You obtain the address for the original window procedure for the window defined by handle _vfp.HWnd

This._WindowProc = apiGetWindowLong(_vfp.HWnd, GWL_WNDPROC)

The handle is a number. For example, the moment you call the GetWindowLong the value is N.

Could you verify that:

1) The same handle N is used when you call the CallWindowProc -- nhWnd parameter must be equal to N.

2) _vfp.HWnd is still equal to N and does not change through the session.

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Btw, is the main VFP window is visible when you run your application? Also I would check the validity of windows involved in the CallWindowProc call using this function

DECLARE INTEGER IsWindow IN user32 INTEGER hWindow

It returns non-zero value for valid window handles.