Max Value with Substring with HTML Agility Pack

Question

I can't seem to get this xpath query to work with the HTMLAgilityPack with this code and I was wondering if anyone had any suggestions.

This is the query I have so far, but I can't seem to get it to return a number.

DocumentNode.GetAttributeValue("max(a[(@class='shackmsg')]/@href/substring-after(.,?id='))", "");

I'm trying to get the MAX value in the href attribute after the = sign on all hrefs with a class of shackmsg.

How long is the beta live before it goes retail? No one knows. We do know t</span> : </a><span class="oneline_user ">legsbrogan</span>
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<a class="shackmsg" rel="nofollow" href="?id=31218936" onclick="return clickItem( 31218933, 31218936);"><span class="oneline_body"><b><u><span class="jt_yellow">Current Multiplayer Servers</span>!</u></b>
<span class="jt_sample"><span class="jt_green">Nighteyes's Japan Server: </span> <span class="jt_lime">(PvE)</span>: <b>211.15.2.34</b></span>
<span class="jt_sample"><span class="jt_green">zolointo's Canada Server: </span> <span class="jt_lime">(</span></span></span> : </a><span class="oneline_user ">legsbrogan</span>
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  <a class="open" rel="nofollow" href="#" onclick="toggle_collapse(31218938); return false;" title="Toggle">toggle</a>
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<a class="shackmsg" rel="nofollow" href="?id=31218938" onclick="return clickItem( 31218933, 31218938);"><span class="oneline_body">Had fun freezing my ass off last night with a bunch of shackers.  Not sure who started the big tower we f...</span> : </a><span class="oneline_user ">legsbrogan</span>
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<a class="shackmsg" rel="nofollow" href="?id=31218966" onclick="return clickItem( 31218933, 31218966);"><span class="oneline_body">wasn't me. I hung out on my ship for a bit listening to your kid play Christmas songs for a bit and then ...</span> : </a><span class="oneline_user ">jonin</span><a class="lightningbolt" rel=\"nofollow\" href="http://www.shacknews.com/user/jonin/posts?result_sort=postdate_asc"><img src="http://cf.shacknews.com/images/bolt.gif" alt="This person is cool!" /></a>
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<a class="shackmsg" rel="nofollow" href="?id=31219008" onclick="return clickItem( 31218933, 31219008);"><span class="oneline_body">haha i heard you guys booby trapped some poor sap's space ship</span> : </a><span class="oneline_user ">Break</span><a class="lightningbolt" rel=\"nofollow\" href="http://www.shacknews.com/user/Break/posts?result_sort=postdate_asc"><img src="http://cf.shacknews.com/images/bolt.gif" alt="This person is cool!" /></a>
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Any suggestions?

Answer 1

There are two problems as far as I can see:

• You're only scanning for anchor tags in the current context. You probably want to extend to scan everywhere (use // in the beginning of your query):

  //a[@class='shackmsg']/@href/substring-after(., '?id=')
  

  Note that I removed a pair of unnecessary parenthesis.

• If I'm not completely mistaken, HTML Agility Pack only supports XPath 1.0 (yet I'm not totally sure). While System.Xml.XPath says it implements the XPath 2.0 data model, it does not actually implement XPath 2.0 (probably this is done so third party APIs can implement this API and offer XPath 2.0/XQuery support at the same time). Also have a look at this discussion on .NET's XPath 2.0 support.

Missing XPath 2.0 support would show up as two problems:

1. Function substring-after(...) does not exist.

   A solution for your problem could be to use string-lenght($string) and substring($string, $start, $length) to extract the last n digits, or translate(...) to remove some characters:

   translate('?id=31219008', '?id=', '')
   

   will remove all occurences in the character class [?id=] (yet it is none, I just want to highlight it does not match strings, but individual characters of this set!).

2. You cannot apply functions in axis steps. This means, you cannot find the maximum value of substrings.

   Possible solution: Only fetch all substrings and find the maximum from outside XPath.

Answer 2

You can combine XPath with HTML Agility Pack and make the following code :

var value = doc.DocumentNode.SelectNodes("//a[@class='shackmsg']").Select(
                   x => x.Attributes["href"].Value.Substring(4)).Max();

Console.WriteLine(value);

And this output :

31219008

In this code I assume to always exist the href attribute and always have the following structure :

"?id=XXXX"