c#调用xmlhttp时报了异常，求解

Question

C# 用System.Net　里的类 or HttpWebResponse
域入类似入下   HttpWebResponse result = (HttpWebResponse)req.GetResponse();
                Stream ReceiveStream = result.GetResponseStream();
                Encoding enc = System.Text.Encoding.GetEncoding("gb2312");
                XmlDocument doc = new XmlDocument();
                doc.Load(ReceiveStream);　　　　　　　　为什么还要用com