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2.3 变量
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2.3 变量
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2006-8-17 16:57:00
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<div style='float:left;width:49%'>·<a href='100162737.htm'>1.8 小结</a></div><div style='float:right;width:49%'>·<a href='100162738.htm'>2.1 引言</a></div><div style='float:left;width:49%'>·<a href='100162739.htm'>2.2 第一个C#程序</a></div><div style='float:right;width:49%'>·<a href='100162741.htm'>2.4 预定义数据类型</a></div><div style='float:left;width:49%'>·<a href='100162742.htm'>2.5 流控制</a></div><div style='float:right;width:49%'>·<a href='100162743.htm'>2.6 枚举</a></div></div>
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<div id="text"> <link href="css.css" rel="stylesheet" type="text/css" /><p class="MsoNormal"><span><span style="FONT-FAMILY: 宋体">在</span><span lang="EN-US">C#</span></span><span style="FONT-FAMILY: 宋体">中声明变量使用下述语法：</span></p>
<p class="a6" style="MARGIN: 4.9pt 0cm 4.9pt 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">datatype identifier;</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">例如</span><span style="FONT-FAMILY: 宋体">：</span></p>
<p class="2" style="MARGIN: 4.9pt 0cm 4.9pt 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">int i; </span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 16pt"><span style="FONT-FAMILY: 宋体">该语句声明</span><span lang="EN-US">int</span><span style="FONT-FAMILY: 宋体">变量</span><span lang="EN-US">i</span><span style="FONT-FAMILY: 宋体">。编译器不会让我们使用这个变量，除非我们用一个值初始化了该变量。但这个声明会在堆栈中给它分配</span><span lang="EN-US">4</span><span style="FONT-FAMILY: 宋体">个字节，以保存其值。</span><span lang="EN-US"> </span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 16pt"><span style="FONT-FAMILY: 宋体">一旦它被声明之后，就可以使用赋值运算符</span><span lang="EN-US">(</span><span style="FONT-FAMILY: 宋体">＝</span><span lang="EN-US">)</span><span style="FONT-FAMILY: 宋体">给它分配一个值：</span></p>
<p class="2" style="MARGIN: 6.5pt 0cm 6.5pt 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">i = 10;</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 16pt"><span style="FONT-FAMILY: 宋体">还可以在一行代码中声明变量，并初始化它的值：</span></p>
<p class="2" style="MARGIN: 6.5pt 0cm 6.5pt 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">int i = 10; </span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 16pt"><span style="FONT-FAMILY: 宋体">其语法与</span><span lang="EN-US">C++</span><span style="FONT-FAMILY: 宋体">和</span><span lang="EN-US">Java</span><span style="FONT-FAMILY: 宋体">语法相同，但与</span><span lang="EN-US">VB</span><span style="FONT-FAMILY: 宋体">中声明变量的语法完全不同。如果你是一位</span><span lang="EN-US">VB6</span><span style="FONT-FAMILY: 宋体">用户，应记住</span><span lang="EN-US">C#</span><span style="FONT-FAMILY: 宋体">不区分对象和简单的类型，所以不需要类似</span><span lang="EN-US">Set</span><span style="FONT-FAMILY: 宋体">的关键字，即使是要把变量指<span style="LETTER-SPACING: -0.1pt">向一个对象，也不需要</span></span><span lang="EN-US" style="LETTER-SPACING: -0.1pt">Set</span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: -0.1pt">关键字。无论变量的数据类型是什么，声明变量的</span><span lang="EN-US" style="LETTER-SPACING: -0.1pt">C#</span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: -0.1pt">语法都是相同的。</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 16pt"><span style="FONT-FAMILY: 宋体">如果在一个语句中声明和初始化了多个变量，那么所有的变量都具有相同的数据类型：</span></p>
<p class="2" style="MARGIN: 6.5pt 0cm 6.5pt 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">int x = 10, y =20; // x and y are both ints</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 16pt"><span style="FONT-FAMILY: 宋体">要声明类型不同的变量，需要使用单独的语句。在多个变量的声明中，不能指定不同的数据类型：</span></p>
<p class="2" style="MARGIN: 8.15pt 0cm 0pt 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">int x = 10;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">bool y = true; // Creates a variable that stores true or false</span></p>
<p class="2" style="MARGIN: 0cm 0cm 8.15pt 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">int x = 10, bool y = true; // This won't compile!</span></p>
<h3 style="MARGIN: 8.15pt 0cm"><span lang="EN-US">2.3.1 </span><span style="FONT-FAMILY: 黑体">变量的初始化</span></h3>
<p class="MsoNormal" style="LINE-HEIGHT: 16pt"><span><span style="FONT-FAMILY: 宋体">变量的初始化是</span><span lang="EN-US">C#</span></span><span style="FONT-FAMILY: 宋体">强调安全性的另一个例子。简单地说，</span><span lang="EN-US">C#</span><span style="FONT-FAMILY: 宋体">编译器需要用某个初始值对变量进行初始化，之后才能在操作中引用该变量。大多数现代编译器把没有初始化标记为警告，但</span><span lang="EN-US">C#</span><span style="FONT-FAMILY: 宋体">编译器把它当作错误来看待。这就可以防止我们无意中从其他程序遗留下来的内存中获取垃圾值。</span></p>
<p class="MsoNormal"><span lang="EN-US">C#</span><span style="FONT-FAMILY: 宋体">有两个方法可确保变量在使用前进行了初始化：</span></p>
<p class="1" style="MARGIN-LEFT: 37.55pt; TEXT-INDENT: -16.1pt"><span lang="EN-US">●<span style="FONT: 7pt 'Times New Roman'"> </span></span><span style="FONT-FAMILY: 宋体">变量是类或结构中的字段，如果没有显式进行初始化，在默认状态下当创建这些变量时，其值就是</span><span lang="EN-US">0</span><span style="FONT-FAMILY: 宋体">。</span></p>
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