从贝宝沙盒获取响应

我正在尝试将贝宝沙盒与我的asp.net代码集成。要求是我必须带用户到贝宝网站，用户将使用他的凭据登录并支付指定的金额。

现在我可以把用户带到贝宝网站，并按照下面的代码成功完成交易。我的下一步是从贝宝那里得到回复。在响应中，我需要状态，并且我将在重定向到我想要的贝宝网站时发送uniqueid，以便我的应用程序可以识别响应来自哪个用户。

private void PayJNP()
    {
        try
        {
            string redirectUrl = "";
            redirectUrl += "https://www.sandbox.paypal.com/cgi-bin/webscr?cmd=_xclick&business=" + ConfigurationManager.AppSettings["paypalemail"].ToString();
            redirectUrl += "&first_name=ATPTrader";
            redirectUrl += "&item_name=JNP";
            redirectUrl += "&amount=100";
            redirectUrl += "&return=" + ConfigurationManager.AppSettings["SuccessURL"].ToString();
            redirectUrl += "&cancel_return=" + ConfigurationManager.AppSettings["FailedURL"].ToString();
            Response.Redirect(redirectUrl);
        }
        catch (Exception Ex)
        {
        }
    }

请帮我获取回复。

TIA-

我已经开始实施IPN，但我遇到了一些问题

我已经在贝宝网站中配置了IPN

从一个通过一些隐藏的控制，我正在发送数据到贝宝沙盒

<form action="https://www.sandbox.paypal.com/cgi-bin/webscr" method="post" id="form1"
        name="form1">
        <input type="hidden" name="cmd" value="_xclick"/>
        <input type="hidden" name="business" value="shantanusenin@gmail.com"/><!--Paypal or sandbox Merchant account -->
        <input type="hidden" name="item_name" value="JNP"/>
        <input type="hidden" name="item_number" value="1"/>
        <input type="hidden" name="amount" value="100"/>
        <input type="hidden" name="return" value="http://alltradepartners.com/test/paypal.aspx"/><!--this page will be your redirection page -->
        <input type="hidden" name="cancel_return" value="http://alltradepartners.com/test/partnerregistration.aspx"/>
        <input type="hidden" name="currency_code" value="USD"/>
        <input type="hidden" name="notify_url" value="http://alltradepartners.com/test/paypal.aspx"/><!--this should be your domain web page where you going to receive all your transaction variables -->
    </form>

现在我有一个收集响应的页面

//Post back to either sandbox or live
        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);
        //Set values for the request back
        req.Method = "POST";
        req.ContentType = "application/x-www-form-urlencoded";
        byte[] param = Request.BinaryRead(HttpContext.Current.Request.ContentLength);
        string strRequest = Encoding.ASCII.GetString(param);
        strRequest += "&cmd=_notify-validate";
        req.ContentLength = strRequest.Length;
        //for proxy
        //WebProxy proxy = new WebProxy(new Uri("http://url:port#"));
        //req.Proxy = proxy;
        //Send the request to PayPal and get the response
        StreamWriter streamOut = new StreamWriter(req.GetRequestStream(), System.Text.Encoding.ASCII);
        streamOut.Write(strRequest);
        streamOut.Close();
        StreamReader streamIn = new StreamReader(req.GetResponse().GetResponseStream());
        string strResponse = streamIn.ReadToEnd();
        streamIn.Close();
        if (strResponse == "VERIFIED")
        {
            Response.Write("VERIFIED");
            //UPDATE YOUR DATABASE
            //TextWriter txWriter = new StreamWriter(Server.MapPath("../uploads/") + Session["orderID"].ToString() + ".txt");
            //txWriter.WriteLine(strResponse);
            //txWriter.Close();
            //check the payment_status is Completed
            //check that txn_id has not been previously processed
            //check that receiver_email is your Primary PayPal email
            //check that payment_amount/payment_currency are correct
            //process payment


            NameValueCollection these_argies = HttpUtility.ParseQueryString(strResponse);
            string user_email = these_argies["payer_email"];
            string pay_stat = these_argies["payment_status"];
            //.
            //.  more args as needed look at the list from paypal IPN doc
            //.

            if (pay_stat.Equals("Completed"))
            {
                Response.Write("Completed");
                //Send_download_link("yours_truly@mycompany.com", user_email, "Your order", "Thanks for your order this the downnload link ... blah blah blah");
            }       
        }
        else if (strResponse == "INVALID")
        {
            Response.Write("INVALID");
            //UPDATE YOUR DATABASE
            //TextWriter txWriter = new StreamWriter(Server.MapPath("../uploads/") + Session["orderID"].ToString() + ".txt");
            //txWriter.WriteLine(strResponse);
            ////log for manual investigation
            //txWriter.Close();
        }
        else
        {
            Response.Write("SUCCESS");
            //UPDATE YOUR DATABASE
            //TextWriter txWriter = new StreamWriter(Server.MapPath("../uploads/") + Session["orderID"].ToString() + ".txt");
            //txWriter.WriteLine("Invalid");
            ////log response/ipn data for manual investigation
            //txWriter.Close();
        }

现在我可以发送请求并得到验证的响应，但我没有得到transactionID。请建议我错过的地方

TIA