How can I catch a value from one form to another before the form closes?

I have two Windows forms opened in a mainform as mdichildren. In Form A, I have a listbox with numbers. In Form B, I want to catch the selected item of the listbox (from form A) when the user pushes a button and then closes form A. The question is, how can I catch a value from one form to another before the form is closed?
Unfortunately, you can't handle an event from one form on another form. What you'll want to do is handle the Close event on Form A and manually tell Form B about the selected item.

This was first published in May 2003