Encoding myEncoding = Encoding.GetEncoding("gb2312");  
string param =   
HttpUtility.UrlEncode("参数一", myEncoding) +   
"=" + HttpUtility.UrlEncode("值一", myEncoding) +   
"&" + HttpUtility.UrlEncode("参数二", myEncoding) +  
"=" + HttpUtility.UrlEncode("值二", myEncoding);  
 
byte[] postBytes = Encoding.ASCII.GetBytes(param);  
 
HttpWebRequest req = (HttpWebRequest)  
HttpWebRequest.Create( "http://www.baidu.com/s" );  
req.Method = "POST";  
req.ContentType =   
"application/x-www-form-urlencoded;charset=gb2312";  
req.ContentLength = postBytes.Length;  
 
using (Stream reqStream = req.GetRequestStream())  
{  
   reqStream.Write(bs, 0, bs.Length);  
}  
using (WebResponse wr = req.GetResponse())  
{  
   //在这里对接收到的页面内容进行处理  
}