When the user selects a filename in the directory window or in the Search Results window, File Manager sends the FMEVENT_SELCHANGE message to an extension DLL. An extension can use this opportunity to send a query message to File Manager to obtain more information about the user's selection. For more information, see Section 16.4, “Extension Messages.”
Because the user can change the selection often, the extension should return promptly after processing the FMEVENT_SELCHANGE message to avoid slowing the user's selection process.