为什么在asp.net中使用c#使用沙盒验证paypal响应时内容长度为零？

我是新的贝宝集成在我的ASP.NET应用程序。以下是我使用C#为ASP.NET编写的paypal代码：

为什么我得到的

ContentLength

值为0

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#"));
    //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")
    {
        //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
    }
    else if (strResponse == "INVALID")
    {
        //log for manual investigation
    }
    else
    {
        //log response/ipn data for manual investigation
    }
}