LINQ to XML: handling nodes that do not exist?

Question

This may be a simple fix (well, it probably is) but for some reason I just can't figure it out.

So, I have some xml that looks something like this:

XElement xml = XElement.Parse (
@"<Alphabet>
     <a name="A" />
     <b name="B" />
     <d name="D" />
     <e name="E" />
</Alphabet>");

So later in my code, I reference a node that may or may not exist in there like so:

var name = (from b in xml.Descendants("c")
            select b.Attribute("name")).FirstOrDefault().Value;

But when it doesn't exist, instead of returning null or "" it throws a NullReferenceException: Object reference not set to an instance of an object.

What's the best way to check and see if a node actually exists in my linq query? Or do I need to check if it exists some other way?

Answer 1

Well, you're selecting the attribute - so just use:

var nameAttribute = xml.Descendants("c").Select(b => b.Attribute("name"))
                                        .FirstOrDefault();
if (nameAttribute != null)
{
    string name = nameAttribute.Value;
}
else
{
    // Whatever...
}

(I've changed it from a query expression to dot notation because the query was trivial - query expression syntax wasn't actually buying you anything.)

One problem with this solution: it doesn't differentiate between there being a "c" element but it not having a "name" attribute, and there not being a "c" element in the first place. Do you need to be able to tell the difference?

Answer 2

I created extension methods to do that for me.

public static string GetAttributeValue(this XElement element, string attributeName)
{
    XAttribute attribute = element.Attribute(attributeName);
    return attribute != null ? attribute.Value : string.Empty;
}

public static string GetElementValue(this XElement element)
{
    return element != null ? element.Value : string.Empty;
}

public static string GetElementValue(this XElement element, string elementName)
{
    XElement child = element.Element(elementName);
    return child != null ? child.Value : string.Empty;
}

Answer 3

You can do something like this:

var name = (from b in xml.Descendants("c")
            select b.Attribute("name").Value).FirstOrDefault();

or if you really need the element:

var name = (from b in xml.Descendants("c")
            select b.Attribute("name")).FirstOrDefault();

if (name != null)
{
    // your logic ...
}

Answer 4

FirstOrDefault returns null or an XAttribute which you can cast to a string to get the value:

var name = (string)((from b in xml.Descendants("c")
                     select b.Attribute("name")).FirstOrDefault());

or

var name = (string)xml.Descendants("c")
                      .Select(b => b.Attribute("name"))
                      .FirstOrDefault();