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function IsFileInUse(Path: string): boolean;
var hFile: THandle;
begin
  Result := False;
    if not FileExists(Path) then Exit;

    hFile := CreateFile(pchar(Path), GENERIC_READ or GENERIC_WRITE or GENERIC_EXECUTE,
                      0, nil, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, 0);
    Result := hFile = INVALID_HANDLE_VALUE;
    if not Result then CloseHandle(hFile);

end;					

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procedure TForm1.Button1Click(Sender: TObject);
begin
  if IsFileInUse(Application.Exename) then
        Showmessage('Die Datei ist in Benutzung');
end;