How can XPath be used to dig through (someone else's) poorly coded HTML?

Question

When I execute this C# code...

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.IO;
using System.Net;
using System.Dynamic;
using HtmlAgilityPack;

namespace entropedizer
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        public String postRequest(string url, string eventTarget)
        {
            // A "pre-request," sent to gather SessionID and POST data parameters for the main request
            HttpWebRequest prequest = (HttpWebRequest)WebRequest.Create("http://www.entropedia.info/Chart.aspx?chart=Chart");
            HttpWebResponse presponse = (HttpWebResponse)prequest.GetResponse();
            Stream pstream = presponse.GetResponseStream();
            StreamReader psr = new StreamReader(pstream);
            string phtml = psr.ReadToEnd();

            Match viewstate = Regex.Match(phtml, "id=\"__VIEWSTATE\".+/>");
            Match eventvalidation = Regex.Match(phtml, "id=\"__EVENTVALIDATION\".+/>");
            ASCIIEncoding encoding = new ASCIIEncoding();
            string postData = "__EVENTTARGET=" + eventTarget + "&__VIEWSTATE=" + Uri.EscapeDataString(viewstate.ToString().Substring(24, viewstate.Length - 28)) + "&__EVENTVALIDATION=" + Uri.EscapeDataString(eventvalidation.ToString().Substring(30, eventvalidation.Length - 34));
            byte[] data = encoding.GetBytes(postData);

            // The main request, intended to retreive the desired HTML
            HttpWebRequest request = (HttpWebRequest)WebRequest.Create("http://www.entropedia.info/Chart.aspx?chart=Chart");
            request.Method = "POST";
            request.ContentType = "application/x-www-form-urlencoded";

            request.CookieContainer = new CookieContainer();
            Cookie sessionId = new Cookie("ASP.NET_SessionId", Regex.Match(presponse.Headers.ToString(), "ASP.NET_SessionId=.+ d").ToString().Substring(18, Regex.Match(presponse.Headers.ToString(), "ASP.NET_SessionId=.+ d").Length - 21), "/", ".entropedia.info");
            request.CookieContainer.Add(new Uri("http://www.entropedia.info/Chart.aspx?chart=Chart"), sessionId);

            Stream stream = request.GetRequestStream();
            stream.Write(data, 0, data.Length);
            stream.Close();

            HttpWebResponse response = (HttpWebResponse)request.GetResponse();
            stream = response.GetResponseStream();

            StreamReader sr = new StreamReader(stream);

            return sr.ReadToEnd();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            System.Net.ServicePointManager.Expect100Continue = false;
            HtmlAgilityPack.HtmlDocument hChart = new HtmlAgilityPack.HtmlDocument();
            hChart.LoadHtml(postRequest("http://www.entropedia.info/Chart.aspx?chart=Chart", "ctl00%24ContentPlaceHolder1%24DG1%24ctl19%24ctl05"));

            HtmlNodeCollection chartStrings = hChart.DocumentNode.SelectNodes("/");

            if (chartStrings != null)
            {
                foreach (HtmlNode i in chartStrings)
                {
                    System.IO.File.WriteAllText("C:/Users/Admin/Desktop/WholeDocument.txt", i.OuterHtml);
                }
            }
            else
            {
                MessageBox.Show("Error:  Null item list.");
            }
        }
    }
}

...the following HTML is written to a text file.

http://pastebin.com/FALerBWR

When I change the line in my C# code to HtmlNodeCollection chartStrings = hChart.DocumentNode.SelectNodes("/html/body"); the 400+ lines of HTML within the body is written to the text file instead.

When I change the line to HtmlNodeCollection chartStrings = hChart.DocumentNode.SelectNodes("/html/body/form"); only a single line of code (the form opening tag with its attributes) is written to the text file. It should write many lines (most of the document). I believe HtmlAgilityPack becomes confused due to the malformed HTML tags. Is there a way to programatically work around this? I do not want to correct the HTML manually every time I run the program!

Answer 1

This is a "by design" behavior. FORM is, by default, considered as an empty HTML element. The reasons are explained here on SO (check my answer): HtmlAgilityPack -- Does <form> close itself for some reason?

But this is also configurable, you just need to instruct the parser to behave differently, like this:

HtmlAgilityPack.HtmlDocument hChart = new HtmlAgilityPack.HtmlDocument();

// remove all specific behaviors for the `FORM` element
HtmlAgilityPack.HtmlNode.ElementsFlags.Remove("form");

hChart.LoadHtml(postRequest("http://www.entropedia.info/Chart.aspx?chart=Chart", "ctl00%24ContentPlaceHolder1%24DG1%24ctl19%24ctl05"));
HtmlNodeCollection chartStrings = hChart.DocumentNode.SelectNodes("/");