(First post, please be gentle!)

I am just learning about LINQ to XML in all its glory and frailty, trying to hack it to do what I want to do:

Given an XML file like this -

<list>
<!-- random data, keys, values, etc.-->

  <key>FIRST_WANTED_KEY</key>
  <value>FIRST_WANTED_VALUE</value>

  <key>SECOND_WANTED_KEY</key>
  <value>SECOND_WANTED_VALUE</value> <!-- wanted because it's first -->

  <key>SECOND_WANTED_KEY</key>
  <value>UNWANTED_VALUE</value>  <!-- not wanted because it's second -->

  <!-- nonexistent <key>THIRD_WANTED_KEY</key> -->
  <!-- nonexistent <value>THIRD_WANTED_VALUE</value> -->

<!-- more stuff-->
</list>

I want to extract the values of a set of known "wanted keys" in a robust fashion, i.e. if SECOND_WANTED_KEY appears twice, I only want SECOND_WANTED_VALUE, not UNWANTED_VALUE. Additionally, THIRD_WANTED_KEY may or may not appear, so the query should be able to handle that as well. I can assume that FIRST_WANTED_KEY will appear before other keys, but can't assume anything about the order of the other keys - if a key appears twice, its values aren't important, I only want the first one. An anonymous data type consisting of strings is fine.

My attempt has centered around something along these lines:

var z = from y in x.Descendants()
        where y.Value == "FIRST_WANTED_KEY"
        select new
        {
          first_wanted_value = ((XElement)y.NextNode).Value,
         //...
        }

My question is what should that ... be? I've tried, for instance, (ugly, I know)

second_wanted_value = ((XElement)y.ElementsAfterSelf()
                      .Where(w => w.Value=="SECOND_WANTED_KEY")
                      .FirstOrDefault().NextNode).Value

which should hopefully allow the key to be anywhere, or non-existent, but that hasn't worked out, since .NextNode on a null XElement doesn't seem to work.

I've also tried to add in a

.Select(t => { 
    if (t==null) 
        return new XElement("SECOND_WANTED_KEY",""); 
    else return t;
})

clause in after the where, but that hasn't worked either.

I'm open to suggestions, (constructive) criticism, links, references, or suggestions of phrases to Google for, etc. I've done a fair share of Googling and checking around S.O., so any help would be appreciated.

Thanks!