delphi中怎样把string转为array of byte

以下为示例代码：
procedure TForm1.Button1Click(Sender: TObject);

var
s:string;
ab:array of byte;
i:integer;
begin
s:='this is a test';
SetLength(ab,Length(s));
for i:=1 to length(s) do
ab[i]:=byte(s[i]);
end;