Thanks Dave. I thought that was perfect, but it turns out that if you open a .dfx, as opposed to a .cb, CADFile.Filename is "none".
If I then look in the .config file, the DefaultDrawingDirector IS updated to the directory of the .dfx I opened.
Also, if I then do a doc.Save, CB prompts me to save the new .cb file (which contains the .dfx data) to the directory from which the .dfx was opened. At that point the doc.Filename does contain the full path I need. However, CB prompts me to confirm the doc.Save and I'm trying to avoid showing ANY prompt dialogs in my script. If I could do a doc.Save and somehow force "no prompt" that would work also, but I don't see that option.
It seems like the info I need is available and being used internally by CB, but I can't seem to get at it.