HttpWebRequest post参数有关问题

       HttpWebRequest req = (HttpWebRequest)WebRequest.Create(url);            req.KeepAlive = false;            req.Method = "POST";            req.UserAgent = "Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.2; .NET CLR 1.1.4322; .NET CLR 2.0.50727)";            req.Credentials = CredentialCache.DefaultCredentials;            req.ContentType = "application/x-form-www-urlencoded";            Encoding encode = new UTF8Encoding();            byte[] b = encode.GetBytes(string.Format("Content={0}&Type={1}&StartDate={2}&EndDate={3}", content, columnname.ToLower() == "macaddress" ? 1 : 0, DateTime.Now.AddDays(-1).Date.ToString("yyyy-MM-dd 00:00:00"), DateTime.Now.Date.ToString("yyyy-MM-dd 00:00:00")));            req.ContentLength = b.Length;            using (Stream stream = req.GetRequestStream())            {                stream.Write(b, 0, b.Length);            }            WebResponse res = req.GetResponse();            StreamReader sr = new StreamReader(res.GetResponseStream(), encode);            string result = sr.ReadToEnd();            res.Close();            sr.Close();