HttpWebRequest 二三事

随着REST风格的流行，直接通过 HttpWebRequest 进行服务调用的客户端应用越来越多。这里总结一些可能需要费时调查的经验，希望能帮助大家。

1. 用完的HttpWebRequest要Abort()或者要把 Response.Close()
否则会导致请求Timeout。 (HttpWebRequest.Method默认是GET)

 static   void  Main( string [] args)
{
 for  ( int  i = 0; i < 10; i++)
    {
         Console.Write( "[{0}] Request - " , i + 1);
         TryGet( "https://login.live.com/" );
    }
    Console.Read();
}
 static   void  TryGet( object  obj)
{
 try
    {
         HttpWebRequest webReq =  null ;
 string  url = ( string )obj;
        webReq = (HttpWebRequest)HttpWebRequest.Create(url);
        webReq.Timeout = 20 * 1000;
         var resp = webReq.GetResponse()  as  HttpWebResponse;
        resp.Close();
         Console.WriteLine( "Get Response StatusCode: {0}({1})" ,
             resp.StatusCode, ( int )resp.StatusCode);
    }
 catch  (WebException we)
    {
         Console.WriteLine( "Get Response StatusCode: {0}({1})" ,
             we.Status, ( int )we.Status);
    }
 catch  (Exception ex)
    {
        Console.WriteLine(ex);
    }
}

static void Main(string[] args) { for (int i = 0; i < 10; i++) { Console.Write("[{0}] Request - ", i + 1); TryGet("https://login.live.com/"); } Console.Read(); } static void TryGet(object obj) { try { HttpWebRequest webReq = null; string url = (string)obj; webReq = (HttpWebRequest)HttpWebRequest.Create(url); webReq.Timeout = 20 * 1000; var resp = webReq.GetResponse() as HttpWebResponse; resp.Close(); Console.WriteLine("Get Response StatusCode: {0}({1})", resp.StatusCode, (int)resp.StatusCode); } catch (WebException we) { Console.WriteLine("Get Response StatusCode: {0}({1})", we.Status, (int)we.Status); } catch (Exception ex) { Console.WriteLine(ex); } }

上面的代码，会从第3次Request开始出现Timeout，因为GetResponse 后 Stream打开未关闭。

解决方法：上面的代码中加上 resp.Close(); 或者 webReq.Abort(); 就能解决。

2. 多线程中调用 HttpWebRequest 时，需要设置 ServicePointManager.DefaultConnectionLimit 数(默认连接数是 2)。
当多线程请求时，同时的连接数超过Limit时，GetResponse会抛出 Timeout WebException。

 // 用多线程同时发出4个请求
 WaitCallback methodTarget =  new  WaitCallback(TryGet);
 ThreadPool.QueueUserWorkItem(methodTarget,  "https://login.live.com/" );
 ThreadPool.QueueUserWorkItem(methodTarget,  "https://login.live.com/" );
 ThreadPool.QueueUserWorkItem(methodTarget,  "https://login.live.com/" );
 ThreadPool.QueueUserWorkItem(methodTarget,  "https://login.live.com/" );

// 用多线程同时发出4个请求 WaitCallback methodTarget = new WaitCallback(TryGet); ThreadPool.QueueUserWorkItem(methodTarget, "https://login.live.com/"); ThreadPool.QueueUserWorkItem(methodTarget, "https://login.live.com/"); ThreadPool.QueueUserWorkItem(methodTarget, "https://login.live.com/"); ThreadPool.QueueUserWorkItem(methodTarget, "https://login.live.com/");

解决方法：在GetResponse()之前设置 ServicePointManager.DefaultConnectionLimit = 100;

3.  当请求一个基于SSL的服务时，默认的验证行为都在 ServicePointManager 定义：
ServicePointManager.CheckCertificateRevocationList = true;

如果请求的服务端证书没有第三方的认证支持，则请求会失败，如果要完全信任服务端证书，则可以将
CheckCertificateRevocationList  设为 false。

4. 可以在 <system.net> 配置节中配置 HttpWebRequest 的属性，包括 WebProxy

 <system.net>
  <connectionManagement>
  </connectionManagement>
  <defaultProxy>
     <proxy proxyaddress= "http://xxx.xxx.xxx.xxx:xxx"  bypassonlocal= "False" />
  </defaultProxy>
  <settings>
       <httpWebRequest useUnsafeHeaderParsing= "true" />
       <servicePointManager checkCertificateName= "true"
                            checkCertificateRevocationList= "true"
                            enableDnsRoundRobin= "true"
                            expect100Continue= "true"
                            useNagleAlgorithm= "true" />
  </settings>
</system.net>

<system.net> <connectionManagement> </connectionManagement> <defaultProxy> <proxy proxyaddress="http://xxx.xxx.xxx.xxx:xxx" bypassonlocal="False"/> </defaultProxy> <settings> <httpWebRequest useUnsafeHeaderParsing="true"/> <servicePointManager checkCertificateName="true" checkCertificateRevocationList="true" enableDnsRoundRobin="true" expect100Continue="true" useNagleAlgorithm="true"/> </settings> </system.net>