银行接口Post方式提交，xml怎么直接通过post提交？

悬赏园豆：5 [待解决问题]

XML模板

XMLModel.xml

<?xml version="1.0" encoding="GBK"?>
<stream>
  <B2CCERTNO>500001</B2CCERTNO>
  <ORDERMODE>01</ORDERMODE>
  <ORDERDATE></ORDERDATE>
  <ORDERTIME></ORDERTIME>
  <ORDERNO></ORDERNO>
  <CURRID>01</CURRID>
  <ORDERAMT></ORDERAMT>
  <MEMO>充值支付</MEMO>
  <NOTIFYMODE>01</NOTIFYMODE>
  <NOTIFYURL>https://abc.com/notify</NOTIFYURL>
    <RISKLEVEL>00</RISKLEVEL>
  <SUPPTCARDTYPE>00</SUPPTCARDTYPE>
  <TTL></TTL>
  <MEMBERID></MEMBERID>
  <NOTIFYSCOPE></NOTIFYSCOPE>
  <ISSAFEINF></ISSAFEINF>
  <REFERER></REFERER>
  <list name="OrderListQue">
    <row>
      <PRODUCTNAME></PRODUCTNAME>
      <PRODUCTCODE></PRODUCTCODE>
      <QUANTITY></QUANTITY>
      <SUBTOTAL></SUBTOTAL>
      <TERM></TERM>
      <NOTE></NOTE>
    </row>
  </list>
</stream>

主程序

        /// <summary>
        /// 获取提交参数、XML模板、提交到银行
        /// </summary>
        public void SubXmlPost(string OrderNo,string OrderRamt)
        {
            //获取XML模板
            XmlDataDocument xmlDoc = new XmlDataDocument();
            xmlDoc.Load(Server.MapPath("XMLModel.xml"));

            //修正XML信息
            XmlNodeList nodeList = xmlDoc.SelectSingleNode("stream").ChildNodes;//获取stream节点下的所有子节点
            foreach(XmlNode xn in nodeList)
            {
                XmlElement xe = (XmlElement)xn;
                //填充订单日期
                if (xe.Name == "ORDERDATE")
                {
                    xe.InnerText = DateTime.Now.ToString("yyyy-MM-dd");
                }

                //填充订单时间
                if (xe.Name == "ORDERTIME")
                {
                    xe.InnerText = DateTime.Now.ToString("hh:mm:ss");
                }

                //订单编号
                if (xe.Name == "ORDERNO")
                {
                    xe.InnerText = OrderNo.ToString();
                }

                //订单金额
                if (xe.Name == "ORDERAMT")
                {
                    xe.InnerText = OrderRamt.ToString();
                }
            }

            //发送到银行
            HttpWebRequest req = (HttpWebRequest)HttpWebRequest.Create("https://ec.test.bank.ecitic.com/pec/e3rdplaceorder.do");
            req.Method = "POST";
            //设置 Content-type HTTP 标头的值
            //req.ContentType = "text/xml";// "application/x-www-form-urlencoded;charset=gb2312";


            using (StreamWriter SubWri = new StreamWriter(req.GetRequestStream()))
            {
                StreamReader reader = new StreamReader(xmlDoc);
                string ret = xmlDoc.
                    //这里就有问题了，xmlDoc是一个XmlDataDocument，我要怎么把一个XmlDataDocument直接通过POST的方式传输出去？
                    //我本来想把xmlDoc放到StreamReader里面然后在变成string，再通过WriteLine出去，然是放不进去，希望指教



                SubWri.WriteLine(ret);
                

            }
        }