Check if an element exists when parsing XML

Question

I'm parsing XML. I normally parse it the way I show in the code below which is straightforward The problem is that I don't own the XML I'm parsing and I can't change it. Sometimes there is no thumbnail element (there are no tags) and I get an Exception.

Is there a way to maintain this simplicity and check if the element exists? Or do I have to get first an XElement list with LINQ, to then check it and fill only the existing object properties?

void wc_DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)
{
    XDocument dataDoc = XDocument.Load(new StringReader(e.Result));

    var listitems = from noticia in dataDoc.Descendants("Noticia")
                    select new News()
                    {
                        id = noticia.Element("IdNoticia").Value,
                        published = noticia.Element("Data").Value,
                        title = noticia.Element("Titol").Value,
                        subtitle = noticia.Element("Subtitol").Value,
                        thumbnail = noticia.Element("Thumbnail").Value
                    };

    itemList.ItemsSource = listitems;
}

Answer 1

[Edit]Jon Skeet's answer should be the accepted answer. It is far more readable and easier to apply.[/edit]

Create an extension method like this :

public static string TryGetElementValue(this XElement parentEl, string elementName, string defaultValue = null)  {     var foundEl = parentEl.Element(elementName);      if (foundEl != null)     {         return foundEl.Value;     }      return defaultValue; } 

then, change your code like this :

select new News() {     id = noticia.TryGetElementValue("IdNoticia"),     published = noticia.TryGetElementValue("Data"),     title = noticia.TryGetElementValue("Titol"),     subtitle = noticia.TryGetElementValue("Subtitol"),     thumbnail = noticia.TryGetElementValue("Thumbnail", "http://server/images/empty.png") }; 

This approach allows you to keep a clean code with isolating the check of element presence. It also allow you to define a default value, which can be helpful

Answer 2

Instead of using the Value property, if you cast to string you'll just get a null reference instead:

void wc_DownloadStringCompleted(object sender,                                 DownloadStringCompletedEventArgs e) {     XDocument dataDoc = XDocument.Load(new StringReader(e.Result));      var listitems = from noticia in dataDoc.Descendants("Noticia")                     select new News()                     {                         id = (string) noticia.Element("IdNoticia"),                         published = (string) noticia.Element("Data"),                         title = (string) noticia.Element("Titol"),                         subtitle = (string) noticia.Element("Subtitol"),                         thumbnail = (string) noticia.Element("Thumbnail")                     };      itemList.ItemsSource = listitems; } 

That uses the explicit conversion from XElement to string, which handles a null input by returning a null output. The same is true for all explicit conversions on XAttribute and XElement to nullable types, including nullable value types such as int? - you just need to be careful if you're using nested elements. For example:

string text = (string) foo.Element("outer").Element("inner"); 

will give a null reference if inner is missing, but will still throw an exception if outer is missing.

If you want a "default" value, you can use the null coalescing operator (??):

string text = (string) foo.Element("Text") ?? "Default value";