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<p class=2 style='margin-left:21.45pt;FTEL:18.45pt'><span lang=EN-US>HttpWebRequest
hwrq = (HttpWebRequest)wrq;</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>listBox1.Items.Add("Request
Timeout (ms) = " + wrq.Timeout);</span></p>
<p class=2 style='margin-left:21.45pt;FTEL:18.45pt'><span lang=EN-US>listBox1.Items.Add("Request
Keep Alive = " + hwrq.KeepAlive);</span></p>
<p class=2 style='margin-top:0cm;margin-right:0cm;FTEL:8.15pt;
margin-left:21.45pt;FTEL:18.45pt'><span lang=EN-US>listBox1.Items.Add("Request
AllowAutoRedirect = " + hwrq.AllowAutoRedirect);</span></p>
<p class=MsoNormal><span lang=EN-US>Timeout</span><span style='font-family:
宋体'>属性的单位是毫秒，其默认值是</span><span lang=EN-US>100 000</span><span style='font-family:
宋体'>。可以设置这个属性，以控制</span><span lang=EN-US>WebRequest</span><span
style='font-family:宋体'>对象在产生</span><span lang=EN-US>WebException</span><span
style='font-family:宋体'>之前要在响应中等待多长时间。可以检查属性</span><span lang=EN-US>WebException.Status</span><span
style='font-family:宋体'>，看看产生异常的原因。这个枚举类型包括超时的状态码、连接失败、协议错误等。</span></p>
<p class=MsoNormal><span lang=EN-US>KeepAlive</span><span style='font-family:
宋体'>属性是对</span><span lang=EN-US>HTTP</span><span style='font-family:宋体'>协议的特定扩展，所以可以通过</span><span
lang=EN-US>HttpWebRequest</span><span style='font-family:宋体'>引用访问这个属性。该属性允许多个请求使用同一个连接，在后续的请求中节省关闭和重新打开连接的时间。其默认值为</span><span
lang=EN-US>true</span><span style='font-family:宋体'>。</span></p>
<p class=MsoNormal><span lang=EN-US>AllowAutoRedirect</span><span
style='font-family:宋体'>属性也是专用于</span><span lang=EN-US>HttpWebRequest</span><span
style='font-family:宋体'>类的，使用这个属性可以控制</span><span lang=EN-US>Web</span><span
style='font-family:宋体'>请求是否应自动跟随</span><span lang=EN-US>Web</span><span
style='font-family:宋体'>服务器上的重定向响应。其默认值也是</span><span lang=EN-US>true</span><span
style='font-family:宋体'>。如果只允许有限的重定向，可以把</span><span lang=EN-US>HttpWebRequest</span><span
style='font-family:宋体'>的</span><span lang=EN-US>MaximumAutomaticRedirections</span><span
style='font-family:宋体'>属性设置为想要的数值。</span></p>
<p class=MsoNormal><span style='font-family:宋体'>请求和响应类把大多数重要的标题显示为属性，也可以使用</span><span
lang=EN-US>Headers</span><span style='font-family:宋体'>属性本身显示标题的总集合。在</span><span
lang=EN-US>GetResponse()</span><span style='font-family:宋体'>方法调用的后面添加如下代码，把所有的标题都放在列表框中：</span></p>
<p class=2 style='margin-top:8.15pt;margin-right:0cm;FTEL:0cm;
margin-left:21.45pt;FTEL:.0001pt;FTEL:18.45pt'><span
lang=EN-US>WebRequest wrq = WebRequest.Create("http://www.wrox.com");</span></p>
<p class=2 style='margin-left:21.45pt;FTEL:18.45pt'><span lang=EN-US>WebResponse
wrs = wrq.GetResponse();</span></p>
<p class=2 style='margin-left:21.45pt;FTEL:18.45pt'><span lang=EN-US>WebHeaderCollection
whc = wrs.Headers;</span></p>
<p class=2 style='margin-left:21.45pt;FTEL:18.45pt'><span lang=EN-US>for(int
i = 0; i < whc.Count; i++)</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>
listBox1.Items.Add("Header " + whc.GetKey(i) + " : " +
whc[i]);</span></p>
<p class=2 style='margin-top:0cm;margin-right:0cm;FTEL:8.15pt;
margin-left:21.45pt;FTEL:18.45pt'><span lang=EN-US>}</span></p>
<p class=MsoNormal><span style='font-family:宋体'>这个示例代码会产生如图</span><span
lang=EN-US>31-2</span><span style='font-family:宋体'>所示的标题列表。</span></p>
<p align=center><span lang=EN-US><img width=238 height=140
src="31/image002.jpg"></span></p>
<p align=center style='FTEL:0cm;FTEL:.0001pt'><span
style='font-family:宋体'>图</span><span lang=EN-US> 31-2</span></p>
<h4 style='FTEL:21.45pt'><span lang=EN-US>2. </span><span
style='font-family:黑体'>身份验证</span></h4>
<p class=MsoNormal><span lang=EN-US>WebRequest</span><span style='font-family:
宋体'>类中的另一个属性是</span><span lang=EN-US>Credentials</span><span style='font-family:
宋体'>。如果需要把身份验证证书附带在请求中，就可以用用户名和密码创建</span><span lang=EN-US>NetworkCredential</span><span
style='font-family:宋体'>类</span><span lang=EN-US>(</span><span style='font-family:
宋体'>也在</span><span lang=EN-US>System.Net</span><span style='font-family:宋体'>命名空间中</span><span
lang=EN-US>)</span><span style='font-family:宋体'>的一个实例。在调用</span><span
lang=EN-US>GetResponse()</span><span style='font-family:宋体'>之前，添加下述代码：</span></p>
<p class=2 style='margin-top:8.15pt;margin-right:0cm;FTEL:0cm;
margin-left:21.45pt;FTEL:.0001pt;FTEL:18.45pt'><span
lang=EN-US>NetworkCredential myCred = new
NetworkCredential("myusername", "mypassword");</span></p>
<p class=2 style='margin-top:0cm;margin-right:0cm;FTEL:8.15pt;
margin-left:21.45pt;FTEL:18.45pt'><span lang=EN-US>wrq.Credentials =
myCred;</span></p>
<h4 style='FTEL:21.45pt'><span lang=EN-US>3. </span><span
style='font-family:黑体'>异步页面请求</span></h4>
<p class=MsoNormal><span lang=EN-US>WebRequest</span><span style='font-family:
宋体'>类的另一个特性就是可以异步请求页面。由于在给主机发送请求到接收响应之间有很长的延迟，因此，异步请求页面就显得比较重要。像</span><span
lang=EN-US>WebClient.</span><span
lang=EN-US>DownloadData()</span><span style='font-family:宋体'>和</span><span
lang=EN-US>WebRequest.GetResponse()</span><span style='font-family:宋体'>等方法，在响应没有从服务器回来之前，是不会返回的。如果不希望在那段时间中应用程序处于等待状态，可以使用</span><span
lang=EN-US>BeginGetResponse() </span><span style='font-family:宋体'>方法和</span> <span lang=EN-US>EndGetResponse()</span><span style='font-family:宋体'>方法，</span><span
lang=EN-US>BeginGetResponse()</span><span style='font-family:宋体'>方法可以异步工作，并立即返回。在底层，运行库会异步管理一个后台线程，从服务器上接收响应。</span><span
lang=EN-US>BeginGetResponse() </span><span style='font-family:宋体'>方法不返回</span><span
lang=EN-US>WebResponse</span><span style='font-family:宋体'>对象，而是返回一个执行</span><span
lang=EN-US>IAsyncResult</span><span style='font-family:宋体'>接口的对象。使用这个接口可以选择或等待可用的响应，然后调用</span><span
lang=EN-US>EndGetResponse()</span><span style='font-family:宋体'>搜集结果。</span></p>
<p class=MsoNormal><span style='font-family:宋体'>也可以把一个回调委托发送给</span><span
lang=EN-US>BeginGetResponse()</span><span style='font-family:宋体'>方法。该回调委托的目的地是一个返回类型为</span><span
lang=EN-US>void</span><span style='font-family:宋体'>并把</span><span lang=EN-US>IAsyncResult</span><span
style='font-family:宋体'>引用作为参数的方法，当工作线程完成了搜集响应的任务后，运行库就调用该回调委托，通知用户工作已完成。如下面的代码所示，在回调方法中调用</span><span
lang=EN-US>EndGetResponse()</span><span style='font-family:宋体'>可以接收</span><span
lang=EN-US>WebResponse</span><span style='font-family:宋体'>对象：</span></p>
<p class=2 style='margin-top:8.15pt;margin-right:0cm;FTEL:0cm;
margin-left:21.45pt;FTEL:.0001pt;FTEL:18.45pt'><span
lang=EN-US>public Form1()</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>
InitializeComponent();</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>
WebRequest wrq = WebRequest.Create("http://www.wrox.com");</span></p>
<p class=2 style='margin-left:21.45pt;FTEL:18.45pt'><span lang=EN-US>
wrq.BeginGetResponse(new AsyncCallback(OnResponse),
wrq); </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>protected
void OnResponse(IAsyncResult ar)</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>
WebRequest wrq = (WebRequest)ar.AsyncState;</span></p>
<p class=2 style='margin-left:21.45pt;FTEL:18.45pt'><span lang=EN-US>
WebResponse wrs = wrq.EndGetResponse(ar);</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>
// read the response ...</span></p>
<p class=2 style='margin-top:0cm;margin-right:0cm;FTEL:8.15pt;
margin-left:21.45pt;FTEL:18.45pt'><span lang=EN-US>}</span></p>
<p class=MsoNormal><span style='font-family:宋体'>注意可以把对象传送为</span><span
lang=EN-US>BeginGetResponse()</span><span style='font-family:宋体'>的第二个参数，检索最初的</span><span
lang=EN-US>WebRequest</span><span style='font-family:宋体'>对象。第三个参数是一个对象引用，称为状态参数，在回调方法中，可以使用</span><span
lang=EN-US>IAsyncResult</span><span style='font-family:宋体'>的</span><span
lang=EN-US>AsyncState</span><span style='font-family:宋体'>属性检索相同的状态对象。</span></p>
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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='100165783.htm'><font color='red'>31.2 WebRequest类和WebResponse类</font></a></h1>
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<div style='float:left;width:49%'>·<a href='100165780.htm'>31.1.1 下载文件</a></div><div style='float:right;width:49%'>·<a href='100165781.htm'>31.1.2 基本的Web客户示例</a></div><div style='float:left;width:49%'>·<a href='100165782.htm'>31.1.3 上传文件</a></div><div style='float:right;width:49%'>·<a href='100165784.htm'>31.3 把输出结果显示为HTML页面</a></div><div style='float:left;width:49%'>·<a href='100165785.htm'>31.4 实用工具类</a></div><div style='float:right;width:49%'>·<a href='100165786.htm'>31.4.1 URI</a></div></div>
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