Paypal Sandbox IPN always response Invalid.

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Data.SqlClient;
using System.IO;
using System.Text;
using System.Threading;
using System.Net;
using System.Net.Mail;


public partial class Paypal : System.Web.UI.Page
{
    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

               Response.Write(strResponse);
        }
        else if (strResponse == "INVALID")
        {
            //log for manual investigation
                 Response.Write(strResponse);
        }
        else
        {
            //log response/ipn data for manual investigation
             Response.Write("Cant Get Response");
        }
    }
}