Develop Reference

Bug in HTMLAgilityPack when getting href attribute value. C#

Found a nasty bug in HTMLAgilityPack whereby some attribute values are NOT returned fully - they are truncated. Specifically, when attempting to get the href value out of an anchor tag, only the root domain is returned, anything following (the query string) is completely ignored. Anyone know a good workaround?
Example:
node.SelectSingleNode("//link").Attributes["href"].Value
returns https://www.example.com
instead of returning https://www.example.com/mypage.php?_src=ffk_title&ffkid=66534&site=data:http%3A%2F%2Fwww.othersite.com%2Frss%2F
the link looks like so
<a class="tlink" href="https://www.example.com/mypage.php?_src=ffk_title&ffkid=66534&site=data:http%3A%2F%2Fwww.othersite.com%2Frss%2F" target="_blank">Click to get feed</a>
Anyway - right now, I'll just get the link tag and parse with old methods - I figure HTMLAgilityPack gets confused if there are atypical characters in the href tag. I hope it's just something I'm doing wrong, but this kind of quirk is really hurts.

For anchor tags, you should use //a XPath expression:
node.SelectSingleNode("//a").Attributes["href"].Value;
Additionally, if you need to reference an anchor with a particular class, you could use:
node.SelectSingleNode("//a[#class='tlink']").Attributes["href"].Value;
A working example can be seem here.

Why is this seemingly simple Xpath navigation not working?

I'm having what seems like a really simple problem. I'm trying to navigate to an element in HTML by Xpath, and can't seem to get it to function properly.
I want to grab a span from the html contents of a page. The page is fairly complex, so I've been using Firebug's "get element by xpath" and pasting the result into my code. I've noticed it's slightly different than the xpath you get from doing the same thing in Chrome, but they both seem to direct to the same place.
The html I'm trying to navigate is found here. The field I'm trying to access via xpath is the first "Results 1 - 10 of n".
Based on FireBug's 'inspect element' the xpath should be: /html/body/div/center/table/tbody/tr[6]/td/table/tbody/tr/td[2]/table/tbody/tr/td/table/tbody/tr/td/table/tbody/tr/td/table/tbody/tr/td/span
However when I try to use this xpath to identify the element in a C# codebehind, it gives me a number of errors that that path cannot be found.
Am I doing something wrong here? I've tried a number of permutations of the xpath and I don't understand why this wouldn't be cooperating within code.
Edit: I'm having this problem both in HTMLAgilityPack (but managed to hack out a bad solution using regexes instead) and a SELECT statement modeled after the answer found here
Edit 2: I'm trying to figure out this issue using Yahoo's free proxy as shown in the example here:
var query = 'SELECT * FROM html WHERE url="http://mattgemmell.com/2008/12/08/what-have-you-tried/" and xpath="//h1" and class="entry-title"';
var url = "http://query.yahooapis.com/v1/public/yql?q=" + query + "&format=json&callback=??";
$.getJSON(url,function(data){
alert(data.query.results.h1.content);
})
I'm having the same problems with HTML agility pack but I'm more interested in getting this part to work. It works for the provided URL that the answerer gave me (seen above). However when I try to use even simple xpath expressions on the http://nl.newsbank.com url, I get errors that no object has been retrieved every time, no matter how basic the xpath.
Edit 3: I thought I'd elaborate a little more on the big picture of the larger problem I'm trying to solve of which this problem is a critical component in the hopes that maybe it provides a little more insight.
To learn basic ASP.NET development skills from scratch, I decided to make a simple web application, based around the news archive search at http://nl.newsbank.com/. In its current iteration, it sends a POST request (although I've now learned you can use a GET request and just dump the body at the end of the URL) to send search criteria, as if the user entered criteria in the search bar. It then searches the response (using RegExes, not Xpath because I couldnt get that working) for the "Results 1-n of n" span, extracts n, and dumps it in a table. It's a cool little tool for looking up news occurrence rates over time.
I wanted to add functionality such that you could enter a date range (say May 2002 - June 2010) and run a frequency search for every month / week in that range. This is very easy to implement conceptually. HOWEVER the problem is, right now all this happens server side, and since there's no API, the HTTP response contains the entire page, and is therefore very bandwidth intensive. Sending dozens of queries at once would swallow absolutely unspeakable amounts of bandwidth and wouldn't be even a little scalable.
As a result I tried rewriting the application to work client-side. However because of the same-origin policy I'm not able to send a request to an external host from the client-side. HOWEVER there is a loophole that I can use a free Yahoo proxy that makes the request and converts it to JSON, and then I can use the JSON exception of the Same-Origin Policy to retrieve that data from the proxy.
Here's where I'm running into these xpath problems specific to http://nl.newsbank.com. I'm not able to retrieve html with any xpath, and I'm not sure why or how I can fix it. When debugging in VS2010, I'll receive the error Microsoft JScript runtime error: Unable to get value of the property 'content': object is null or undefined

As paul t. already mentioned in a comment, the TBODY elements are generated by the webkit engine. The next problem is that the DIV between the BODY and CENTER does not exist on the page by default. It is added by an JS statement on line 119.
After stripping out the DIV and TBODY elements like
/html/body/center/table/tr[6]/td/table/tr/td[2]/table/tr/td/table/tr/td/table/tr/td/table/tr/td/span
i can successfull select a node with the HthmlAgilityPack.
Edit: don't use tools like Firebug for getting an XPath value on a website. Don't even use it if you just want wo look at the source of the page. The problem with Firebug is, that it will show you the current DOM document tree which probably on almost every is already (heavily) modified by JS.

Your sample HTML page's elements haven't got many classes to select on, but if you're interested in the first <span> element that contains "Results: 1 - 10 of n", you can use an XPath expression that explicitly targets this textual content.
For example:
//table//span[starts-with(., "Results:")]
will select all <span> elements, contained in a <table>, and that contain text beginning with "Results:" (the //table is not strictly necessary in your case I think, but might as well restrict a little)
You want the first one of these <span>, so you can use this expression:
(//table//span[starts-with(., "Results:")])[1]
Note the brackets around the whole previous expression and then [1] to select the first of all the <span> matching the text

It may sound kind of simplistic, but the element you are looking for is the only doc element that is using the css class "basic-text-white". I would think this would be a lot easier to find and extract than a long xpath. Web-scraping is never a stable thing, but I would think this is probably as stable as the xpath. Trying to debug the xpath just about makes my eyes bleed.

HtmlAgilityPack and large HTML Documents

I have built a little crawler and now when trying it out i found that when crawling certain sites my crawler uses 98-99% CPU.
I used dotTrace to see what the problem could be and it pointed me towards my httpwebrequest method - i optimised it a bit with the help of some previous questions here on stackoverflow.. but the problem was still there.
I then went to see what URLs that were causing the CPU load and found that it was actually sites that are extremely large in size - go figure :)
So, now i am 99% certain it has to do with the following piece of code:
HtmlAgilityPack.HtmlDocument documentt = new HtmlAgilityPack.HtmlDocument();
HtmlAgilityPack.HtmlNodeCollection list;
HtmlAgilityPack.HtmlNodeCollection frameList;
documentt.LoadHtml(_html);
list = documentt.DocumentNode.SelectNodes(".//a[#href]");
All that i want to do is to extract the links on the page, so for large sites.. is there anyway i can get this to not use so much CPU?
I was thinking maybe limit what i fetch? What would be my best option here?
Certainly someone must have run into this problem before :)

Have you tried dropping the XPath and using the LINQ functionality?
var list = documentt.DocumentNode.Descendants("a").Select(n => n.GetAttributeValue("href", string.Empty);
That'll pull a list of the href attribute of all anchor tags as a List<string>.

If you aren't heavily invested in Html Agility Pack, try using CsQuery instead. It builds an index when parsing the documents, and selectors are much faster than HTML Agility Pack. See a comparison.
CsQuery is a .NET jQuery port with a full CSS selector engine; it lets you use CSS selectors as well as the jQuery API to access and manipulate HTML. It's on nuget as CsQuery.

".//a[#href]" is extremely slow XPath. Tried to replace with "//a[#href]" or with code that simply walks whole document and checks all A nodes.
Why this XPath is slow:
"." starting with a node
"//" select all descendent nodes
"a" - pick only "a" nodes
"#href" with href.
Portion 1+2 ends up with "for every node select all its descendant nodes" which is very slow.

XML: Searching elements for specific text using C#

I'm trying to get a list of PDF links from different websites. First I'm using the Web client class to download the page source. I then use sgmlReader to convert the HTML to XML. So for one particular site, I'll get a tag that looks like this:
<p>1985 to 1997 Board Action Summary</p>
I need to grab all the links that contain ".pdf". Obviously not all websites are laid out the same, so just searching for a <p> tag, wont be dynamic enough. I'd rather not use linq, but I will if I have to. Thanks in advance.

Linq makes this easy...
var hrefs = doc.Root.Descendants("a")
.Where(a => a.Attrib("href").Value.ToUpper().EndsWith(".PDF"))
.Select(a => a.Attrib("href"));
away you go! (note: did this from memory, so you might have to fix it somewhat)
This will break down for <a/> tags that don't have an href (anchors) but you can fix that surely...

I think you have 2 options here. If you need only the links, you can use Regular Expressions to find the matches for strings ending with .pdf. If you need to manipulate the XML structure or get other values from the XML, it would be better to use XmlDocument and use an XPath query to find out the nodes which have a link to a pdf file in it. Using LINQ to XML just reduces the number of lines of code you have to write.

Getting a "summary" of a webpage

I have something of a a hairy problem, I'd like to generate a couple of paragraphs of "description" of a given url, normally the start of an article. The Meta description field is one way to go but it isn't always good or set properly.
It's fair to say it's a bit problematic to accomplish this from the screenscraped HTML. I had a general idea that perhaps one could scan the HTML for the first "appropriate" segment but it's hard to say what that is, perhaps something like the first paragraph containing a certain amount of text...
Anyone have any good ideas? :) It doesn't have to be foolproof

So, you wanna become a new Google, heh? :-)
Many sites are "SEO friendly" these days. This enables you to go for the headings and then look for paragraphs bellow.
Also, look for lists. There is a lot of content in some sort of tab-like (tabs, accordions...) interfaces that is done using ordered or unordered lists.
If that fails, maybe look for a div with class "content" or "main" or a combination and start from there.
If you use different approaches, make sure you keep statistics of what worked and what didn't (maybe even save a full page), so you can review and tweak your parsing and searching methods.
As a side note, I've used htmlagilitypack to parse and search through html with success. Well, at leasts it beats parsing with regex :-)

Perhaps look for the div element that contains the most p elements, and then grab the first p child. If no div, get the first p from the body element.
This will always have its problems.

You can strip the HTML tags using this regular expression
string stripped = Regex.Replace(textBox1.Text,#"<(.|\n)*?>",string.Empty)
You will them get the content text you can use to generate your paragraphs.