C# HttpWebRequest如何使用Post提交网页？

请教各位高手，这个网页
http://www.taifex.com.tw/chinese/3/3_1_1.asp?syear=2010&smonth=10&sday=7
如何通过Post的方法修改契约，然后提交，谢谢！
我修改了契约但是得到的页面还是旧的，不知道什么原因。

代码：

String strUrl = @"http://www.taifex.com.tw/chinese/3/3_1_1.asp?syear=2010&smonth=10&sday=7";

HttpWebRequest req = (HttpWebRequest)HttpWebRequest.Create(strUrl);

string PostData = @"commodity_idt=CPF";
WriteLogFile(PostData);
byte[] bs = Encoding.ASCII.GetBytes(PostData);

req.Method = "POST";
req.ContentType = "application/x-www-form-urlencoded";
req.ContentLength = bs.Length;

Stream outStream = req.GetRequestStream();
outStream.Write(bs,0,bs.Length);
outStream.flush();
outStream.Close();

WebResponse myResp = null;
try { //接收HTTP做出的响应
myResp = req.GetResponse();
}
catch(Exception e)
{
int ii =0;
}
Stream ReceiveStream = myResp.GetResponseStream();

StreamReader sReader = new StreamReader(ReceiveStream);
string strContent = sReader.ReadToEnd();

ReceiveStream.Close();
sReader.Close();
myResp.Close();