Question

I’m trying to select nodes starting from nodes that are not root nodes of XML documents. The code the following:

 foreach (XmlNode xmlIter in root.SelectNodes("/refentry/refsect1[@id='parameters']/variablelist/*")) {
    Parameter p = new Parameter();

    xmlNode = xmlIter.SelectSingleNode("varlistentry/term/parameter");
    p.Identifier = xmlNode.InnerText;

    xmlNode = xmlIter.SelectSingleNode("varlistentry/listitem");
    p.Documentation = xmlNode.InnerText;
}

The XML document (here is only a snippet) is the following:

<refsect1 id="parameters"><title>Parameters</title>
    <variablelist>
    <varlistentry>
        <term><parameter>pipeline</parameter></term>
        <listitem>
        <para>

        </para>
        </listitem>
    </varlistentry>
    <varlistentry>
        <term><parameter>program</parameter></term>
        <listitem>
        <para>

        </para>
        </listitem>
    </varlistentry>
    </variablelist>
</refsect1>

But the above code doesn’t work: the xmlNode variable is always null… What’s wrong?

Answer 1

The leading “/” means “start at the root of the document”. Try just:

xmlNode = xmlIter.SelectSingleNode("varlistentry/term/parameter");
p.Identifier = xmlNode.InnerText;

xmlNode = xmlIter.SelectSingleNode("varlistentry/term/listitem");
p.Documentation = xmlNode.InnerText;

(If you’re using .NET 3.5 or later I’d personally use LINQ to XML instead – I find it easier to use than XPath, but that’s a different matter. I can provide the equivalent LINQ to XML code if you want.)

EDIT: Got it. Your [stuff]/variablelist/* query is already going into the varlistentry node – you’re then trying to find another varlistentry node beneath it. If you select all the varlistentry elements instead, you only need to find the term/parameter and listitem elements under that varlistentry.

This works:

foreach (XmlNode xmlIter in doc.SelectNodes
         ("/refentry/refsect1[@id='parameters']/variablelist/varlistentry"))
{
    XmlNode xmlNode = xmlIter.SelectSingleNode("term/parameter");
    Console.WriteLine("Identifier = {0}", xmlNode.InnerText);

    xmlNode = xmlIter.SelectSingleNode("listitem");
    Console.WriteLine("Documentation = {0}", xmlNode.InnerText);
}

Mind you, it would probably still be worth validating that xmlNode is non-null before using it.