Delphi thread exception mechanism

http://www.techques.com/question/1-3627743/Delphi-thread-exception-mechanism

i have a dilema on how threads work in delphi, and why at a moment when a thread should raise an exception,

the exception is not showed. bellow is the code with comments, maybe somebody cand explain to me how that thread,

or delphi, is managing access violations

//thread code

unit Unit2;

interface

uses
  Classes,
  Dialogs,
  SysUtils,
  StdCtrls;

type
  TTest = class(TThread)
  private
  protected
    j: Integer;
    procedure Execute; override;
    procedure setNr;
  public
    aBtn: tbutton;
  end;

implementation


{ TTest }

procedure TTest.Execute;
var
  i                 : Integer;
  a                 : TStringList;
begin
 // make severals operations only for having something to do
  j := 0;
  for i := 0 to 100000000 do
    j := j + 1;
  for i := 0 to 100000000 do
    j := j + 1;
  for i := 0 to 100000000 do
    j := j + 1;
  for i := 0 to 100000000 do
    j := j + 1;
  for i := 0 to 100000000 do
    j := j + 1;
  for i := 0 to 100000000 do
    j := j + 1;
  for i := 0 to 100000000 do
    j := j + 1;
  for i := 0 to 100000000 do
    j := j + 1;

  Synchronize(setnr);
  a[2] := 'dbwdbkbckbk'; //this should raise an AV!!!!!!

end;

procedure TTest.setNr;
begin
  aBtn.Caption := IntToStr(j)
end;

end.

project's code

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs,
  Unit2, StdCtrls;

type
  TForm1 = class(TForm)
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
    procedure FormCreate(Sender: TObject);
  private
  public
    nrthd:Integer;
    acrit:TRTLCriticalSection;
    procedure bla();
    procedure bla1();
    function bla2():boolean;
    procedure onterm(Sender:TObject);
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.bla;
begin
 try
  bla1;
 except on e:Exception do
   ShowMessage('bla '+e.Message);
 end;
end;

procedure TForm1.bla1;
begin
 try
  bla2
 except on e:Exception do
   ShowMessage('bla1 '+e.Message);
 end;
end;

function TForm1.bla2: boolean;
var ath:TTest;
begin
 try
  ath:=TTest.Create(true);
   InterlockedIncrement(nrthd);
  ath.FreeOnTerminate:=True;
  ath.aBtn:=Button1;
  ath.OnTerminate:=onterm; 
   ath.Resume;
 except on e:Exception do
  ShowMessage('bla2 '+e.Message);
 end;
end;

procedure TForm1.Button1Click(Sender: TObject);

begin
//
 try
   bla;
   while nrthd>0 do
    Application.ProcessMessages;
 except on e:Exception do
  ShowMessage('Button1Click '+e.Message);
 end;
 ShowMessage('done with this');
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
 nrthd:=0;
end;

procedure TForm1.onterm(Sender: TObject);
begin
 InterlockedDecrement(nrthd)
end;

end.

the purpose of this application is only to know where the access violation is catch, and how the code should be written.
i can not understand why in the line "a[2] := 'dbwdbkbckbk';" the AV is not raised.

thanks in advance and best regards!

hreading is one place where you should swallow exceptions.

The gist of handling Exceptions in threads is that if you want the exception to be shown to the end user,

you should capture it and pass it on to the main thread where it can safely be shown.

You'll find some examples in this EDN thread How to Handle exceptions in TThread Objects.

procedure TMyThread.DoHandleException;
begin
  // Cancel the mouse capture
  if GetCapture <> 0 then SendMessage(GetCapture, WM_CANCELMODE, 0, 0);
  // Now actually show the exception
  if FException is Exception then
    Application.ShowException(FException)
  else
    SysUtils.ShowException(FException, nil);
end;

procedure TMyThread.Execute;
begin
  FException := nil;
  try
    // raise an Exception
    raise Exception.Create('I raised an exception');
  except
    HandleException;
  end;
end;

procedure TMyThread.HandleException;
begin
  // This function is virtual so you can override it
  // and add your own functionality.
  FException := Exception(ExceptObject);
  try
    // Don't show EAbort messages
    if not (FException is EAbort) then
      Synchronize(DoHandleException);
  finally
    FException := nil;
  end;
end;

In Delphi 2005 — and probably most other versions — if an exception escapes from the Execute method without being handled,

then it is caught by the function that called Execute and stored in the thread's FatalException property.

(Look in Classes.pas, ThreadProc.)

Nothing further is done with that exception until the thread is freed, at which point the exception is also freed.

It's your responsibility, therefore, to check that property and do something about it.

You can check it in the thread's OnTerminate handler.

If it's non-null, then the thread terminated due to an uncaught exception. So, for example:

procedure TForm1.onterm(Sender: TObject);
var
  ex: TObject;
begin
  Assert(Sender is TThread);
  ex := TThread(Sender).FatalException;
  if Assigned(ex) then begin
    // Thread terminated due to an exception
    if ex is Exception then
      Application.ShowException(Exception(ex))
    else
      ShowMessage(ex.ClassName);
  end else begin
    // Thread terminated cleanly
  end;
  Dec(nrthd);
end;

We can also reraise FatalException.

Reraising seems not logical but if you have an central exception/error handler in your code and

and if you just want to include thread exceptions into that mechanisim, you can reraise on some rare situation :

procedure TForm1.onterm(Sender: TObject);
var
  ex: Exception;
begin
  Assert(Sender is TThread);
  ex := Exception(TThread(Sender).FatalException);
  if Assigned(ex) then
    // Thread terminated due to an exception
    raise ex;
  Dec(nrthd);
end;

How to handle exceptions in TThread objects

By: Corbin Dunn

Abstract: This document describes how to properly handle and show an exception that happens in a thread.

Question: 
I have a TThread object which may raise an exception in the Execute procedure.

When an exception is raised, I want to be able to show that exception to the end user.

How do I go about doing this in the most efficient way? 

Answer:
With a TThread object, if you don't catch an exception in the Execute procedure of a TThread,

you may get access violations.

The Delphi IDE may break fine on the exception, but often when the application is run outside of the IDE

you get an "Application error has occurred" exception and your application stops running.

If you don't care about showing the end user that an exception occurred,

you can simply wrap your Execute procedure with a try..finally block such as:

procedure TMyThread.Execute;
begin
  try
    // Do your thread stuff here
  except // Eat all exceptions
  end;
end;

Quite often, this isn't the best solution and you will want to show the message to the end user,

or allow your application to further process the message.

The easiest way to do this, is to add an Exception object to your TThread class,

and call the appropriate handler based on the type of exception.

Here is an example of how to do this.

The project consists of one form with a Button placed on it:

unit Unit1;

interface

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

type
  TForm1 = class(TForm)
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

  TMyThread = class(TThread)
  private
    FException: Exception;
    procedure DoHandleException;
  protected
    procedure Execute; override;
    procedure HandleException; virtual;
  public
  end;

var
  Form1: TForm1;

implementation

{$R *.DFM}

procedure TMyThread.DoHandleException;
begin
  // Cancel the mouse capture
  if GetCapture <> 0 then SendMessage(GetCapture, WM_CANCELMODE, 0, 0);
  // Now actually show the exception
  if FException is Exception then
    Application.ShowException(FException)
  else
    SysUtils.ShowException(FException, nil);
end;

procedure TMyThread.Execute;
begin
  FException := nil;
  try
    // raise an Exception
    raise Exception.Create('I raised an exception');
  except
    HandleException;
  end;
end;

procedure TMyThread.HandleException;
begin
  // This function is virtual so you can override it
  // and add your own functionality.
  FException := Exception(ExceptObject);
  try
    // Don't show EAbort messages
    if not (FException is EAbort) then
      Synchronize(DoHandleException);
  finally
    FException := nil;
  end;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  // Create an instance of the TMyThread
  with TMyThread.Create(True) do
  begin
    FreeOnTerminate := True;
    Resume; 
  end;
end;

end.

vv