This message returns the current active multiple document interface (MDI) child window, along with a flag indicating whether the child is maximized or not.
wParam
This parameter is not used.
lParam
This parameter is not used.
The return value contains the handle of the active MDI child window.
The return value used to include a flag indicating whether the MDI child window was maximized. It no longer does so. To determine the MDI child window's maximized state, you can use the GetWindowLong function and a Boolean test:
fMaximized = GetWindowLong(hwndMdiChild, GWL_STYLE) & WS_MAXIMIZE;