A by file lookup is to take an mp3 that has ID3 tags in it or the file is named with "Artist - SongName"
you can get the CDDB dll to look at the tags, and the file name and look it up on the CDDB database. My program MP3 Tag Grabber (that i took off my web page last month) did this along with tag editing, playing etc... using a batch process without user input.
the problem is CDDB has a very restrictive license. I have a License for it, but i can not sell the program or advertize. if i did i would need to come up with about $10,000 first.
you can also use the cddb dll to write id3v2 tags it will not write id3v1 or id31.1 tags.
one thing i was looking into is taking the A music database and braking it down into a form i can use. then do byfile lookups from the database.
I would like to find a dll \ service that is free like
www.FreeDB.org (that is not the same company as
www.freeDB.com) that provices a ByFile lookUp as Gracenote CDDB.
This is just something to play with when drinking beer however.