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Tip of the Day
Language: VB5,VB6
Expertise: Intermediate
Aug 7, 2000

Intercepting MouseEnter and MouseExit events without subclassing

Visual Basic raises the MouseMove event when the mouse cursor is within a control, so it's easy to know when the cursor "enters" a control. However, determining when the mouse leaves the control is more difficult.

You can solve this problem by monitoring the first time the control raises the MouseMove event, which corresponds to the MouseEnter event; in this occasion you capture the mouse input with SetCapture API function, so that your control will receive MouseMove messages even if the cursor is outside the control's window.

Once you have captured the mouse input you can check the coordinates of the cursor and verify if it's still in the control area. When mouse leaves the control's window, it's time for the MouseExit event, and you must release the mouse capture with ReleaseCapture(). Here's the code that does the trick for the Command1 control, but that can be easily modified for any control:

Private Declare Function SetCapture Lib "user32" (ByVal hwnd As Long) As Long
Private Declare Function ReleaseCapture Lib "user32" () As Long
Private Declare Function GetCapture Lib "user32" () As Long

Private Sub Command1_MouseMove(Button As Integer, Shift As Integer, X As Single, _
    Y As Single)

    If (X < 0) Or (Y < 0) Or (X > Command1.Width) Or (Y > Command1.Height) Then
        ' the MOUSELEAVE pseudo-event
        ' in this example revert the caption to normal
        Command1.Font.Bold = False
    ElseIf GetCapture() <> Command1.hwnd Then
        ' the MOUSEENTER pseudo-event
        SetCapture Command1.hwnd
        ' in this example, make the caption bold
        Command1.Font.Bold = True
    End If

End Sub

You can apply this tip to all controls that have the hWnd property, such as PictureBox, ListBox, etc, for example to easily implement hot-tracking effects. Here is another example that selects the text of a textbox (Text1) when the cursor is over.

Private Sub Text1_MouseMove(Button As Integer, Shift As Integer, X As Single, _
    Y As Single)
    If (X Or Y) < 0 Or (X > Text1.Width) Or (Y > Text1.Height) Then
        Text1.SelLength = 0
    ElseIf GetCapture() <> Text1.hWnd Then
        SetCapture Text1.hWnd
        Text1.SelStart = 0
        Text1.SelLength = Len(Text1)
    End If
End Sub
UPDATE: The original example for the Command1 button didn't work well, and the mouseover functionality was disabled after you click on the button. The new version shown above uses the GetCapture API to solve this problem, and gets rid of the bCaptured Static variable, so it's also simpler than the original code. Big thanks to Sergio Perciballi from UK for letting us know about the mistake.
Alberto Falossi
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