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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"> class NamespaceExample</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"> // Code for the class here...</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: 0cm 0cm 8.15pt 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">}</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 17pt"><span style="FONT-FAMILY: 宋体">注意不允许在另一个嵌套的命名空间中声明多部分的命名空间。</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 17pt"><span style="FONT-FAMILY: 宋体">命名空间与程序集无关。同一个程序集中可以有不同的命名空间，也可以在不同的程序集中定义同一个命名空间中的类型。</span></p>
<h3 style="MARGIN: 8.15pt 0cm"><span lang="EN-US">2.8.1 using</span><span style="FONT-FAMILY: 黑体">语句</span></h3>
<p class="MsoNormal" style="LINE-HEIGHT: 17pt"><span><span style="FONT-FAMILY: 宋体">显然，命名空间相当长，键入起来很繁琐，用这种方式指定某个特定的类也是不必要的。如本章开头所述，</span><span lang="EN-US">C#</span></span><span style="FONT-FAMILY: 宋体">允许简写类的全名。为此，要在文件的顶部列出类的命名空间，前面加上</span><span lang="EN-US">using</span><span style="FONT-FAMILY: 宋体">关键字。在文件的其他地方，就可以使用其类型名称来引用命名空间中的类型了：</span></p>
<p class="2" style="MARGIN: 8.15pt 0cm 0pt 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">using System;</span></p>
<p class="2" style="MARGIN: 0cm 0cm 8.15pt 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">using Wrox.ProCSharp;</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 17pt"><span style="FONT-FAMILY: 宋体">如前所述，所有的</span><span lang="EN-US">C#</span><span style="FONT-FAMILY: 宋体">源代码都以语句</span><span lang="EN-US">using System;</span><span style="FONT-FAMILY: 宋体">开头，这仅是因为</span><span lang="EN-US">Microsoft</span><span style="FONT-FAMILY: 宋体">提供的许多有用的类都包含在</span><span lang="EN-US">System</span><span style="FONT-FAMILY: 宋体">命名空间中。</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 17pt"><span style="FONT-FAMILY: 宋体">如果</span><span lang="EN-US">using</span><span style="FONT-FAMILY: 宋体">指令引用的两个命名空间包含同名的类，就必须使用完整的名称</span><span lang="EN-US">(</span><span style="FONT-FAMILY: 宋体">或者至少较长的名称</span><span lang="EN-US">)</span><span style="FONT-FAMILY: 宋体">，确保编译器知道访问哪个类型，例如，类</span><span lang="EN-US">NamespaceExample</span><span style="FONT-FAMILY: 宋体">同时存在于</span><span lang="EN-US">Wrox.ProCSharp.Basics</span><span style="FONT-FAMILY: 宋体">和</span><span lang="EN-US" style="LETTER-SPACING: 0.2pt">Wrox.ProCSharp.OOP</span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: 0.2pt">命名空间中，如果要在命名空间</span><span lang="EN-US">Wrox.ProCSharp</span><span style="FONT-FAMILY: 宋体">中创建一个类</span><span lang="EN-US">Test</span><span style="FONT-FAMILY: 宋体">，并在该类中实例化一个</span><span lang="EN-US">NamespaceExample</span><span style="FONT-FAMILY: 宋体">类，<span style="LETTER-SPACING: 0.2pt">就需要指定使用哪个类：</span></span></p>
<p class="2" style="MARGIN: 8.15pt 0cm 0pt 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">using Wrox.ProCSharp;</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">class Test</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"> public static int 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"> Basics.NamespaceExample nSEx = new Basics.NamespaceExample();</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US"> //do something with the nSEx variable</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US"> return 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: 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">因为</span><span lang="EN-US">using</span><span style="FONT-FAMILY: 宋体">语句在</span><span lang="EN-US">C#</span><span style="FONT-FAMILY: 宋体">文件的开头，</span><span lang="EN-US">C</span><span style="FONT-FAMILY: 宋体">和</span><span lang="EN-US">C++</span><span style="FONT-FAMILY: 宋体">也把</span><span lang="EN-US">#include</span><span style="FONT-FAMILY: 宋体">放在这里，所以从</span><span lang="EN-US">C++</span><span style="FONT-FAMILY: 宋体">迁移到</span><span lang="EN-US">C#</span><span style="FONT-FAMILY: 宋体">的程序员常把命名空间与</span><span lang="EN-US">C++</span><span style="FONT-FAMILY: 宋体">风格的头文件相混淆。不要犯这种错误，</span><span lang="EN-US">using</span><span style="FONT-FAMILY: 宋体">语句在这些文件之间并没有真正建立物理链接。</span><span lang="EN-US">C#</span><span style="FONT-FAMILY: 宋体">也没有对应于</span><span lang="EN-US">C++</span><span style="FONT-FAMILY: 宋体">头文件的部分。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">公司应花一定的时间开发一种命名空间模式，这样其开发人员才能快速定位他们所需要的功能，而且公司内部使用的类名也不会与外部的类库相冲突。本章后面将介绍建立命名空间模式的规则和其他命名约定。</span></p>
<h3 style="MARGIN: 8.15pt 0cm"><span lang="EN-US">2.8.2 </span><span style="FONT-FAMILY: 黑体">命名空间的别名</span></h3>
<p class="MsoNormal"><span><span lang="EN-US">using</span></span><span style="FONT-FAMILY: 宋体">关键字的另一个用途是给类和命名空间指定别名。如果命名空间的名称非常长，又要在代码中使用多次，但不希望该命名空间的名称包含在</span><span lang="EN-US">using</span><span style="FONT-FAMILY: 宋体">指令中</span><span lang="EN-US">(</span><span style="FONT-FAMILY: 宋体">例如，避免类名冲突</span><span lang="EN-US">)</span><span style="FONT-FAMILY: 宋体">，就可以给该命名空间指定一个别名，其语法如下：</span></p>
<p class="a6" style="MARGIN: 8.15pt 0cm 8.15pt 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">using alias = NamespaceName;</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">下面的例子</span><span lang="EN-US">(</span><span style="FONT-FAMILY: 宋体">前面例子的修订版本</span><span lang="EN-US">)</span><span style="FONT-FAMILY: 宋体">给</span><span lang="EN-US">Wrox.ProCSharp.Basics</span><span style="FONT-FAMILY: 宋体">命名空间指定别名</span><span lang="EN-US">Introduction</span><span style="FONT-FAMILY: 宋体">，并使用这个别名实例化了一个</span><span lang="EN-US">NamespaceExample</span><span style="FONT-FAMILY: 宋体">对象，这个对象是在该命名空间中定义的。它有一个方法</span><span lang="EN-US">GetNamespace()</span><span style="FONT-FAMILY: 宋体">，该方法调用每个类都有的</span><span lang="EN-US">GetType()</span><span style="FONT-FAMILY: 宋体">方法，以访问表示类的类型的</span><span lang="EN-US">Type</span><span style="FONT-FAMILY: 宋体">对象。下面使用这个对象来返回类的命名空间名：</span></p>
<p class="2" style="MARGIN: 8.15pt 0cm 0pt 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">using System;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">using Introduction = Wrox.ProCSharp.Basics;</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">class Test</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"> public static int Main()</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"> Introduction.NamespaceExample NSEx =</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US"> new Introduction.NamespaceExample();</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US"> Console.WriteLine(NSEx.GetNamespace());</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US"> return 0;</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-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">namespace Wrox.ProCSharp.Basics</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"> class NamespaceExample</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"> public string GetNamespace()</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"> return this.GetType().Namespace;</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"> }</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; TEXT-INDENT: 18.45pt"><span lang="EN-US">}</span></p></div>
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<h1 id="divCurrentNode2" style="color: #b83507; width: 100%; text-align: left; font-size: 12px; padding-left: 2px">当前章节:<a href='100162745.htm'><font color='red'>2.8 命名空间</font></a></h1>
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<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 style='float:left;width:49%'>·<a href='100162744.htm'>2.7 数组</a></div><div style='float:right;width:49%'>·<a href='100162746.htm'>2.9 Main()方法</a></div><div style='float:left;width:49%'>·<a href='100162747.htm'>2.10 有关编译C#文件的更多内容</a></div><div style='float:right;width:49%'>·<a href='100162748.htm'>2.11 控制台I/O</a></div></div>
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