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<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"> isZero = false;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US"> Console.WriteLine("i is Non-zero");</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">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">类似，但与</span><span lang="EN-US">VB</span><span style="FONT-FAMILY: 宋体">不同。</span><span lang="EN-US">VB</span><span style="FONT-FAMILY: 宋体">开发人员注意，</span><span lang="EN-US">C#</span><span style="FONT-FAMILY: 宋体">中没有与</span><span lang="EN-US">VB</span><span style="FONT-FAMILY: 宋体">的</span><span lang="EN-US">EndIf</span><span style="FONT-FAMILY: 宋体">对应的语句，其规则是</span><span lang="EN-US">if</span><span style="FONT-FAMILY: 宋体">的每个子句都只包含一个语句。如果需要多个语句，如上面的例子所示，就应把这些语句放在花括号中，这会把整组语句当作一个语句块来处理。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">还可以单独使用</span><span lang="EN-US">if</span><span style="FONT-FAMILY: 宋体">语句，不加</span><span lang="EN-US">else</span><span style="FONT-FAMILY: 宋体">语句。也可以合并</span><span lang="EN-US">else if</span><span style="FONT-FAMILY: 宋体">子句，测试多个条件。</span></p>
<p class="a6" style="MARGIN: 8.15pt 0cm 0pt 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">using System;</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US"> </span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">namespace Wrox.ProCSharp.Basics</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">{</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US"> class MainEntryPoint</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US"> {</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US"> static void Main(string[] args)</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US"> {</span></p>
<p class="2" style="MARGIN: 8.15pt 0cm 0pt 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US"> Console.WriteLine("Type in a string");</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US"> string input;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US"> input = Console.ReadLine();</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US"> if (input == "")</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"> Console.WriteLine("You typed in an empty string");</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"> else if (input.Length < 5)</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"> Console.WriteLine("The string had less than 5 characters");</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"> else if (input.Length < 10)</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"> Console.WriteLine("The string had at least 5 but less than 10</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US"> characters");</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"> Console.WriteLine("The string was " + input);</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US"> }</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US"> }</span></p>
<p class="a6" 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">添加到</span><span lang="EN-US">if</span><span style="FONT-FAMILY: 宋体">子句中的</span><span lang="EN-US">else if</span><span style="FONT-FAMILY: 宋体">语句的个数没有限制。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">注意在上面的例子中，我们声明了一个字符串变量</span><span lang="EN-US">input</span><span style="FONT-FAMILY: 宋体">，让用户在命令行上输入文本，把文本填充到</span><span lang="EN-US">input</span><span style="FONT-FAMILY: 宋体">中，然后测试该字符串变量的长度。代码还说明了在</span><span lang="EN-US">C#</span><span style="FONT-FAMILY: 宋体">中如何进行字符串处理。例如，要确定</span><span lang="EN-US">input</span><span style="FONT-FAMILY: 宋体">的长度，可以使用</span><span lang="EN-US">input.Length</span><span style="FONT-FAMILY: 宋体">。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">对于</span><span lang="EN-US">if</span><span style="FONT-FAMILY: 宋体">，要注意的一点是如果条件分支中只有一条语句，就无需使用花括号：</span></p>
<p class="2" style="MARGIN: 8.15pt 0cm 0pt 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">if (i == 0)</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US"> Console.WriteLine("i is Zero"); // This will only execute if i == 0</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">Console.WriteLine("i can be anything"); // Will execute whatever the</span></p>
<p class="2" style="MARGIN: 0cm 0cm 8.15pt 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US"> // value of i</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">但是，为了保持一致，许多程序员只要使用</span><span lang="EN-US">if</span><span style="FONT-FAMILY: 宋体">语句，就使用花括号。</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 18pt"><span style="FONT-FAMILY: 宋体">前面介绍的</span><span lang="EN-US">if</span><span style="FONT-FAMILY: 宋体">语句还演示了比较值的一些</span><span lang="EN-US">C#</span><span style="FONT-FAMILY: 宋体">运算符。特别注意，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">一样，</span><span lang="EN-US">C#</span><span style="FONT-FAMILY: 宋体">使用“</span><span lang="EN-US">==</span><span style="FONT-FAMILY: 宋体">”对变量进行等于比较。此时不要使用“</span><span lang="EN-US">=</span><span style="FONT-FAMILY: 宋体">”。“</span><span lang="EN-US">=</span><span style="FONT-FAMILY: 宋体">”用于赋值。</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 18pt"><span style="FONT-FAMILY: 宋体">在</span><span lang="EN-US">C#</span><span style="FONT-FAMILY: 宋体">中，</span><span lang="EN-US">if</span><span style="FONT-FAMILY: 宋体">子句中的表达式必须等于布尔值。</span><span lang="EN-US">C++</span><span style="FONT-FAMILY: 宋体">程序员应特别注意这一点；与</span><span lang="EN-US">C++</span><span style="FONT-FAMILY: 宋体">不同，</span><span lang="EN-US">C#</span><span style="FONT-FAMILY: 宋体">中的</span><span lang="EN-US">if</span><span style="FONT-FAMILY: 宋体">语句不能直接测试整数</span><span lang="EN-US">(</span><span style="FONT-FAMILY: 宋体">例如从函数中返回的值</span><span lang="EN-US">)</span><span style="FONT-FAMILY: 宋体">。在</span><span lang="EN-US">C#</span><span style="FONT-FAMILY: 宋体">中，必须明确地把返回的整数转换为布尔值</span><span lang="EN-US">true </span><span style="FONT-FAMILY: 宋体">或</span><span lang="EN-US"> false</span><span style="FONT-FAMILY: 宋体">，例如，比较值</span><span lang="EN-US">0</span><span style="FONT-FAMILY: 宋体">和</span><span lang="EN-US">null</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">if (DoSomething() != 0)</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"> // Non-zero value returned</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">else</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"> // Returned zero</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">这个限制用于防止</span><span lang="EN-US">C++</span><span style="FONT-FAMILY: 宋体">中某些常见的运行错误，特别是在</span><span lang="EN-US">C++</span><span style="FONT-FAMILY: 宋体">中，当应使用“</span><span lang="EN-US">==</span><span style="FONT-FAMILY: 宋体">”时，常常误输入“＝”，导致不希望的赋值。在</span><span lang="EN-US">C#</span><span style="FONT-FAMILY: 宋体">中，这常常会导致一个编译错误，因为除非在处理</span><span lang="EN-US">bool</span><span style="FONT-FAMILY: 宋体">值，否则“＝”不会返回</span><span lang="EN-US">bool</span><span style="FONT-FAMILY: 宋体">。</span></p>
<h4 style="TEXT-INDENT: 21.45pt"><span lang="EN-US">2. switch</span><span style="FONT-FAMILY: 黑体">语句</span></h4>
<p class="MsoNormal"><span></span><span lang="EN-US">switch<sup>...</sup>case</span><span style="FONT-FAMILY: 宋体">语句适合于从一组互斥的分支中选择一个执行分支。</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">程序员应很熟悉它，该语句类似于</span><span lang="EN-US">VB</span><span style="FONT-FAMILY: 宋体">中的</span><span lang="EN-US">Select Case</span><span style="FONT-FAMILY: 宋体">语句。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">其形式是</span><span lang="EN-US">switch</span><span style="FONT-FAMILY: 宋体">参数的后面跟一组</span><span lang="EN-US">case</span><span style="FONT-FAMILY: 宋体">子句。如果</span><span lang="EN-US">switch</span><span style="FONT-FAMILY: 宋体">参数中表达式的值等于某个</span><span lang="EN-US">case</span><span style="FONT-FAMILY: 宋体">子句旁边的某个值，就执行该</span><span lang="EN-US">case</span><span style="FONT-FAMILY: 宋体">子句中的代码。此时不需要使用花括号把语句组合到块中；只需使用</span><span lang="EN-US">break</span><span style="FONT-FAMILY: 宋体">语句标记每个</span><span lang="EN-US">case</span><span style="FONT-FAMILY: 宋体">代码的结尾即可。也可以在</span><span lang="EN-US">switch</span><span style="FONT-FAMILY: 宋体">语句中包含一个</span><span lang="EN-US">default</span><span style="FONT-FAMILY: 宋体">子句，如果表达式不等于其他</span><span lang="EN-US">case</span><span style="FONT-FAMILY: 宋体">子句的值，就执行</span><span lang="EN-US">default</span><span style="FONT-FAMILY: 宋体">子句的代码。下面的</span><span lang="EN-US">switch</span><span style="FONT-FAMILY: 宋体">语句测试</span><span lang="EN-US">integerA</span><span style="FONT-FAMILY: 宋体">变量的值</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">switch (integerA)</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt; LINE-HEIGHT: 14.5pt"><span lang="EN-US">{</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt; LINE-HEIGHT: 14.5pt"><span lang="EN-US"> case 1:</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt; LINE-HEIGHT: 14.5pt"><span lang="EN-US"> Console.WriteLine("integerA =1");</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt; LINE-HEIGHT: 14.5pt"><span lang="EN-US"> break;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt; LINE-HEIGHT: 14.5pt"><span lang="EN-US"> case 2:</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt; LINE-HEIGHT: 14.5pt"><span lang="EN-US"> Console.WriteLine("integerA =2");</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt; LINE-HEIGHT: 14.5pt"><span lang="EN-US"> break;</span></p>
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