没有得到IPN的响应

付款成功后，它在成功URL重定向我，但我没有得到IPN的响应。

下面是notifypage.aspx.cs页面代码:

protected void Page_Load(object sender, EventArgs e)
{
    //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#80"));
    //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")
    {
        File.WriteAllText(HttpContext.Current.Server.MapPath("~") + @"'Notify.txt", "Student1111" + Environment.NewLine + "----------------------------------------" + Environment.NewLine);
    }
    else if (strResponse == "INVALID")
    {
        File.WriteAllText(HttpContext.Current.Server.MapPath("~") + @"'Notify.txt", "Problem with payment : " + Environment.NewLine + "----------------------------------------" + Environment.NewLine);
    }
    else
    {
        File.WriteAllText(HttpContext.Current.Server.MapPath("~") + @"'Notify.txt", "not any" + Environment.NewLine + "----------------------------------------" + Environment.NewLine);
    }
}

那么请帮助我如何才能得到IPN的响应?