How to get the IDataObject interface from the VB DataObject class

VB stores a pointer to the IDataObject interface in the 4th Long from the object pointer. Copying that pointer to a IDataObject variable you can use it.
 

Private Sub Form_OLEDragDrop(Data As DataObject, Effect As Long, _
                             Button As Integer, Shift As Integer, _
                             X As Single, Y As Single)
Dim MyDataObject as IDataObject

    ' Get an uncounted reference to the IDataObject 
    MoveMemory MyDataObject, ByVal ObjPtr(Data) + 16, 4

    ' Use the IDataObject interface

    ' Release the IDataObject
    ' You can't use Set = Nothing because
    ' the reference is not counted
    MoveMemory MyDataObject, 0&, 4

End Sub

The DataObject class in Common Controls implements the IDataObject interface. To get it just asign the object to the IDataObject variable:
 

Private Sub ListView1_OLEDragDrop(Data As ComctlLib.DataObject, _
                                  Effect As Long, _
                                  Button As Integer, Shift As Integer, _
                                  x As Single, y As Single)
Dim MyDataObject As IDataObject

    Set MyDataObject = Data

End Sub