The ComboBox control exposes the DropDown event, that lets you determine when its list area is being opened. However, VB doesn’t offer any simple way to understand when the list area is being closed. You can achieve this by intercepting the WM_COMMAND message that the ComboBox control sends to its container (can be the parent form, or a container control such as a PictureBox control). The following code shows how you can trap this message using the MsgHook component (that you can download from the File Bank section of this site):
' REQUIRES THE MSGHOOK.DLL COMPONENTConst WM_COMMAND = &H111Const CBN_DROPDOWN = 7Const CBN_CLOSEUP = 8Dim WithEvents FormHook As MsgHookPrivate Sub Form_Load() ' subclass the form with events Set FormHook = New MsgHook ' you should pass the hWnd of the combobox container FormHook.StartSubclass Me.hWndEnd SubPrivate Sub FormHook_AfterMessage(ByVal uMsg As Long, ByVal wParam As Long, _ ByVal lParam As Long, retValue As Long) Dim Ctrl As Control Dim ctrlIndex As Long Dim cmdCode As Long If uMsg = WM_COMMAND Then ' on entry, lParam holds the hWnd of the control ' the high word of wParam contains the actual command code ' the low word of wParam contains the control ID ' ( this is the index of the control in the Controls collection + 1) ctrlIndex = (wParam And &HFFFF&) - 1 Set Ctrl = Me.Controls(ctrlIndex) cmdCode = (wParam And &HFFFF0000) &H10000 If Not TypeOf Ctrl Is ComboBox Then ' this should never happen ElseIf cmdCode = CBN_DROPDOWN Then ' the list area is being opened Debug.Print Ctrl.Name & " is being opened" ElseIf cmdCode = CBN_CLOSEUP Then ' the list area is being closed Debug.Print Ctrl.Name & " is being closed" End If End IfEnd Sub
