string postData = "user=123&pass=456"; // 要发放的数据

    byte[] byteArray = Encoding.UTF8.GetBytes(postData);

    HttpWebRequest objWebRequest = (HttpWebRequest)WebRequest.Create("http://www.abc.com/a.aspx");

    objWebRequest.Method = "POST";

    objWebRequest.ContentType = "application/x-www-form-urlencoded";

    objWebRequest.ContentLength = byteArray.Length;

    Stream newStream = objWebRequest.GetRequestStream();

    // Send the data.

    newStream.Write(byteArray, , byteArray.Length); //写入参数

    newStream.Close();

    HttpWebResponse response = (HttpWebResponse)objWebRequest.GetResponse();

    StreamReader sr = new StreamReader(response.GetResponseStream(), Encoding.Default);

    string textResponse = sr.ReadToEnd(); // 返回的数据