Question

I have the following simple code for testing a webservice :

using System;
using System.Linq;
using System.Xml;
using System.Xml.Linq;
using System.Collections.Generic;

namespace Testing_xmlReturn
{
    class MainClass
    {
        public static void Main (string[] args)
        {   
        // Default namespaces
        XNamespace df = @"http://oss.dbc.dk/ns/opensearch";
        XNamespace dkdcplus = @"http://biblstandard.dk/abm/namespace/dkdcplus/";
        XNamespace ac = @"http://biblstandard.dk/ac/namespace/";
        XNamespace dcterms = @"http://purl.org/dc/terms/";
        XNamespace dkabm = @"http://biblstandard.dk/abm/namespace/dkabm/";
        XNamespace dc = @"http://purl.org/dc/elements/1.1/";
        XNamespace oss = @"http://oss.dbc.dk/ns/osstypes";
        XNamespace xsi = @"http://www.w3.org/2001/XMLSchema-instance";

        XDocument xd = new XDocument();
        xd = XDocument.Load(@"http://opensearch.addi.dk/next_2.0/?action=search&query=mad&stepValue=1&sort=date_descending&outputType=xml");


        var q = from result in xd.Descendants(dkabm + "record").Elements(dc + "title")
            where result.Attribute(xsi + "type").Value == "dkdcplus:full"
            select result;

        foreach(XElement xe in q)
                Console.WriteLine("Name: " + xe.Name +" Value: " + xe.Value);

        Console.ReadLine();

        }
    }
}

The XElement i need to get from the response is :

<dc:title xsi:type="dkdcplus:full">Dynastiet præsenterer D-Dag!</dc:title>

I keep getting a System.NullReferenceException. Clearly I don’t get the XElement but why ?

It’s easy to get all the dc:title elements out by removing “where” so thats got to be the problem.

I’m no Linq-to-Xml master but this namespace business with attributes is really confusing.

Answer 1

It’s because there are 2 dc:title elements returned by the Descendants(). One with a xsi:type attribute and one without. When you call .Value on the one without in your where it gives you a null reference exception. You need to check for the attribute been null before you inspect the value.

Here’s some code that works:

var q = from result in xd.Descendants(dc + "title")
    where (String)result.Attribute(xsi + "type")  == "dkdcplus:full"
    select result;