The FILEOKSTRING message is sent by the Open dialog box or Save As dialog box to the application's hook function when the user has selected a filename and chosen the OK button. The message lets the application accept or reject the user-selected filename.
wParam
Not used.
lParam
Points to an OPENFILENAME structure containing information about the user's selection. (This information includes the filename for the selection.)
The hook function should return 1 if it rejects the user-selected filename. In this case, the dialog box remains open and the user must select another filename. The hook function should return 0 if it accepts the user-selected filename or does not process the message.
To use this message, the application must create a message identifier by using the RegisterWindowMessage function and passing the FILEOKSTRING constant as the function's single parameter.