The CDN_TYPECHANGE notification message is sent by an Explorer-style Open or Save As common dialog box when the user selects a new file type from the file types combo box. Your OFNHookProc hook procedure receives this message in the form of a WM_NOTIFY message.
CDN_TYPECHANGE lpon = (LPOFNOTIFY) lParam;
The OFNOTIFY structure contains an NMHDR structure whose code member indicates the CDN_TYPECHANGE notification code.
The OFNOTIFY structure also contains a pointer to an OPENFILENAME structure whose nFilterIndex member indicates the one-based index of the newly selected file type filter.
The return value is ignored.
The system sends this notification only if the dialog box was created using the OFN_EXPLORER value.
GetOpenFileName, GetSaveFileName, NMHDR, OFNHookProc, OFNOTIFY, OPENFILENAME, WM_NOTIFY