NullReferenceException in C# Dealing with XML

Question

I'm getting the NRE error that says: "Object reference not set to an instance of an object."

From the following code:

  select new
                  {
                      ICAO = station.Element("icao").Value,
                  };

The entire script is:

XDocument xmlDoc = XDocument.Load(@"http://api.wunderground.com/auto/wui/geo/GeoLookupXML/index.xml?query=94107");

    var stations = from station in xmlDoc.Descendants("station")
                  select new
                  {
                      ICAO = station.Element("icao").Value,
                  };
    lblXml.Text = "";
    foreach (var station in stations)
    {
        lblXml.Text = lblXml.Text + "ICAO: " + station.ICAO + "<br />";
    }

    if (lblXml.Text == "")
        lblXml.Text = "No Results.";
    }

I don't understand why it isn't creating the station object and setting the ICAO value. Any ideas/tips for future XML and C# reference?

Why do you have a comma after the ICAO = station.Element("Icao").Value line? You're not getting multiple elements... — curtisk
– curtisk, Commented Jun 30, 2009 at 16:04
It doesn't hurt, and it may be that the real portion of code is much larger. If this is actually all that is done, you wouldn't need it within new{ ... } either, you could select the icao directly. — Ryan Versaw
– Ryan Versaw, Commented Jun 30, 2009 at 16:14

Ryan Versaw · Accepted Answer · 2009-06-30 16:11:27Z

9

It appears that only the airport stations have the icao element. This should work for you:

var stations = from airport in xmlDoc.Descendants("airport")
               from station in airport.Elements("station")
               select new
               {
                   ICAO = station.Element("icao").Value,
               };

You could instead add a where condition to get around the exception:

var stations = from station in xmlDoc.Descendants("station")
               where station.Element("icao") != null
               select new
               {
                   ICAO = station.Element("icao").Value,
               };

Also, you can pull a value like this to prevent an exception, though it will return numerous null records you may or may not want:

ICAO = (string)station.Element("icao")

You can do that for various other types, not only for strings.

edited Jun 30, 2009 at 16:11

answered Jun 30, 2009 at 16:02

Ryan Versaw

6,4853 gold badges33 silver badges31 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

user7116 Over a year ago

Not to mention protecting against station.Element("icao") being null with either an extension method or a where clause.

GalacticCowboy Over a year ago

The second route is probably better, since you're not imposing an artificial business rule that may or may not be true.

Ryan Versaw Over a year ago

Yeah, a combination of both may be the ideal situation.

VVS · Accepted Answer · 2009-06-30 16:07:17Z

1

The XML file in your sample returns some station elements with no icao descendant so sometimes station.Element("icao") will return null.

answered Jun 30, 2009 at 16:07

VVS

19.6k5 gold badges50 silver badges66 bronze badges

Comments

Keltex · Accepted Answer · 2009-06-30 15:54:05Z

0

I don't think that xmlDoc.Descendants("station") is returning what you are expecting. You should examine the results here. This is why station.Element("icao") is returning null.

answered Jun 30, 2009 at 15:54

Keltex

26.5k12 gold badges83 silver badges112 bronze badges

Comments

Lazarus · Accepted Answer · 2009-06-30 15:55:38Z

0

That URL doesn't appear to be returning XML data, I suspect that's causing your node references to return nulls.

answered Jun 30, 2009 at 15:55

Lazarus

43.2k4 gold badges47 silver badges54 bronze badges

Comments

Timothy S. Van Haren · Accepted Answer · 2009-06-30 16:00:39Z

0

Try something like this:

var stations = from station in xmlDoc.Elements("station")
select new
{
    ICAO = station.Element("icao").Value,
};

answered Jun 30, 2009 at 16:00

Timothy S. Van Haren

8,9762 gold badges32 silver badges34 bronze badges

Collectives™ on Stack Overflow

NullReferenceException in C# Dealing with XML

5 Answers 5

3 Comments

Comments

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

3 Comments

Comments

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related