If you mean a listbox in a dialog, $did(id).sel will return the line number of the
selected line. I'm assuming that, to double click a line, you automatically select on your first click.
You could use the dclick event together with the $did(id).sel, I think it can work
Good luck,
Zyzzyx
Edit: didnt see it was MDX - I know nothing about ii, so the above might not work 
