procedure TForm1.DoFileOpen(fn: TFileName);
var
  ts: TTabSheet;
  se: TSynEdit;
begin
  ts := TTabSheet.Create(PageControl1);
  ts.PageControl := PageControl1;
  if fn = ''
    then ts.Caption := Format('Neu [%d]', [PageControl1.PageCount])
    else ts.Caption := ExtractFileName(fn);
  ts.Hint := fn;
  se := TSynEdit.Create(ts);
  se.Parent := ts;
  se.Align := alClient;
    se.ActiveLineColor:= $00FFFF80;
  se.Gutter.ShowLineNumbers:= TRUE;
  if fn = ''
    then se.Lines.Clear
    else se.Lines.LoadFromFile(fn);
  Files.AddObject(fn, se);
  PageControl1.ActivePage := ts;
  se.SetFocus;
end;

procedure TForm1.DoFileSaveAs(fn: TFileName);
var
  se: TSynEdit;
  i: Integer;
begin
  with PageControl1 do
    i := ActivePageIndex;
  se := TSynEdit(Files.Objects[i]);
  se.Lines.SaveToFile(fn);
  Files[i] := fn;
  if se.Modified then
    se.Modified := False;
end;

begin
  with Sender as TFileSaveAs do
    DoFileSaveAs(Dialog.FileName);
end;

begin
  with Sender as TFileOpen do
    DoFileOpen(Dialog.FileName);
end;