Occurs when the list portion of a ComboBox control is about to drop down; this event doesn't occur if a ComboBox control's Style property is set to 1 (Simple Combo).
Private Sub object_DropDown( )
Use a DropDown event procedure to make final updates to a ComboBox list before the user makes a selection. This enables you to add or remove items from the list using the AddItem or RemoveItem methods. This flexibility is useful when you want some interplay between controls — for example, if what you want to load into a ComboBox list depends on what the user selects in an OptionButton group.