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8.1.1 创建字符串
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8.1.1 创建字符串
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2006-10-13 14:41:00
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<div style='float:left;width:49%'>·<a href='100165351.htm'>7.3.2 使用指针优化性能</a></div><div style='float:right;width:49%'>·<a href='100165352.htm'>7.4 小结</a></div><div style='float:left;width:49%'>·<a href='100165353.htm'>8.1 System.String类</a></div><div style='float:right;width:49%'>·<a href='100165355.htm'>8.1.2 格式化字符串</a></div><div style='float:left;width:49%'>·<a href='100165356.htm'>8.2 正则表达式</a></div><div style='float:right;width:49%'>·<a href='100165357.htm'>8.3 小结</a></div></div>
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<div id="text"> <link href="css.css" rel="stylesheet" type="text/css" /><h3 style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 0cm; MARGIN-RIGHT: 0cm; FTEL: 8.15pt"><a ftel="_Toc507815065"><span lang="EN-US">8.1.1 </span></a><span style="FONT-FAMILY: 黑体">创建字符串</span></h3>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">如上所述，</span><span lang="EN-US">string</span><span style="FONT-FAMILY: 宋体">类是一个功能非常强大的类，它执行许多很有用的方法。但是，</span><span lang="EN-US">string</span><span style="FONT-FAMILY: 宋体">类存在一个问题：重复修改给定的字符串，效率会很低，它实际上是一个不可变的数据类型，一旦对字符串对象进行了初始化，该字符串对象就不能改变了。修改字符串内容的方法和运算符实际上<span style="LETTER-SPACING: -0.1pt">是创建一个新的字符串，如果必要，可以把旧字符串的内容复制到新字符串中。例如，下面的代码：</span></span></p>
<p class="2" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">string greetingText = "Hello from all the guys at Wrox Press. ";</span></p>
<p class="2" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">greetingText += "We do hope you enjoy this book as much as we enjoyed writing it.";</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">在执行这段代码时，首先，创建一个</span><span lang="EN-US">System.String</span><span style="FONT-FAMILY: 宋体">类型的对象，并初始化为文本“</span><span lang="EN-US">Hello from all the people at Wrox Press. </span><span style="FONT-FAMILY: 宋体">”。注意该行最后的空格。此时</span><span lang="EN-US">.NET </span><span style="FONT-FAMILY: 宋体">运行库会为该字符串分配足够的内存来保存这个文本</span><span lang="EN-US">(39</span><span style="FONT-FAMILY: 宋体">个字符</span><span lang="EN-US">)</span><span style="FONT-FAMILY: 宋体">，再设置变量</span><span lang="EN-US">greetingText</span><span style="FONT-FAMILY: 宋体">，表示这个字符串实例。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">从语法上看，下一行代码是把更多的文本添加到字符串中。实际上并非如此，而是创建一个新字符串实例，给它分配足够的内存，以保存合并起来的文本</span><span lang="EN-US">(</span><span style="FONT-FAMILY: 宋体">共</span><span lang="EN-US">103</span><span style="FONT-FAMILY: 宋体">个字符</span><span lang="EN-US">)</span><span style="FONT-FAMILY: 宋体">。最初的文本“</span><span lang="EN-US">Hello from all the people at Wrox Press. </span><span style="FONT-FAMILY: 宋体">”复制到这个新字符串中，再加上额外的文本“</span><span lang="EN-US">We do hope you enjoy this book as much as we enjoyed writing it</span><span style="FONT-FAMILY: 宋体">”。然后更新存储在变量</span><span lang="EN-US">greetingText</span><span style="FONT-FAMILY: 宋体">中的地址，使变量正确地指向新的字符串对象。旧的字符串对象被撤销了引用——不再有变量引用它，下一次垃圾收集器清理应用程序中所有未使用的对象时，就会删除它。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">这本身还不坏，但假定要对这个字符串加密，在字母表中，用</span><span lang="EN-US">ASCII</span><span style="FONT-FAMILY: 宋体">码中的字符替代其中的每个字母</span><span lang="EN-US">(</span><span style="FONT-FAMILY: 宋体">标点符号除外</span><span lang="EN-US">)</span><span style="FONT-FAMILY: 宋体">，以便以后作为非常简单的加密模式的一部分，就会把该字符串变成“</span><span lang="EN-US">Ifmmp gspn bmm uif hvst bu Xspy Qsftt. Xf ep ipqf zpv fokpz uijt cppl bt nvdi bt xf fokpzfe xsjujoh ju.</span><span style="FONT-FAMILY: 宋体">”。完成这个任务有好几种方式，但最简单、最高效的一种</span><span lang="EN-US">(</span><span style="FONT-FAMILY: 宋体">假定只使用</span><span lang="EN-US">String</span><span style="FONT-FAMILY: 宋体">类</span><span lang="EN-US">)</span><span style="FONT-FAMILY: 宋体">是使用</span><span lang="EN-US">String.Replace()</span><span style="FONT-FAMILY: 宋体">方法，把字符串中指定的子字符串用另一个子字符串代替。使用</span><span lang="EN-US">Replace()</span><span style="FONT-FAMILY: 宋体">，加密文本的代码如下所示：</span></p>
<p class="a6" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><a ftel="Replace1"><span lang="EN-US">string greetingText = "Hello from all the guys at Wrox Press. ";</span></a></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">greetingText += "We do hope you enjoy this book as much as we enjoyed writing it.";</span></p>
<p class="a6" 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">for(int i = (int)'z'; i>=(int)'a' ; i</span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: -0.6pt">––</span><span lang="EN-US">)</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"> char old1 = (char)i;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> char new1 = (char)(i+1);</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> greetingText = greetingText.Replace(old1, new1);</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"> </span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">for(int i = (int)'Z'; i>=(int)'A' ; i</span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: -0.8pt">––</span><span lang="EN-US">)</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"> char old1 = (char)i;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> char new1 = (char)(i+1);</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> greetingText = greetingText.Replace(old1, new1);</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">}</span></p>
<p class="2" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">Console.WriteLine("Encoded:\n" + greetingText);</span></p>
<p class="a3" style="MARGIN-TOP: 8.15pt; FTEL: 21.45pt"><span style="FONT-FAMILY: 黑体">注意：</span></p>
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