HRESULT GetWindow( HWND* phwnd );
See IOleWindow::GetWindow in the Win32 SDK.
Remarks
The container calls this function to get the window handle of the control. Some containers will not work with a control that has been windowless, even if it is currently windowed. In ATL's implementation, if the control class's data member m_bWasOnceWindowless is TRUE, the function returns E_FAIL. Otherwise, if phwnd is not NULL, GetWindow sets *phwnd to the control class's data member m_hWnd and returns S_OK.
IOleInPlaceObjectWindowlessImpl Overview | Class Members
See Also CComControl::m_bWasOnceWindowless