Comments by Cnight Stalker (Top 1 by date)

Thanks for your efforts, but the problem remains.

I have various functions located on ContextMenuStrip, including 'MainFrm.Close()'. When a user presses the ShortcutKey for closing the main form, that event must not fire, not while there is a modal dialog. This is the whole point of a modal dialog - to suspend main form input, until the dialog is closed.

Unfortunately, your solution does not result the problem.