function RandString(s: string; Min, Max: Integer): string;

var

count, i, r : Integer;

begin

Result := '';

if ((Max - Min) > 0) then

begin

  count := Random(Max - Min + 1) + Min;

  for i := 0 to count - 1 do

  begin

   r := Random(Length(s)) + 1;

   Result := Result + s[r];

  end;

end;

end;

procedure TForm1.Button1Click(Sender: TObject);

begin

showmessage(RandString('ABCDEFGHIJKLMN',9,10));

end;