGIN: 0cm 0cm 8.15pt 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">}</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">注意，我们把所有的代码都放在一个</span><span lang="EN-US">try</span><span style="FONT-FAMILY: 宋体">块中，来捕获在数据类型转换过程中发生的任何异常。在</span><span lang="EN-US">checked</span><span style="FONT-FAMILY: 宋体">块中还添加了把超出范围的值转换为</span><span lang="EN-US">Currency</span><span style="FONT-FAMILY: 宋体">的测试代码，所以，负值是肯定会被捕获的。运行这段代码，得到如下所示的结果：</span></p>
<p class="a6" style="MARGIN: 8.15pt 0cm 6pt 9.95pt; TEXT-INDENT: 9.95pt"><strong><span lang="EN-US">SimpleCurrency</span></strong></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">50.35</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">Balance is $50.35</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">Balance is (using ToString()) $50.35</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">After converting to float, = 50.35</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">After converting back to Currency, = $50.34</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">Now attempt to convert out of range value of</span><span style="FONT-FAMILY: 宋体">&ndash;</span><span lang="EN-US">$100.00 to a Currency:</span></p>
<p class="a6" style="MARGIN: 0cm 0cm 8.15pt 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">Result is $4294967246.60486</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 17pt"><span style="FONT-FAMILY: 宋体">这个结果表示代码并没有像我们希望的那样工作。首先，从</span><span lang="EN-US">float</span><span style="FONT-FAMILY: 宋体">转换回</span><span lang="EN-US">Currency</span><span style="FONT-FAMILY: 宋体">得到一个错误的结果</span><span lang="EN-US">$50.34</span><span style="FONT-FAMILY: 宋体">，而不是</span><span lang="EN-US">$50.35</span><span style="FONT-FAMILY: 宋体">。其次，在试图转换明显超出范围的值时，没有生成异常。</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 17pt"><span style="FONT-FAMILY: 宋体">第一个问题是由圆整错误引起的。如果类型转换用于把</span><span lang="EN-US">float</span><span style="FONT-FAMILY: 宋体">转换为</span><span lang="EN-US">uint</span><span style="FONT-FAMILY: 宋体">，计算机就会截去多余的数字，而不是圆整它。计算机以二进制方式存储数字，而不是十进制，小数部分</span><span lang="EN-US">0.35</span><span style="FONT-FAMILY: 宋体">不能用二进制小数来表示</span><span lang="EN-US">(</span><span style="FONT-FAMILY: 宋体">像</span><span lang="EN-US">1/3</span><span style="FONT-FAMILY: 宋体">这样的小数不能表示为小数部分，它应等于循环小数</span><span lang="EN-US">0.3333)</span><span style="FONT-FAMILY: 宋体">。所以，计算机最后存储了一个略小于</span><span lang="EN-US">0.35</span><span style="FONT-FAMILY: 宋体">的值，它可以用二进制格式表示。把该数字乘以</span><span lang="EN-US">100</span><span style="FONT-FAMILY: 宋体">，就会得到一个小于</span><span lang="EN-US">35</span><span style="FONT-FAMILY: 宋体">的数字，截去了美分</span><span lang="EN-US">34</span><span style="FONT-FAMILY: 宋体">，显然在本例中，这种由截去引起的错误是很严重的，避免该错误的方式是确保在数字转换过程中执行智能圆整操作。</span><span lang="EN-US">Microsoft</span><span style="FONT-FAMILY: 宋体">编写了一个类</span><span lang="EN-US">System.Convert</span><span style="FONT-FAMILY: 宋体">来完成该任务。</span><span lang="EN-US">System.Convert</span><span style="FONT-FAMILY: 宋体">包含大量的静态方法来执行各种数字转换，我们需要使用的是</span><span lang="EN-US">Convert.To UInt16()</span><span style="FONT-FAMILY: 宋体">。注意，在使用</span><span lang="EN-US">System.Convert</span><span style="FONT-FAMILY: 宋体">方法时会产生额外的性能损失，所以只应在需要时才使用它们。</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 17pt"><span></span><span><span style="FONT-FAMILY: 宋体">下面看看为什么没有抛出期望的溢出异常。此处的问题是异常实际发生的位置根本不在</span><span lang="EN-US">Main()</span></span><span style="FONT-FAMILY: 宋体">例程中&mdash;&mdash;这是在转换运算符的代码中发生的，该代码在</span><span lang="EN-US">Main()</span><span style="FONT-FAMILY: 宋体">方法中调用，而且没有标记为</span><span lang="EN-US">checked</span><span style="FONT-FAMILY: 宋体">。</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 17pt"><span style="FONT-FAMILY: 宋体">其解决方法是确保类型转换本身也在</span><span lang="EN-US">checked</span><span style="FONT-FAMILY: 宋体">环境下进行。进行了这两个修改后，修订后的转换代码如下所示。</span></p>