12 April 2012

Ocx background color

Not all OCX controls support a .BackColor property to change the background color, although the control itself allows changing it. Here GB32 conforms to VB too much. Rather than adding a property to the implementation – which would require another patch in executable code – we can use a different approach.

All OCX controls share a user-defined structure to store the current settings of the wrapped control. Setting and getting property values is sometimes nothing more than changing and setting a member of the structure. Setting and obtaining the background value is one of them. The BackColor property stores its value in the member at offset $B8 of the start of object’s data block. The following code changes the background of a StatusBar Ocx.

Form frm1
Ocx StatusBar sb1
Trace SetBkColorControl(sb1, RGB(23, 56, 229))
Until Me Is Nothing

Function SetBkColorControl(Ctrl As Control, Color As Long) As Long
  Const OffBackColor = $B8
  Local Long ObjPtr  = Long{*Ctrl}
  SetBkColorControl  = Long{ObjPtr + OffBackColor}
  {ObjPtr + OffBackColor} = Color
  ~InvalidateRect(Ctrl.hwnd, 0, 1)

  ' Notes
  ' 1 Using the IDispatch interface does not work
  '   when the GetTypeInfo doesn't support the
  '   the .BackColor property.
  ' BackColor is not a property of StatusBar
  If 0 Then Ctrl.BackColor = Color

  ' 2 When using XP Visual Themes the Visual Style
  '   theme takes over and gets precedence over
  '   GB OCX propery settings.

Note – Not all controls let their background set this way. Just experiment. 

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