本人初学delphi,运行了下面这段程序的时候，总弹出 I/O error 6.请问各位大虾问题所在

uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, StdCtrls;
type
TForm1 = class(TForm)
OpenDialog1: TOpenDialog;
Memo1: TMemo;
Button1: TButton;
procedure Button1Click(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;
var
Form1: TForm1;
implementation
{$R *.dfm}
procedure TForm1.Button1Click(Sender: TObject);
var
f:textfile;
a:string;
begin
if opendialog1.execute then
memo1.clear;
assignfile(f,opendialog1.filename);
reset(f);
repeat
readln(f,a);
application.ProcessMessages;
memo1.Lines.Add(a);
until
eof(f);
closefile(f);
end;
end.

請補上粗體紅字的部份,因為當你按取消的時候你也執行了讀檔的程式
可是實際上並無法讀檔了

if opendialog1.execute then
begin
memo1.clear;
assignfile(f,opendialog1.filename);
reset(f);
repeat
readln(f,a);
application.ProcessMessages;
memo1.Lines.Add(a);
until
eof(f);
closefile(f);
end;

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