IOleInPlaceObjectWindowless interface

Lets a container send window messages to an OLE object that is active in place and has no window of its own, and ask the object for its drop target.

IOleInPlaceObjectWindowless has the interface identifier 1C2056CC-5EF4-101B-8BC8-00AA003E3B29 and extends IOleInPlaceObject, so it also has the methods of that interface and of IOleWindow. It is declared [OleAutomation(False)]. Microsoft Learn describes the Windows interface: IOleInPlaceObjectWindowless.

A small object, such as a control, does not need a window. The container receives the window messages for it and passes them on with OnWindowMessage, and the object takes part in drag and drop through the target that GetDropTarget returns.

The declaration is complete: it has both methods of the Windows interface. Both are marked [PreserveSig] and return the raw HRESULT as a Long, so a class cannot implement this interface; it is for calling. IOleInPlaceObjectWindowlessVB declares OnWindowMessage in a form a class can implement.

Methods

OnWindowMessage 

[PreserveSig]
Function OnWindowMessage(ByVal wMsg As Long, ByVal wParam As LongPtr, ByVal lParam As LongPtr, ByRef Result As LongPtr) As Long

Sends a window message from the container to the object. wMsg is the message identifier, and wParam and lParam are the parameters of the message, as the container received them. A position in wParam or lParam is in client coordinates of the container’s window. Result receives the result code of the message.

The result of the call is the raw HRESULT: S_OK (0) when the object processed the message, and S_FALSE (1) when it did not. The container then processes the message itself or passes it to the default window procedure.

The container sends the mouse messages to the object that has captured the mouse, or else to the object under the pointer, and the keyboard messages to the object that has the focus.

GetDropTarget 

[PreserveSig]
Function GetDropTarget(ByRef lppDropTarget As LongPtr) As Long

Returns the object’s IDropTarget interface in lppDropTarget. A windowless object cannot register a drop target, because it has no window to register, so the container gets the interface here instead. The package does not declare IDropTarget, so the pointer is a LongPtr. The container gives the pointer back with Release, and may keep it for later drag operations until then.

The result is the raw HRESULT: S_OK (0) on success, and E_NOTIMPL (&H80004001) when the object does not support drag and drop.

Example

This function sends a message to a windowless object and returns whether the object processed it. The result code of the message is not used.

Private Function Deliver(ByVal Obj As IOleInPlaceObjectWindowless, ByVal Msg As Long, ByVal WParam As LongPtr, ByVal LParam As LongPtr) As Boolean
    Dim result As LongPtr
    Return Obj.OnWindowMessage(Msg, WParam, LParam, result) = 0
End Function

See Also