WebClient client = new WebClient();
clinet.DownloadString(@"http://PageA.aspx");
clinet.DownloadString(@"http://PageB.aspx");

CookieContainer cc = new CookieContainer();
for(int i=0;i<100;i++)
{
HttpWebRequest myReq = (HttpWebRequest)WebRequest.Create("http://localhost/AspxApp/MainForm.aspx");
myReq.CookieContainer = cc;
HttpWebResponse resp = myReq.GetResponse() as HttpWebResponse;
Stream s = resp.GetResponseStream();
StreamReader sr = new StreamReader(s); String text = sr.ReadToEnd();
sr.Close();
s.Close();
} 

    这样当然可以解决问题,但是有一个不好地方,就是HttpWebRequest的功能比较弱,只能返回流,下载文件时不能异步,没有进度.
我通过使用Reflector查看了一下WebClient的代码,发现WebClient每次发请求(如DownloadString)时都会调用自己的一个叫GetWebRequest的方法,查MSDN,发现GetWebRequest 方法签名如下

protected virtual WebRequest GetWebRequest ( Uri address ) 

    也就意味着,如果我们重写这个方法,把CookeContainer给这个WebRequest加上,那么WebClinet就支持会话了. 实现代码如下:

public class HttpWebClient:WebClient
{
private CookieContainer cookie ;
protected override WebRequest GetWebRequest(Uri address)
{
//throw new Exception(); 
WebRequest request ;
request = base.GetWebRequest(address);
//判断是不是HttpWebRequest.只有HttpWebRequest才有此属性 
if (request is HttpWebRequest)
{
HttpWebRequest httpRequest = request as HttpWebRequest;
httpRequest.CookieContainer = cookie;
}
return request;
}
public HttpWebClient(CookieContainer cookie)
{
this.cookie = cookie;
}
} 

    调用代码如下:

ttpWebClient httpClient = new HttpWebClient(new CookieContainer());
String s = httpClient.DownloadString(@"http://localhost/TestWS/Default.aspx");
s = httpClient.DownloadString(String.Format(@"http://localhost/TestWS/Test.aspx?Test={0}",s));
Console.WriteLine(s);