public CookieCollection cookie = new CookieCollection();
string post(string url, string postData)
{
    ASCIIEncoding encoding = new ASCIIEncoding();
    byte[] bytesToPost = encoding.GetBytes(postData);
    CookieContainer cookieCon = new CookieContainer();
    if (cookie.Count > 0)
    {
        cookieCon.Add(new Uri(url), cookie);
    }
    HttpWebRequest httpRequest = (HttpWebRequest)WebRequest.Create(url);
    httpRequest.CookieContainer = cookieCon;
    httpRequest.UserAgent = "Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.2; .NET CLR 1.1.4322; .NET CLR 2.0.50727)";
    httpRequest.ContentType = "application/x-www-form-urlencoded";
    httpRequest.Method = "POST";
    httpRequest.ContentLength = bytesToPost.Length;
    Stream requestStream = httpRequest.GetRequestStream();
    requestStream.Write(bytesToPost, 0, bytesToPost.Length);
    requestStream.Close();
    Stream responseStream = httpRequest.GetResponse().GetResponseStream();
    string stringResponse = string.Empty;
    StreamReader responseReader = new StreamReader(responseStream, Encoding.UTF8);
    stringResponse = responseReader.ReadToEnd();
    responseStream.Close();
    cookie = httpRequest.CookieContainer.GetCookies(new Uri(url));
    return stringResponse;
}

public CookieCollection cookie = new CookieCollection();
string post(string url, string postData)
{
    ASCIIEncoding encoding = new ASCIIEncoding();
    byte[] bytesToPost = encoding.GetBytes(postData);
    CookieContainer cookieCon = new CookieContainer();
    if (cookie.Count > 0)
    {
        cookieCon.Add(new Uri(url), cookie);
    }
    HttpWebRequest httpRequest = (HttpWebRequest)WebRequest.Create(url);
    httpRequest.CookieContainer = cookieCon;
    httpRequest.UserAgent = "Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.2; .NET CLR 1.1.4322; .NET CLR 2.0.50727)";
    httpRequest.ContentType = "application/x-www-form-urlencoded";
    httpRequest.Method = "POST";
    httpRequest.ContentLength = bytesToPost.Length;
    Stream requestStream = httpRequest.GetRequestStream();
    requestStream.Write(bytesToPost, 0, bytesToPost.Length);
    requestStream.Close();
    Stream responseStream = httpRequest.GetResponse().GetResponseStream();
    string stringResponse = string.Empty;
    StreamReader responseReader = new StreamReader(responseStream, Encoding.UTF8);
    stringResponse = responseReader.ReadToEnd();
    responseStream.Close();
    cookie = httpRequest.CookieContainer.GetCookies(new Uri(url));
    return stringResponse;
}