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function IsFileOpen(const fName: TFileName): Boolean;
var 
  HFileRes: HFILE; 
begin 
  Result := False; 
  HFileRes := CreateFile(PChar(fName),
                         GENERIC_READ or GENERIC_WRITE,
                         0,
                         nil,
                         OPEN_EXISTING, 
                         FILE_ATTRIBUTE_NORMAL, 
                         0); 
  Result := (HFileRes = INVALID_HANDLE_VALUE); 
  if not Result then
    Application.MessageBox('offen', 'Ende', MB_OK)
  else
    Application.MessageBox('geschlossen', 'Ende', MB_OK);
end;