internal static PropertyKey UI_PKEY_ItemsSource = new PropertyKey(new Guid(Convert.ToString(101, 16).PadLeft(8, '0') + UI_PropertyKey), (int)VarEnum.VT_UNKNOWN); // IEnumUnknown or IUICollection

    internal static PropertyKey UI_PKEY_Categories = new PropertyKey(new Guid(Convert.ToString(102, 16).PadLeft(8, '0') + UI_PropertyKey), (int)VarEnum.VT_UNKNOWN); // IEnumUnknown or IUICollection

    public interface IEnumUnknown

        void Clone(out IEnumUnknown ppenum);

Interface IEnumUnknown

     int EnumObjects([In, MarshalAs(UnmanagedType.U4)] int grfFlags, out IEnumUnknown ppenum);

     int EnumObjects([In, MarshalAs(UnmanagedType.U4)] int grfFlags, out IEnumUnknown ppenum);