protected void Page_Load(object sender, EventArgs e)
    {
        
        string authToken = "XXXXXX";
        string txToken = Request.QueryString["tx"];
        string query = "cmd=_notify-synch&tx=" + txToken + "&at=" + authToken;
        string strSandbox = "https://www.sandbox.paypal.com/cgi-bin/webscr";
        string strLive = "https://www.paypal.com/cgi-bin/webscr";
        HttpWebRequest req = (HttpWebRequest)WebRequest.Create(strSandbox);

        //公司用代理服务器上网
        WebProxy proxy = new WebProxy("XXXXX:XXX", false);
        proxy.Credentials = new NetworkCredential("XXXXX", "XXXXX"); 
        req.AllowAutoRedirect = false;
        req.Proxy = proxy;

        
        req.Method = "POST";
        req.ContentType = "application/x-www-form-urlencoded";
        req.ContentLength = query.Length;

        StreamWriter streamOut = new StreamWriter(req.GetRequestStream(), System.Text.Encoding.ASCII);
        streamOut.Write(query);
        streamOut.Close();
        StreamReader streamIn = new StreamReader(req.GetResponse().GetResponseStream());
        string strResponse = streamIn.ReadToEnd();
        streamIn.Close();

        Dictionary<string, string> results = new Dictionary<string, string>();
        if (strResponse != "")
        {
            StringReader reader = new StringReader(strResponse);
            string line = reader.ReadLine();
            
            //支付成功后进行状态修改
            if (line == "SUCCESS")
            {
                UserOrder order = UserOrderDAL.m_UserOrderDAL.GetModel(string.Format("OrderCode='{0}'", txToken));
                order.IsPay = 1;
                order.Status = 2;
                UserOrderDAL.m_UserOrderDAL.Update(order);
                Response.Write(string.Format("Order Number：{0} Pay for success. The total amount of：{1}", txToken, order.SubPrice + 5));
            }
            else if (line == "FAIL")
            {
                Response.Write("Order payment failure");
            }
        }
        else
        {
            Response.Write("ERROR");
        }           
    }


报错：