How to fix EConvertError handling in Delphi 7? - delphi

Question

so i tried to make simple app to check if entered number is odd or even. Also i wanted to handle EConvertError by entering Try and Except.
unit Unit1;
interface
uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs;
type
TForm1 = class(TForm)
procedure FormCreate(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;
var
Form1: TForm1;
var x:integer;
implementation
{$R *.dfm}
procedure TForm1.FormCreate(Sender: TObject);
begin
try
x:=StrToInt(InputBox('Zadávanie','Napíš číslo',''));
if x mod 2 = 0 then ShowMessage('Zadané číslo je párne')
else ShowMessage('Zadané číslo je nepárne');
except
on E: EConvertError do
ShowMessage('Zadávaj len čísla!');
end;
end;
end.
But this don't work and still showing the same exact Project1.exe raised exception class EConvertError with message ''' is not a vaild integer value instead of 'Zadávaj len čísla!'. Why?

Answer 1

Your try..except code is fine.
You are simply experiencing what happens when you run your app inside of the IDE's debugger. The debugger sees the exception before your app does. You are seeing a popup message from the debugger. Simply dismiss the popup and either press the "Run" button in the IDE, or press F9 on the keyboard, to continue execution and the exception will be passed to your app for normal handling, calling your except block. The popup will not happen when you run your app outside of the debugger, the except will just be called immediately.
If you don't want the debugger to popup a message on the exception, you can add EConvertError to the debugger's list of exceptions that it ignores. Or you can place breakpoints around the code that instruct the debugger to ignore exceptions for just this block of code.
Or, you can simply use TryStrToInt() instead of StrToInt(). TryStrToInt() does not raise an exception on a conversion error.

Answer 2

Just use TryStrToInt() which returns false if the input was no valid integer, something like that:
procedure TForm1.FormCreate(Sender: TObject);
var x: integer;
begin
try
if not TryStrToInt(InputBox('Zadávanie','Napíš číslo',''),x) then begin
ShowMessage('Zadávaj len čísla!');
exit;
end;
if x mod 2 = 0 then ShowMessage('Zadané číslo je párne')
else ShowMessage('Zadané číslo je nepárne');
end;
So no exception will be raised on input error.
IMHO exceptions should be exceptional - I don't like using EConvertError at all in my code.
BTW it is not a very good idea to put some UI code in the OnCreate event - better use OnShow for that.