XPath String that grabs an element with a specific id value

Question

I am trying to create an XPath query/string that grabs a specific element from a XML document. I am attempting to grab the element with the id=38 but my code always returns nothing for some reason.

If you look at my code & the organisation of my XML file can you tell me what XPath I need to grab the element with the id=38?

My code is:

XmlDocument xdoc = new XmlDocument();
xdoc.Load(getProductURL());
XmlNode node = xdoc.DocumentElement.SelectSingleNode("id('38')");

// node always is null for some reason?

The way the xml is organised is like so:

<courseg>
    <group isempty="False" isbranch="true" id="1" name="abc">
        <group isempty="False" isbranch="true" id="38" name="def"></group>
    </group>
</courseg>

Answer 1

The XPath you need is

//*[@id='38']

Here is the example with XDocument:

        XDocument xdoc = XDocument.Parse(@"
<courseg>
    <group isempty=""False"" isbranch=""true"" id=""1"" name=""abc"">
        <group isempty=""False"" isbranch=""true"" id=""38"" name=""def""></group>
    </group>
</courseg>");
        XElement node = xdoc.Root.XPathSelectElement("//*[@id='38']");
        Console.WriteLine(node);

Answer 2

The function id('P38') would select an element with an ID value of P38. But this doesn't just mean "an attribute named 'id'". It means an attribute declared in the DTD or schema as being of type ID. You haven't shown a DTD or schema, and I suspect you don't have one. If you did, and if it declared the id attribute as being of type ID, then your document would be invalid, because an ID value cannot be all-numeric (for legacy SGML reasons, it has to take the form of a name).

In practice the id() function is probably best avoided unless you have severe performance requirements. It's too fragile - it only works when you are validating the source document against a schema or DTD. In XSLT, use key() instead. Alternatively, many processors now recognize the attribute name xml:id as a 'self declaring' ID value without reference to a schema or DTD: use that if your processor supports it.

Answer 3

Use this XPath query:

//*[@id = 38]

It selects every node with id attribute equals to 38. If you have to be more specific, i.e. select group with id attribute equals to 38, use this one:

//group[@id = 38]