'以下在.bas
Type RECT
        Left As Long
        Top As Long
        Right As Long
        Bottom As Long
End Type
Declare Function SetWindowLong Lib "user32" Alias "SetWindowLongA" _
  (ByVal hwnd As Long, ByVal nIndex As Long, ByVal dwNewLong As Long) As Long
Declare Function GetWindowLong Lib "user32" Alias "GetWindowLongA" _
  (ByVal hwnd As Long, ByVal nIndex As Long) As Long
Declare Function CallWindowProc Lib "user32" Alias "CallWindowProcA" _
  (ByVal lpPrevWndFunc As Long, ByVal hwnd As Long, ByVal Msg As Long, _
   ByVal wParam As Long, ByVal lParam As Long) As Long
Declare Function GetWindowRect Lib "user32" (ByVal hwnd As Long, lpRect As RECT) As Long
Declare Function MoveWindow Lib "user32" (ByVal hwnd As Long, ByVal x As Long, ByVal y As Long, ByVal nWidth As Long, ByVal nHeight As Long, ByVal bRepaint As Long) As Long

Public Const WM_CTLCOLORLISTBOX = &H134
Public Const GWL_WNDPROC = (-4)


 Public preWinProc As Long
 Private hwndList As Long
 Private EverChange As Boolean
 Public AddOnWidth As Long
 Public Function wndproc(ByVal hwnd As Long, ByVal Msg As Long, _
                         ByVal wParam As Long, ByVal lParam As Long) As Long
 '以下程式会设定ListBox的大小，再将之送往原来的Window Procedure
 If Msg = WM_CTLCOLORLISTBOX Then
    '请处理Mouse Move的动作
    If Not EverChange Then
       Dim rect5 As RECT
       hwndList = lParam '当收到WM_CTLCOLORLISTBOX时，lParam是ListBox的hwmd
       EverChange = True
       Call GetWindowRect(hwndList, rect5)
       x = rect5.Left
       y = rect5.Top
       dx = rect5.Right - rect5.Left + AddonWidth
       dy = rect5.Bottom - rect5.Top
       Call MoveWindow(hwndList, x, y, dx, dy, 1)
    End If
 End If
 '将之送往原来的Window Procedure
 wndproc = CallWindowProc(preWinProc, hwnd, Msg, wParam, lParam)
 End Function

 '以下程式在Form1, form1中有一Combo1
 Sub Form_Load()
 Dim ret As Long
 AddonWidth = 50 '设定ComboBox中ListBox的宽度加50 Pixel
 '记录原本的Window Procedure的位址
 preWinProc = GetWindowLong(Combo1.hwnd, GWL_WNDPROC)
 '设定Combo1的window Procedure到wndproc
 ret = SetWindowLong(Combo1.hwnd, GWL_WNDPROC, AddressOf wndproc)
 End Sub

 Private Sub Form_Unload(Cancel As Integer)
 Dim ret As Long
 '取消Message的截取，而使之又只送往原来的Window Procedure
 ret = SetWindowLong(Combo1.hwnd, GWL_WNDPROC, preWinProc)
 End Sub