//Url请求地址,postString发送的请求数据
 private string HttpPost(string Url, string postString)
 {
     byte[] byteArray = Encoding.UTF8.GetBytes(postString);//postString请求数据
     HttpWebRequest objWebRequest = (HttpWebRequest)WebRequest.Create(Url);//Url请求地址
     
     objWebRequest.Method = "POST";
     objWebRequest.ContentType = "application/x-www-form-urlencoded";
     objWebRequest.ContentLength = byteArray.Length;
     Stream newStream = objWebRequest.GetRequestStream();
     newStream.Write(byteArray, 0, byteArray.Length);
     newStream.Close();
     HttpWebResponse response = (HttpWebResponse)objWebRequest.GetResponse();
     StreamReader sr = new StreamReader(response.GetResponseStream(), Encoding.UTF8);
     
     // 返回的数据
     string postStringBack= sr.ReadToEnd(); 
     return postStringBack;
 }

 //调用
 string postBack= HttpPost(url, postString);