WebRequest Post Xml

WebRequest request = WebRequest.Create ("http://www.contoso.com/PostAccepter.aspx ");
            // Set the Method property of the request to POST.
            request.Method = "POST";
            // Create POST data and convert it to a byte array.
            string postData = "This is a test that posts this string to a Web server.";
            byte[] byteArray = Encoding.UTF8.GetBytes (postData);
            // Set the ContentType property of the WebRequest.
            request.ContentType = "application/x-www-form-urlencoded";
            // Set the ContentLength property of the WebRequest.
            request.ContentLength = byteArray.Length;
            // Get the request stream.
            Stream dataStream = request.GetRequestStream ();
            // Write the data to the request stream.
            dataStream.Write (byteArray, 0, byteArray.Length);
            // Close the Stream object.
            dataStream.Close ();
            // Get the response.
            WebResponse response = request.GetResponse ();
            // Display the status.
            Console.WriteLine (((HttpWebResponse)response).StatusDescription);
            // Get the stream containing content returned by the server.
            dataStream = response.GetResponseStream ();
            // Open the stream using a StreamReader for easy access.
            StreamReader reader = new StreamReader (dataStream);
            // Read the content.
            string responseFromServer = reader.ReadToEnd ();
            // Display the content.
            Console.WriteLine (responseFromServer);
            // Clean up the streams.
            reader.Close ();
            dataStream.Close ();
            response.Close ();
Q.这是msdn 上面的一个例子，用这个方法post 一般的字符 没有问题，但是post一个xml串 如下：
      System.Xml.XmlDocument xmlpostdata = new System.Xml.XmlDocument();
        xmlpostdata.Load("d:\\ss\\test.xml");
        string postData = xmlpostdata.InnerXml;

在 WebResponse response = request.GetResponse ();

提示错误  WebExceptionStatus ProtocolError

A.将string postData = xmlpostdata.InnerXml;
改为
string postData = HttpUtility.HtmlEncode(xmlpostData.InnerXml);