Question

I am trying to screen scrape using C#.It works for few times,after which i receive Session expired error.Any help will be appreciated.

Answer 1

Here is the set of classes I am using for screen scraping. (I wrote these classes, feel free to use however you want.)

There may be some bugs in it, but every usage I have for it it works quite flawlessly.

It also handles SSL websites fine, works with redirects, and captures the original pages that caused a redirect as well in the WebPage class.

using System;
using System.Collections.Generic;
using System.IO;
using System.Net;
using System.Text;
using System.IO.Compression;
using System.Threading;
using System.Web;
using System.Security.Cryptography.X509Certificates;

public class PostValue
{
    public PostValue( String key, String value )
    {
        Key = key;
        Value = value;
    }


    public String Key { get; set; }

    public String Value { get; set; }
}

[Serializable]
public class WebPage
{
    public WebPage(String html)
    {
        Html = html;
    }

    public WebPage(String html, WebPage parent)
    {
        Html = html;
        Parent = parent;
    }

    public String Html { get; set; }
    public WebPage Parent { get; set; }
}


internal class AcceptAllCertificatePolicy : ICertificatePolicy
{
    public AcceptAllCertificatePolicy()
    {
    }

    public bool CheckValidationResult(ServicePoint sPoint,
       X509Certificate cert, WebRequest wRequest, int certProb)
    {
        // Always accept
        return true;
    }
}

public class WebSession
{
    public String BaseUrl { get; set; }
    public String LastUrl { get; set; }
    public String UserAgent { get; set; }

    public int PageReattempts { get; set; }
    public WebProxy Proxy { get; set; }
    public String CookieString { get; set; }
    public Dictionary<String, String> Cookies { get; set; }

    private static WebSession instance { get; set; }
    public static WebSession Instance { get { if (instance == null) instance = new WebSession(); return instance; } }

    public const String DefaultAgent = "Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US; rv:1.9.0.8) Gecko/2009032609 Firefox/3.0.8";

    public WebSession()
        : this(DefaultAgent, null)
    {
    }


    public WebSession(String baseUrl)
        : this(DefaultAgent, null)
    {
        BaseUrl = baseUrl;
    }

    public WebSession(String userAgent, WebProxy proxy)
    {
        ServicePointManager.CertificatePolicy = new AcceptAllCertificatePolicy();
        CookieString = "";
        Cookies = new Dictionary<string, string>();

        if (userAgent == "")
            UserAgent = DefaultAgent;
        else
            UserAgent = userAgent;

        Proxy = proxy;
        LastUrl = "";
        PageReattempts = 4;
        ServicePointManager.Expect100Continue = false;
    }


    public WebPage RequestPage(string URL)
    {
        return RequestPage(new Uri(BaseUrl + URL));
    }

    public WebPage RequestPage(string URL, string Values, string Method)
    {
        return RequestPage(new Uri(BaseUrl + URL), Values, Method);
    }

    public WebPage RequestPage(string URL, string Values, string Method, string ContentType)
    {
        return RequestPage(new Uri(BaseUrl + URL), Values, Method, "application/x-www-form-urlencoded");
    }

    public WebPage RequestPage(Uri URL)
    {
        return RequestPage(URL, "", "GET");
    }


    public WebPage RequestPage(String URL, params PostValue[] postValues)
    {
        String totalString = "";

        if (postValues.Length > 0)
        {
            for (int count = 0; count < postValues.Length; count++)
            {
                if (count > 0)
                    totalString += "&";

                totalString += postValues[count].Key + "=" + HttpUtility.UrlEncode(postValues[count].Value);
            }
        }

        return RequestPage(URL, totalString);
    }


    public WebPage RequestPage(string URL, string Values)
    {
        return RequestPage(new Uri(BaseUrl + URL), Values);
    }


    public WebPage RequestPage(Uri URL, string Values)
    {
        return RequestPage(URL, Values, "POST");
    }


    public WebPage RequestPage(Uri URL, string Values, string Method)
    {
        return RequestPage(URL, Values, Method, "application/x-www-form-urlencoded");
    }


    public WebPage RequestPage(Uri url, string content, string method, string contentType)
    {
        string htmlResult;
        HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(url);
        HttpWebResponse response = null;
        ASCIIEncoding encoding = new ASCIIEncoding();
        byte[] contentData = encoding.GetBytes(content);

        request.Proxy = Proxy;
        request.Timeout = 60000;
        request.Method = method;
        request.AllowAutoRedirect = false;
        request.Accept = "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8";
        request.Referer = LastUrl;
        request.KeepAlive = false;

        request.UserAgent = UserAgent;

        request.Headers.Add("Accept-Language", "en-us,en;q=0.5");
        //request.Headers.Add("UA-CPU", "x86");
        request.Headers.Add("Cache-Control", "no-cache");
        request.Headers.Add("Accept-Encoding", "gzip,deflate");

        String cookieString = "";
        foreach (KeyValuePair<String, String> cookiePair in Cookies)
            cookieString += cookiePair.Key + "=" + cookiePair.Value + ";";

        if (cookieString.Length > 2)
        {
            String cookie = cookieString.Substring(0, cookieString.Length - 1);
            request.Headers.Add("Cookie", cookie);
        }

        if (method == "POST")
        {
            request.ContentLength = contentData.Length;
            request.ContentType = contentType;

            Stream contentWriter = request.GetRequestStream();
            contentWriter.Write(contentData, 0, contentData.Length);
            contentWriter.Close();
        }

        int attempts = 0;

        while (true)
        {
            try
            {
                response = (HttpWebResponse)request.GetResponse();
                if (response == null)
                    throw new WebException();

                break;
            }
            catch (WebException)
            {
                if (response != null)
                    response.Close();

                if (attempts == PageReattempts)
                    throw;

                // Wait three seconds before trying again
                Thread.Sleep(3000);
            }

            attempts += 1;
        }

        // Tokenize cookies
        if (response.Headers["Set-Cookie"] != null)
        {
            String headers = response.Headers["Set-Cookie"].Replace("path=/,", ";").Replace("HttpOnly,", "");
            foreach (String cookie in headers.Split(';'))
            {
                if (cookie.Contains("="))
                {
                    String[] splitCookie = cookie.Split('=');
                    String cookieKey = splitCookie[0].Trim();
                    String cookieValue = splitCookie[1].Trim();

                    if (Cookies.ContainsKey(cookieKey))
                        Cookies[cookieKey] = cookieValue;
                    else
                        Cookies.Add(cookieKey, cookieValue);
                }
                else
                {
                    if (Cookies.ContainsKey(cookie))
                        Cookies[cookie] = "";
                    else
                        Cookies.Add(cookie, "");
                }
            }
        }

        htmlResult = ReadResponseStream(response);
        response.Close();

        if (response.Headers["Location"] != null)
        {
            response.Close();
            Thread.Sleep(1500);
            String newLocation = response.Headers["Location"];
            WebPage result = RequestPage(newLocation);
            return new WebPage(result.Html, new WebPage(htmlResult));
        }

        LastUrl = url.ToString();

        return new WebPage(htmlResult);
    }

    public string ReadResponseStream(HttpWebResponse response)
    {
        Stream responseStream = null;
        StreamReader reader = null;

        try
        {
            responseStream = response.GetResponseStream();
            responseStream.ReadTimeout = 5000;

            if (response.ContentEncoding.ToLower().Contains("gzip"))
                responseStream = new GZipStream(responseStream, CompressionMode.Decompress);
            else if (response.ContentEncoding.ToLower().Contains("deflate"))
                responseStream = new DeflateStream(responseStream, CompressionMode.Decompress);

            reader = new StreamReader(responseStream);

            return reader.ReadToEnd();
        }
        finally
        {
            reader.Close();
            responseStream.Close();
        }
    }
}