Why do I/O errors fail to raise exceptions?

Question

I'm using old style Pascal I/O routines and expect that calls to I/O functions that fail should raise an EInOutError. When I try this I do not see an exception raised and I have no clue why.

procedure TForm1.Button1Click(Sender: TObject);
//var i: integer;
begin
id:=(strtoint(Edit1.Text)-1)*4;

AssignFile(plik,'\klienci\'+linia_klient[id]+'.txt');
try
Reset(plik);
except
  on EInOutError do Rewrite(plik);
  end;
edit2.Text:=linia_klient[id+1];
edit3.Text:=linia_klient[id+2];
//ListBox1.Clear;
//ListBox1.Items.Add();
end;

Entire code:

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TForm1 = class(TForm)
    Label1: TLabel;
    Edit1: TEdit;
    Button1: TButton;
    Label2: TLabel;
    Label3: TLabel;
    Edit2: TEdit;
    Edit3: TEdit;
    ListBox1: TListBox;
    Label4: TLabel;
    procedure FormCreate(Sender: TObject);
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;
  plik:TextFile;
  linia_klient,linia_video:array[0..20] of string;
  id:integer;

implementation

{$R *.dfm}
{$IOCHECKS ON}
procedure TForm1.FormCreate(Sender: TObject);
var i:integer;
begin
Edit1.Text:='Witaj, Podaj ID klienta';
Label1.Caption:='ID';
AssignFile(plik,'klienci.txt');
Reset(plik);
i:=0;
While Not Eof(plik) do
  begin
    Readln(plik,linia_klient[i]);
    inc(i);
  end;

CloseFile(plik);
AssignFile(plik,'video.txt');
Reset(plik);
i:=0;
While Not Eof(plik) do
  begin
    Readln(plik,linia_video[i]);
    inc(i);
  end;

CloseFile(plik);
end;

procedure TForm1.Button1Click(Sender: TObject);
//var i: integer;
begin
id:=(strtoint(Edit1.Text)-1)*4;

AssignFile(plik,'\klienci\'+linia_klient[id]+'.txt');
try
Reset(plik);
except
  on EInOutError do Rewrite(plik);
  end;
edit2.Text:=linia_klient[id+1];
edit3.Text:=linia_klient[id+2];
//ListBox1.Clear;
//ListBox1.Items.Add();
end;

end.

Answer 1

The exception EInOutError will only be raised if I/O checking is enabled. To make sure it is enabled do the following:

• In your project options, in "Delphi Compiler Options" check the checkbox I/O checking
• Remove any {$IOCHECKS OFF} or {$I-} directives from your code as they disable I/O checking

This should give you a proper exception if the file doesn't exist.

Now if (for whatever reason) you cannot enable I/O checking:

With disabled I/O checking you won't get an EInOutError if something goes wrong. Instead you have to check the value of IOResult after every I/O operation. It's like in old Pascal times: If IOResult <> 0 then an error happened. This (slightly adapted) excerpt from the Delphi docs shows how to work with IOResult:

  AssignFile(F, FileName);
  {$I-}
  Reset(F);
  {$I+}
  if IOResult = 0 then
  begin
    MessageDlg('File size in bytes: ' + IntToStr(FileSize(F)),
      mtInformation, [mbOk], 0);
    CloseFile(F);
  end
  else
    MessageDlg('File access error', mtWarning, [mbOk], 0);

However, nowadays you should use TFileStream to access/create files and don't use the old style Pascal routines anymore. An example how this could look:

filename := '\klienci\'+linia_klient[id]+'.txt';
if not FileExists(filename) then
  // "Create a file with the given name. If a file with the given name exists, open the file in write mode."
  fs := TFileStream.Create(filename, fmCreate) else
  // "Open the file to modify the current contents rather than replace them."
  fs := TFileStream.Create(filename, fmOpenReadWrite);