Proper Catastrophic Error Handling

Question

There's something I keep running into that I really haven't solved with Delphi programs and was wondering if anyone could instruct me on it. As the topic says, how do you do proper catastrophic error handling? For instance:

// is file necessary for the program present?
if not FileExists(FilePath1) then
   begin
     raise Exception.Create(FilePath1 + ' does not exist and is required for this program to function.');
   // I obviously need to do something here to make the program QUIT and not have
   // any more code run.

     Application.Terminate;
     Abort;
   end;

I can use the exception unit there as well and throw out an exception, but the program continues as before. I've used the halt call in the past, but it seems to not do any cleanup or the like so I end up making a big procedure with close and free calls for everything I've done just to be sure (and even then I'm not sure of any of the behind the scenes stuff).

So what is the right way to handle such things?

Edit: To clarify, I'm wanting to know about how to make the program do what clean-up it needs to do and then EXIT NOW and not do any other code.

Answer 1

To perform abnormal termination call Halt() passing the exit code.

if CatastropicErrorDetected then
begin
  ... show error message
  Halt(1);
end;

On Windows this results in a call to TerminateProcess and will stop execution there and then.

You note that no cleanup is performed and usually that's what you want. Since you are performing this check at application startup there should be nothing to cleanup.

Answer 2

IMHO the only clean way would be checking for "Fatal conditions" before Application is running.

program Project2;

uses
  Forms,Dialogs,
  Unit1 in 'Unit1.pas' {Form1};

{$R *.res}

begin
  ReportMemoryLeaksOnShutDown := true;
  Application.Initialize;
  if True then // your condition here
    begin
      MessageDLG('Fatal Error',mtError,[mbok],0);
    end
  else
    begin
      Application.CreateForm(TForm1, Form1);
      Application.Run;
    end;
end.

Any other approach will have side effects

unit Unit1;

interface

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

type
  TForm1 = class(TForm)
    procedure FormCreate(Sender: TObject);
  private
    { Private-Deklarationen }
    FSL:TStringList;
  public
   Destructor Destroy;override;
    { Public-Deklarationen }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

destructor TForm1.Destroy;
begin
  FreeAndNil(FSL);
  Showmessage('Will never be seen with Application.Terminate + HALT');
  inherited;
end;

procedure TForm1.FormCreate(Sender: TObject);
const
  Testing=0; // try 1 and 2 too
begin

   FSL:=TStringList.Create;
   Try
      raise Exception.Create('Terminating now');
   except
     case Testing of
         0: begin
             // exception object will not be freed other code will be prevented, Form won't be shown
             Application.Terminate;
             HALT;
            end;
         1: begin
             // exception object will not be freed Form won't be shown
             HALT;
            end;
         2: begin
             // clean but Form will be shown
             Application.Terminate;
            end;

     end;
   end;

end;

end.