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31.1 WebClient类
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C#高级编程（第3版）【全文连载】
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31.1 WebClient类
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<p class=MsoNormal><span style='font-family:宋体'>在第</span><span lang=EN-US>25~27</span><span
style='font-family:宋体'>章中讨论了怎样使用</span><span lang=EN-US>C#</span><span
style='font-family:宋体'>、</span><span lang=EN-US>ASP.NET</span><span
style='font-family:宋体'>和</span><span lang=EN-US>XML Web</span><span
style='font-family:宋体'>服务编写功能强、效率高的动态</span><span lang=EN-US>Web</span><span
style='font-family:宋体'>页。大多数情况下，访问</span><span lang=EN-US>ASP.NET</span><span
style='font-family:宋体'>页面的客户一般使用的是</span><span lang=EN-US>Internet Explorer</span><span
style='font-family:宋体'>或其他</span><span lang=EN-US>Web</span><span
style='font-family:宋体'>浏览器。但是，有时需要把</span><span lang=EN-US>Web</span><span
style='font-family:宋体'>浏览特性添加到自己的应用程序中，或者需要让自己的应用程序从某个</span><span lang=EN-US>Web</span><span
style='font-family:宋体'>站点获取信息，在后一种情况下，对于站点来说，比较好的解决方案通常是实现</span><span
lang=EN-US>Web</span><span style='font-family:宋体'>服务，但是，如果访问公共的</span><span
lang=EN-US>Internet</span><span style='font-family:宋体'>站点，就不能控制站点的执行方式。</span></p>
<p class=MsoNormal><span style='font-family:宋体'>本章将讨论通过</span><span lang=EN-US>.NET</span><span
style='font-family:宋体'>基类提供的工具，使用各种网络协议</span><span lang=EN-US>(</span><span
style='font-family:宋体'>尤其是</span><span lang=EN-US>HTTP</span><span
style='font-family:宋体'>和</span><span lang=EN-US>TCP)</span><span
style='font-family:宋体'>访问网络和</span><span lang=EN-US>Internet</span><span
style='font-family:宋体'>。本章将讨论的内容包括：</span></p>
<p class=1 style='margin-left:37.55pt;FTEL:-16.1pt'><span lang=EN-US>●<span
style='font:7.0pt "Times New Roman"'> </span></span><span
style='font-family:宋体'>从</span><span lang=EN-US>WWW</span><span
style='font-family:宋体'>下载文件</span></p>
<p class=1 style='margin-left:37.55pt;FTEL:-16.1pt'><span lang=EN-US>●<span
style='font:7.0pt "Times New Roman"'> </span></span><span
style='font-family:宋体'>把</span><span lang=EN-US>Internet Explorer</span><span
style='font-family:宋体'>用作</span><span lang=EN-US>ActiveX</span><span
style='font-family:宋体'>控件</span></p>
<p class=1 style='margin-left:37.55pt;FTEL:-16.1pt'><span lang=EN-US>●<span
style='font:7.0pt "Times New Roman"'> </span></span><span
style='font-family:宋体'>操纵</span><span lang=EN-US>IP</span><span
style='font-family:宋体'>地址，执行</span><span lang=EN-US>DNS</span><span
style='font-family:宋体'>查询</span></p>
<p class=1 style='margin-left:37.55pt;FTEL:-16.1pt'><span lang=EN-US>●<span
style='font:7.0pt "Times New Roman"'> </span></span><span
style='font-family:宋体'>用</span><span lang=EN-US>TCP</span><span
style='font-family:宋体'>、</span><span lang=EN-US>UDP</span><span
style='font-family:宋体'>和套接字类进行套接字编程</span></p>
<p class=MsoNormal><span style='font-family:宋体'>在网络环境下，我们最感兴趣的两个命名空间是</span><span
lang=EN-US>System.Net </span><span style='font-family:宋体'>和</span><span lang=EN-US>System.Net.Sockets</span><span style='font-family:宋体'>。</span><span
lang=EN-US>System.Net</span><span style='font-family:宋体'>命名空间通常与较高层的操作有关，例如下载和上传文件，使用</span><span
lang=EN-US>HTTP</span><span style='font-family:宋体'>和其他协议进行</span><span
lang=EN-US>Web</span><span style='font-family:宋体'>请求等，而</span><span lang=EN-US>System.Net.Sockets</span><span
style='font-family:宋体'>命名空间包含的类通常与较低层的操作有关。如果要直接使用套接字</span><span lang=EN-US>(Socket)</span><span
style='font-family:宋体'>或</span><span lang=EN-US>TCP/IP</span><span
style='font-family:宋体'>之类的协议，这个命名空间中的类是非常有用的，这些类中的方法与</span><span lang=EN-US>Windows</span><span
style='font-family:宋体'>套接字</span><span lang=EN-US>(Winsock)API</span><span
style='font-family:宋体'>函数</span><span lang=EN-US>(</span><span
style='font-family:宋体'>派生自</span><span lang=EN-US>Berkeley</span><span
style='font-family:宋体'>套接字</span><span lang=EN-US>)</span><span
style='font-family:宋体'>非常类似。</span></p>
<p class=MsoNormal><span style='font-family:宋体'>本章将采取实用的方法，结合示例讨论相关理论和相应的网络概念。本章并不是计算机网络的指南，但介绍了如何使用</span><span
lang=EN-US>.NET Framework</span><span style='font-family:宋体'>进行网络通信。</span></p>
<p class=MsoNormal><span style='font-family:宋体'>首先，从最简单的示例开始，这个示例阐明了怎样给服务器发送请求和保存返回的信息</span><span
lang=EN-US>(</span><span style='font-family:宋体'>与其他章节一样，本章的示例代码也可以从</span><span
lang=EN-US>Wrox</span><span style='font-family:宋体'>网站</span><span lang=EN-US>www.wrox.com</span><span
style='font-family:宋体'>上下载</span><span lang=EN-US>)</span><span
style='font-family:宋体'>。</span></p>
<h3 style='margin-top:11.4pt;margin-right:0cm;FTEL:11.4pt;margin-left:
0cm'><a FTEL="_Toc535009257"></a><a FTEL="_Toc507815528"><span lang=EN-US>31.1 WebClient</span></a><span
style='font-family:楷体_GB2312'>类</span></h3>
<p class=MsoNormal><span style='font-family:宋体'>如果只想从特定的</span><span
lang=EN-US>URI</span><span style='font-family:宋体'>请求文件，则可以使用的最简单</span><span
lang=EN-US>.NET</span><span style='font-family:宋体'>基类就是</span><span lang=EN-US>System.Net.WebClient</span><span
style='font-family:宋体'>。这个类是非常高层的类，它主要用于执行带有一个或两个命令的操作。</span><span lang=EN-US>.NET
Framework</span><span style='font-family:宋体'>目前支持以</span><span lang=EN-US>http:</span><span
style='font-family:宋体'>、</span><span lang=EN-US>https:</span><span
style='font-family:宋体'>和</span><span lang=EN-US>file:</span><span
style='font-family:宋体'>标识符开头的</span><span lang=EN-US>URI</span><span
style='font-family:宋体'>。</span></p>
<p class=a3 style='margin-top:8.15pt;FTEL:21.45pt'><span
style='font-family:黑体'>注意：</span></p>
<p class=a1 style='FTEL:8.15pt;FTEL:21.45pt'><span
style='font-family:楷体_GB2312'>术语</span><span lang=EN-US>URL(</span><span
style='font-family:楷体_GB2312'>统一资源定位符</span><span lang=EN-US>)</span><span
style='font-family:楷体_GB2312'>在新的技术规范中已不再使用，现在使用的是</span><span lang=EN-US>URI(</span><span
style='font-family:楷体_GB2312'>统一资源标识符</span><span lang=EN-US>)</span><span
style='font-family:楷体_GB2312'>。</span><span lang=EN-US>URI</span><span
style='font-family:楷体_GB2312'>的含义大致与</span><span lang=EN-US>URL</span><span
style='font-family:楷体_GB2312'>相同，但</span><span lang=EN-US>URI</span><span
style='font-family:楷体_GB2312'>更通用，因为它不隐含正在使用的协议，如</span><span lang=EN-US>HTTP</span><span
style='font-family:楷体_GB2312'>或</span><span lang=EN-US>FTP</span><span
style='font-family:楷体_GB2312'>。</span></p>
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