Get XML elements with a particular attribute value

Question

I have the following code to find xml elements that have a particular attribute value:

IEnumerable<XElement> elems = 
    xmlDoc.Descendants("elemName")
    .Where(x => 
        x.Attribute("attrName").Value
        .Equals(
            filename,
            StringComparison.OrdinalIgnoreCase
        )
    );

The problem is that if the element does not have that attribute, according to the docs, it will return null and we'll get a crash when we try to do null.Value.

I've rewritten it like this:

IEnumerable<XElement> elems = 
     xmlDoc.Descendants("elemName")
     .Where(x => 
         ((x.Attribute("attrName") == null) 
         ? "" 
         : x.Attribute("attrName").Value).Equals(
             filename,
             StringComparison.OrdinalIgnoreCase
         )
     );

But I think it's a bit ugly. Is there an idiomatic way of doing this - I imagine this is a fairly common operation? I suppose I could just do a regular loop.

Answer 1

I think you could use the null-conditional operator for x.Attribute("attrName").Value and swap arguments of String.Equals to avoid the NullReferenceException as follows:

IEnumerable<XElement> elems =
     xmlDoc.Descendants("elemName")
     .Where(x =>
         filename.Equals(
             x.Attribute("attrName")?.Value,
             StringComparison.OrdinalIgnoreCase
         )
     );

Answer 2

There is a trick - you can cast XAttribute to some data types: string, int, long, Guid, DateTime, bool etc and their nullable brothers. And if type supports null value there will be no NullReferenceException. You will simply get null as result:

(string)x.Attribute("attrName")

Next: I suggest you to use more meaningful names for variables, even if it's one-letter variable. I.e. instead of x for element you can use e.

And last, you can write following extension method to nicely compare strings

public static bool EqualsIgnoreCase(this string s, string value)
{
     return String.Equals(s, value, StringComparison.OrdinalIgnoreCase);
}

And finally your parsing query will look like

from e in xmlDoc.Descendants("elemName")
where filename.EqualsIgnoreCase((string)e.Attribute("attrName"))
select e;