>Initially, I construct a form, with an empty private data enviroment.
>The controls are data bound to a database, not opened yet.
>Then, I use the Load event to open the database and use table.
>This form run OK!
>Then I save this form like a class, and create a form derived for that class. Now, I do form this form, and an error appears! Apparently, something is checking the undefined ControlSources from the databound controls, before the Load event!

did u put anything in the sub-classes for load event?
if so try adding a call to the formet load event (with the :: operator)

Arnon