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5.2 类型的安全性
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5.2 类型的安全性
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2006-10-13 14:50:00
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<div style='float:left;width:49%'>·<a href='100165818.htm'>4.3 修饰符</a></div><div style='float:right;width:49%'>·<a href='100165819.htm'>4.4 接口</a></div><div style='float:left;width:49%'>·<a href='100165820.htm'>5.1 运算符</a></div><div style='float:right;width:49%'>·<a href='100165822.htm'>5.3 对象的相等比较</a></div><div style='float:left;width:49%'>·<a href='100165823.htm'>5.4 运算符重载</a></div><div style='float:right;width:49%'>·<a href='100165824.htm'>5.5 用户定义的数据类型转换</a></div></div>
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<div id="text"> <link href="css.css" rel="stylesheet" type="text/css" /><p class="MsoNormal"><span style="FONT-FAMILY: 宋体">第</span><span lang="EN-US">1</span><span style="FONT-FAMILY: 宋体">章提到中间语言</span><span lang="EN-US">(IL)</span><span style="FONT-FAMILY: 宋体">可以对其代码强制加上强类型安全性。强类型支持</span><span lang="EN-US">.NET</span><span style="FONT-FAMILY: 宋体">提供的许多服务，包括安全性和语言的交互性。因为</span><span lang="EN-US">C#</span><span style="FONT-FAMILY: 宋体">这种语言将编译为</span><span lang="EN-US">IL</span><span style="FONT-FAMILY: 宋体">，所以</span><span lang="EN-US">C#</span><span style="FONT-FAMILY: 宋体">也是强类型的。这说明数据类型并不总是可互换的。本节将介绍基本类型之间的转换。</span></p>
<p class="a3" style="MARGIN-TOP: 8.15pt; TEXT-INDENT: 21.45pt"><span style="FONT-FAMILY: 黑体">注意：</span></p>
<p class="a1" style="MARGIN-BOTTOM: 8.15pt; TEXT-INDENT: 21.45pt"><span lang="EN-US">C#</span><span style="FONT-FAMILY: 楷体_GB2312">还支持在不同引用类型之间的转换，允许指定自己创建的数据类型如何与其他类型进行相互转换。这些论题将在本章后面讨论。</span></p>
<h3 style="MARGIN: 8.15pt 0cm"><span lang="EN-US">5.2.1 </span><span style="FONT-FAMILY: 黑体">类型转换</span></h3>
<p class="MsoToc1" style="LINE-HEIGHT: 15pt"><span><span style="FONT-FAMILY: 宋体">我们常常需要把数据从一种类型转换为另一种类型。考虑下面的代码：</span></span></p>
<p class="2" style="MARGIN: 8.15pt 0cm 0pt 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">byte value1 = 10;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt; LINE-HEIGHT: 13.5pt"><span lang="EN-US">byte value2 = 23;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt; LINE-HEIGHT: 13.5pt"><span lang="EN-US">byte total;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt; LINE-HEIGHT: 13.5pt"><span lang="EN-US">total = value1 + value2;</span></p>
<p class="2" style="MARGIN: 0cm 0cm 8.15pt 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">Console.WriteLine(total);</span></p>
<p class="MsoToc1" style="LINE-HEIGHT: 15pt"><span style="FONT-FAMILY: 宋体">在编译这些代码时，会产生一个错误：</span></p>
<p class="a6" style="MARGIN: 8.15pt 0cm 8.15pt 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">Cannot implicitly convert type 'int' to 'byte' (</span><span style="FONT-FAMILY: 宋体">不能把</span><span lang="EN-US">int</span><span style="FONT-FAMILY: 宋体">类型隐式地转换为</span><span lang="EN-US">byte</span><span style="FONT-FAMILY: 宋体">类型</span><span lang="EN-US">)</span><span style="FONT-FAMILY: 宋体">。</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 15pt"><span style="FONT-FAMILY: 宋体">问题是，我们把两个</span><span lang="EN-US">byte</span><span style="FONT-FAMILY: 宋体">加在一起时，应返回</span><span lang="EN-US">int</span><span style="FONT-FAMILY: 宋体">型结果，而不是另一个</span><span lang="EN-US">byte</span><span style="FONT-FAMILY: 宋体">。这是因为</span><span lang="EN-US">byte</span><span style="FONT-FAMILY: 宋体">包含的数据只能为</span><span lang="EN-US">8</span><span style="FONT-FAMILY: 宋体">位，所以把两个</span><span lang="EN-US">byte</span><span style="FONT-FAMILY: 宋体">加在一起很容易得到不能存储在一个</span><span lang="EN-US">byte</span><span style="FONT-FAMILY: 宋体">中的值。如果要把结果存储在一个</span><span lang="EN-US">byte</span><span style="FONT-FAMILY: 宋体">变量中，就必须把它转换回</span><span lang="EN-US">byte</span><span style="FONT-FAMILY: 宋体">。有两种转换方式，隐式转换方式和显式转换方式。</span></p>
<h4 style="TEXT-INDENT: 21.45pt"><span lang="EN-US">1. </span><span style="FONT-FAMILY: 黑体">隐式转换方式</span></h4>
<p class="MsoNormal" style="LINE-HEIGHT: 15pt"><span style="FONT-FAMILY: 宋体">如果能保证值不会发生任何变化，类型转换就可以自动进行。这就是前面代码失败的原因：试图从</span><span lang="EN-US">int</span><span style="FONT-FAMILY: 宋体">转换为</span><span lang="EN-US">byte</span><span style="FONT-FAMILY: 宋体">，而潜在地丢失了</span><span lang="EN-US">3</span><span style="FONT-FAMILY: 宋体">个字节的数据。编译器不会告诉我们该怎么做，除非我们显式地告诉它这就是我们希望的！如果在</span><span lang="EN-US">long</span><span style="FONT-FAMILY: 宋体">型变量中存储结果，而不是</span><span lang="EN-US">byte</span><span style="FONT-FAMILY: 宋体">型变量中，就不会有问题了：</span></p>
<p class="a6" style="MARGIN: 8.15pt 0cm 0pt 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">byte value1 = 10;</span></p>
<p class="a6" style="MARGIN: 0cm 0cm 6pt 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">byte value2 = 23;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">long total; // this will compile fine</span></p>
<p class="a6" style="MARGIN: 6pt 0cm 0pt 21.45pt; TEXT-INDENT: 18.45pt; LINE-HEIGHT: 13pt"><span lang="EN-US">total = value1 + value2;</span></p>
<p class="a6" style="MARGIN: 0cm 0cm 8.15pt 21.45pt; TEXT-INDENT: 18.45pt; LINE-HEIGHT: 13pt"><span lang="EN-US">Console.WriteLine(total);</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 15pt"><span style="FONT-FAMILY: 宋体">这是因为</span><span lang="EN-US">long</span><span style="FONT-FAMILY: 宋体">型变量包含的数据字节比</span><span lang="EN-US">int</span><span style="FONT-FAMILY: 宋体">型多，所以数据没有丢失的危险。在这些情况下，编译器会很顺利地进行转换，我们也不需要显式提出要求。</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 15pt"><span style="FONT-FAMILY: 宋体">表</span><span lang="EN-US">5-4</span><span style="FONT-FAMILY: 宋体">介绍了</span><span lang="EN-US">C#</span><span style="FONT-FAMILY: 宋体">支持的隐式类型转换。</span></p>
<p class="a5" style="MARGIN-TOP: 1pt"><span style="FONT-FAMILY: 黑体">表</span><span lang="EN-US"> 5-4</span></p>
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<p class="a2" style="LINE-HEIGHT: 13.5pt"><span style="FONT-FAMILY: 黑体">源</span><span style="FONT-FAMILY: 'Times New Roman'"> </span><span style="FONT-FAMILY: 黑体">类</span><span style="FONT-FAMILY: 'Times New Roman'"> </span><span style="FONT-FAMILY: 黑体">型</span></p>
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<p class="a2" style="LINE-HEIGHT: 13.5pt"><span style="FONT-FAMILY: 黑体">目</span><span style="FONT-FAMILY: 'Times New Roman'"> </span><span style="FONT-FAMILY: 黑体">的</span><span style="FONT-FAMILY: 'Times New Roman'"> </span><span style="FONT-FAMILY: 黑体">类</span><span style="FONT-FAMILY: 'Times New Roman'"> </span><span style="FONT-FAMILY: 黑体">型</span></p>
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<p class="a4" style="MARGIN-LEFT: 10.7pt; LINE-HEIGHT: 13.5pt"><span lang="EN-US">sbyte</span></p>
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