Why can I not find the XElements in my XML document?

Question

I know this is a repeated question over SO, but I those answers haven't really helped out. I am trying to get details of a book using Classify API with C#. It gives an XML output like:

<classify xmlns="http://classify.oclc.org">
    <response code="2"/>
    <!--
        Classify is a product of OCLC Online Computer Library Center: http://classify.oclc.org
    -->
    <work author="Coelho, Paulo | Clarke, Alan [Author; Translator] | Sampere, Daniel, 1985- [Illustrator; Draftsman] | Lemmens, Harrie, 1953- [Translator] | Smith, James Noel | Ruiz, Derek, 1980- [Adapter]" editions="176" format="Book" holdings="7530" itemtype="itemtype-book" pswid="1151562357" title="The alchemist">123115868</work>
    <authors>
        <author lc="n94113544" viaf="65709090">Clarke, Alan [Author; Translator]</author>
        <author lc="no2005013941" viaf="12032914">Lemmens, Harrie, 1953- [Translator]</author>
        <author lc="no2010190009" viaf="160023476">Sampere, Daniel, 1985- [Illustrator; Draftsman]</author>
        <author lc="n86099875" viaf="52700">Coelho, Paulo</author>
    </authors>
    <orderBy>hold desc</orderBy>
    <input type="stdnbr">9780754073963</input>
    <!-- Snip -->
</classify>

I have worked with XML but in PHP, not C# so I am new to it and the tutorials regarding this everywhere tell different things making me confused about what to do and how to do it. I simply want these things from the XML:

Title Of the Book ( in Work Element )
Authors of Book ( in Work Element )
ISBN of the Book ( in Input Element - type stdnbr )
Most Popular DDC ( in recommendations > ddc > mostPopular - attribute [nsfa] )

But with what the examples I have seen over here and on other forums I get NullReferenceException.

Code:

PS: It's not homework.

public frmAddItem(string xml)
{
    InitializeComponent();
    string itemName = null, itemAuth = null, itemISBN = null;
    XDocument xdoc = new XDocument();
    xdoc = XDocument.Parse(xml);    
    string itemName = xdoc.Element("classify").Descendants("work").Where(s => s.Attributes() == "title");
    string itemAuth = xdoc.Element("classify").Descendants("work").Attributes("author").ToString();
    string itemISBN = xdoc.Element("classify").Descendants("input").ToString();
    txtTitle.Text = itemName;
    txtAuth.Text = itemAuth;
    txtISBN.Text = itemISBN;
    string itemDDC = xdoc.Element("ddc").Descendants("latestEdition").Attributes("sfa");
}

EDIT Duplicate: The link mentioned as duplicate is not same/similar to my XML string. It uses a different approach that didn't work as mentioned in the start of the question.

Perhaps this can help: http://stackoverflow.com/questions/7119806/c-sharp-reading-data-from-xml — ispiro, Jun 08 '14 at 09:47
Please see the structure of the XML, its Different. I tried that and It didn't work. — echo_salik, Jun 08 '14 at 09:48
Please give an example of what you actually tried. Just a guess here, do you know what an xml namespace is? Have you tried specifying the namespace? — Mike Zboray, Jun 08 '14 at 10:06

Mike Zboray · Accepted Answer · 2014-06-09T20:20:51.153

3

Since your XML document has a namespace defined, the APIs require you to qualify your element names with namespaces. Your code is only using the "local" names. There are a couple ways to qualify them. An example is below which gets the 'title' attribute. It is simple to expand on it to get whatever other content/attributes you want.

using System.Xml;
using System.Xml.Linq;
using System.Xml.XPath;

public class Program
{
    public static void Main(string[] args)
    {
        var doc = XDocument.Load("http://classify.oclc.org/classify2/Classify?stdnbr=9780754073963");
        var nsManager = new XmlNamespaceManager(new NameTable());
        nsManager.AddNamespace("c", "http://classify.oclc.org");

        var work = doc.XPathSelectElement("//c:work", nsManager);
        Console.WriteLine(work.Attribute("title").Value);

        var mostPopular = doc.XPathSelectElement("//c:ddc/c:mostPopular", nsManager);
        Console.WriteLine(mostPopular.Attribute("nsfa").Value);            
    }
}

edited Jun 09 '14 at 20:20

answered Jun 08 '14 at 21:05

Mike Zboray

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Hi. Thanks for replying, It works. Thanks for the explanation too. – echo_salik Jun 09 '14 at 09:20
I am stuck at ddc thing, would you be able to help out with that? I tried `//c:ddc/mostPopular` but it gave an **NullException** Error. – echo_salik Jun 09 '14 at 09:28
1

@ZackValentine See my edit, all the element names need namespace qualification. – Mike Zboray Jun 09 '14 at 16:15
oh... Just to be clear for future use, I have to use c:_____ for any child element too? Thanks a lot! – echo_salik Jun 09 '14 at 18:18
1

@ZackValentine Yes, it would be nice if there was a way to ignore namespaces, but there isn't AFAIK. – Mike Zboray Jun 09 '14 at 20:20

Why can I not find the XElements in my XML document?

1 Answers1