OnFileOkClick occurs before a directory is opened or closed from the dialog. This can happen when the user double-clicks on a directory, clicks the Up arrow, or uses the list box at the top of the dialog to navigate through the directory structure. If an event handler is defined, the property contains a pointer to a TFileDialogFolderChangingEvent. The DoOnFolderChanging method triggers this event. 

DoOnFolderChanging 

TFileDialogFolderChangingEvent