why HTML Agility Pack HtmlDocument.DocumentNode is null?

Question

I'm using this code to change the href attribute of a HTML stream.

first I download a full html page using this code:(URL is webpage address)

HttpWebRequest myHttpWebRequest = (HttpWebRequest)WebRequest.Create(URL);
HttpWebResponse myHttpWebResponse = 
                         (HttpWebResponse)myHttpWebRequest.GetResponse();

Stream s = myHttpWebResponse.GetResponseStream();

then I process this:

HtmlDocument doc = new HtmlDocument();

doc.Load(s);
foreach (HtmlNode link in doc.DocumentNode.SelectNodes("/a"))
{
    string att = link.Attributes["href"].Value;
    link.Attributes["href"].Value = "http://ahmadalli.somee.com/default.aspx?url=" + att;
}
doc.Save(s);

s is html stream.

but I've got an exception that says doc.DocumentNode is null!

i tried many sites but doc.DocumentNode is null to

Answer 1

This works for me.

using(WebClient client = new WebClient())
{
    client.Encoding = System.Text.Encoding.UTF8;
    var doc = new HtmlAgilityPack.HtmlDocument();
    doc.LoadHtml(client.DownloadString("http://www.google.com?q=stackoverflow"));
    foreach (var href in doc.DocumentNode.Descendants("a").Select(x => x.Attributes["href"]))
    {
        if (href == null) continue;
        href.Value = "http://ahmadalli.somee.com/default.aspx?url=" + HttpUtility.UrlEncode(href.Value);
    }
    StringWriter writer = new StringWriter();
    doc.Save(writer);
    var finalHtml = writer.ToString();
}

Also see the HttpUtility.UrlEncode to be able to get the url back correctly. Otherwise, some parameters in original url may cause problem.

Use HttpUtility.UrlDecode to decode it.

Answer 2

Try using //a instead of /a.

In XPath, this basically means give me all the links in the document, as opposed to give me all the links in the document root.

Update:

The following code works fine:

        var myHttpWebRequest = (HttpWebRequest)WebRequest.Create("http://google.com");
        var myHttpWebResponse = (HttpWebResponse)myHttpWebRequest.GetResponse();

        var s = myHttpWebResponse.GetResponseStream();

        var doc = new HtmlDocument();

        doc.Load(s);
        foreach (var link in doc.DocumentNode.SelectNodes("//a"))
        {
            var att = link.Attributes["href"].Value;
            link.Attributes["href"].Value = "http://ahmadalli.somee.com/default.aspx?url=" + att;

            Console.WriteLine(link.Attributes["href"].Value);
        }

Answer 3

Here is your answer: HTML Agility Pack Null Reference.

Answer 4

Try using the below code:

HtmlDocument htmlDoc = new HtmlDocument
        {
            OptionAddDebuggingAttributes = false,
            OptionAutoCloseOnEnd = true,
            OptionFixNestedTags = true,
            OptionReadEncoding = true
        };
        try
        {
            using (Stream reader = myHttpWebResponse.GetResponseStream())
            {
                reader.Seek(0, SeekOrigin.Begin);
                htmlDoc.Load(reader, true);
            }
            HtmlNode node = htmlDoc.DocumentNode;
            if (node != null)
            {
                foreach (var href in doc.DocumentNode.Descendants("a").Select(x =>x.Attributes["href"]))
                 {
                     href.Value = "http://ahmadalli.somee.com/default.aspx?url=" +HttpUtility.UrlEncode(href.Value);
                 }
            }
        }
        catch { }

I am using HtmlAgility pack version: 1.4.0

Solved your problem? If no, please comment. Else mark as answer.

Answer 5

Anchor tag reference is an incorrectly escaped string:

...doc.DocumentNode.SelectNodes("/a")    //incorrect
...doc.DocumentNode.SelectNodes("//a")   //correct
...doc.DocumentNode.SelectNodes(@"/a")   //also correct

The original code fails to select any nodes and evaluates to null; this should be checked against to prevent failing on, say, a document where there are no links at all (however unlikely that is :)

var anchors = doc.DocumentNode.SelectNodes("//a");
if (anchors != null)
{
    foreach (HtmlNode link in anchors)
    {
        /*do stuff*/
    } 
}