linq NullReferenceException question

Question

I have a linq query to a XML dataset, which when executed is generating a NullReferenceException.

XDocument dataDoc = XDocument.Load(new StringReader(e.Result));

var Genres = from genre in dataDoc.Descendants("genres")
  where (!genre.Element("ID").IsEmpty)
  select (string)genre.Element("id").Value + ',' + (string)genre.Attribute("name").Value + ',' + (string)genre.Attribute("url").Value;

foreach (string myGenre in Genres)
{
}

When executed, the Linq query works fine, but when the code attempts to iterate through the foreach loop, the NullReferenceException occurs.

Now, i think that the issue has to do with the XML data I am reading, which looks like the following:

<genres>
  <translated>true</translated>
  <genre name="1">
    <id>28</id>
    <url>http://url1</url>
  </genre>
  <genre name="2">
    <id>12</id>
    <url>http://url2</url>
  </genre>
</genres>

Is the first child node, which is different in structure, causing the issue? My class behind this shouldn't be an issue, but is the following (just in case):

public class Genre
{
    public string ID { get; set; }
    public string Name { get; set; }
    public string URL { get; set; }
}

Your where clause should probably be lowercase.

SLaks
– SLaks

2011-02-01 17:00:45 +00:00
Commented Feb 1, 2011 at 17:00 — SLaks
– SLaks, Commented Feb 1, 2011 at 17:00

SLaks · Accepted Answer · 2011-02-01 17:49:00Z

3

genre.Attribute("url") returns null, since there is no url attribute.
You need to call Element, not Attribute.

EDIT: Calling dataDoc.Descendants("genres") returns the single <genres> element, which is not what you want.
You need to call Descendants("genre") (singular) to get the individual <genre ...> elements.
You could also call dataDoc.Descendants("genres").Elements to get the elements inside the <genres> element.

edited Feb 1, 2011 at 17:49

answered Feb 1, 2011 at 16:59

SLaks

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2 Comments

pearcewg Over a year ago

Good catch. I made that (boneheaded mistake) correction, but I still get the same result. I also removed the where clause, with no difference.

pearcewg Over a year ago

Aha! The edit was the difference! I think I need to spend some good quality time learning more than just the basics on Linq. Very useful technology. Thank you for the help!

Jon Skeet · Accepted Answer · 2011-02-01 17:06:19Z

1

SLaks has pointed out the mistake in using Attribute rather than Element, but there's another improvement you can make in your code. Currently you're using the Value property and then redundantly casting to string. If you just cast an XAttribute or XElement to string, then if the original reference is null, the result will be null as well, rather than an exception being thrown. There's no point in using Value and casting.

answered Feb 1, 2011 at 17:06

Jon Skeet

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