C# post servlet中 Stream stream = HttpWebRequest.GetRequestStream()出错

 WebRequest HttpWebRequest = null;
                WebResponse HttpWebResponse = null;
                HttpWebRequest = WebRequest.Create(uri);
                //HttpWebRequest.ContentType = "application/x-www-form-urlencoded";
                HttpWebRequest.ContentType = "text/xml";
                HttpWebRequest.Method = "Post";
                XmlDocument doc = new XmlDocument();
                doc.Load("D://test.xml");
                string strXml = doc.InnerXml;
                strXml = "xml=" + strXml;
                //strXml = HttpUtility.UrlEncode(strXml, System.Text.Encoding.UTF8);

                byte[] byteRequest = Encoding.UTF8.GetBytes(strXml);
                //byte[] byteRequest = Encoding.GetEncoding("GB2312").GetBytes(strXml);   
                HttpWebRequest.ContentLength = byteRequest.Length;
                Stream stream = HttpWebRequest.GetRequestStream();
                stream.Write(byteRequest, 0, byteRequest.Length);
                stream.Close();
                HttpWebResponse = HttpWebRequest.GetResponse();
                Stream res = HttpWebResponse.GetResponseStream();
                StreamReader sr = new StreamReader(res);
                string resultstring = sr.ReadToEnd();
                XmlDocument xdoc = new XmlDocument();
                xdoc.LoadXml(resultstring);

出错代码为：
Length = “stream.Length”引发了“System.NotSupportedException”类型的异常
Position = “stream.Position”引发了“System.NotSupportedException”类型的异常
求高手帮助 stream C# length --------------------编程问答-------------------- 没有高人指点吗，自己先顶一下 --------------------编程问答-------------------- 求助！！！！！！！！！！！

补充：.NET技术 ,  C#