Linq to XML "Dynamic" query

[CSharpUser]

Hi folks,
I’m struggling with what I thought would be a relatively simple process. In my last thread (http://www.daniweb.com/software-development/csharp/threads/379458), I got your help with the placement of parentheses in a Linq to XML query. I thought it was all going to be easy from there.
Now I am trying to use a method to build the query string; but cannot find a way to get the returned string to execute as a query; or find a way to use a method to build a proper query. This hard-coded query works and I get the expected output:

var contList = criteria;
        (from c in contDoc.Descendants("Content")
         where (string)c.Attribute("Type") == (str)
         from d in c.Descendants("Video")
         where (string)d.Attribute("Codec") == ("H264")
         from f in c.Descendants("Audio")
         where (string)f.Attribute("Codec") == ("Mpeg1L1")
         select (string)c.Attribute("Id"));

But, when the same query is built via a method and returned as a string, it is visually and seemingly syntactically correct; but I cannot find how to get it to execute (note: the XDocument being passed is for future use):

.....
        criteria = buildLiveCriteria(profDoc);
        var contList = criteria;
.....
}

    private static string buildLiveCriteria(XDocument docIn)
    {
      string practContent = "Content";
      string practType = "Type";
      string practVideo = "Video";
      string practCodec = "Codec";
      string practCodecVal = "H264";
      string practAudio = "Audio";
      string practAudioCod = "Mpeg1L1";
      string practId = "Id";
      string practLastLine = "select (string)c.Attribute(\"" + practId + "\"));";
      string retVal = "";

      retVal += ("(from c in contDoc.Decendants (\"" + practContent + "\")");
      retVal += ("where (string)c.Attribute(\"" + practType + "\") == (\"Live\")");
      retVal += ("from d in c.Descendants(\"" + practVideo + "\")");
      retVal += ("where (string) d.Attribute(\"" + practCodec + "\") == (\"" + practCodecVal + "\")");
      retVal += ("from f in c.Descendants(\"" + practAudio + "\")");
      retVal += ("where (string) f.Attribute(\"" + practCodec + "\") == (\"" + practAudioCod + "\")");
      retVal += (practLastLine);
      return retVal;
    }

When the query string is returned and the 'var contList = criteria' line is executed, all it get in 'contList' is the query itself. Now, I can logically see how that is happening; but can someone point me to how to properly use a method to build a query like I am trying to do?

Thanks for any help!

[Momerath]

Take a look at this.

[CSharpUser]

Thanks for the suggestion Momerath; I did see that. What I am not getting out of it, and may not have made clear in my post, is how - or if even - I can build a query with a variable number of "from" and "where" clauses. The query may need to include one to five different elements with one or more attributes to qualify against for each.

Ideas?
Thanks!