Raising Exception in TThread Execute?

Question

I just realized that my exceptions are not being shown to the user in my threads!

At first I used this in my thread for raising the exception, which does not work:

except on E:Exception do
begin
  raise Exception.Create('Error: ' + E.Message);
end;

The IDE shows me the exceptions, but my app does not!

I have looked around for a solution, this is what I found:

Delphi thread exception mechanism

http://www.experts-exchange.com/Programming/Languages/Pascal/Delphi/Q_22039681.html

And neither of these worked for me.

Here's my Thread unit:

unit uCheckForUpdateThread;

interface

uses
  Windows, IdBaseComponent, IdComponent, IdTCPConnection, IdTCPClient,
  IdHTTP, GlobalFuncs, Classes, HtmlExtractor, SysUtils, Forms;

type
  TUpdaterThread = class(TThread)
  private
    FileGrabber : THtmlExtractor;
    HTTP : TIdHttp;
    AppMajor,
    AppMinor,
    AppRelease : Integer;
    UpdateText : string;
    VersionStr : string;
    ExceptionText : string;
    FException: Exception;
    procedure DoHandleException;
    procedure SyncUpdateLbl;
    procedure SyncFinalize;
  public
    constructor Create;

  protected
    procedure HandleException; virtual;

    procedure Execute; override;
  end;

implementation

uses
  uMain;

{ TUpdaterThread }

constructor TUpdaterThread.Create;
begin
  inherited Create(False);
end;

procedure TUpdaterThread.Execute;
begin
  inherited;
  FreeOnTerminate := True;

  if Terminated then
    Exit;

  FileGrabber           := THtmlExtractor.Create;
  HTTP                  := TIdHTTP.Create(nil);
  try
    try
      FileGrabber.Grab('http://jeffijoe.com/xSky/Updates/CheckForUpdates.php');
    except on E: Exception do
    begin
      UpdateText := 'Error while updating xSky!';
      ExceptionText := 'Error: Cannot find remote file! Please restart xSky and try again! Also, make sure you are connected to the Internet, and that your Firewall is not blocking xSky!';
      HandleException;
    end;
    end;

    try
      AppMajor      := StrToInt(FileGrabber.ExtractValue('AppMajor[', ']'));
      AppMinor      := StrToInt(FileGrabber.ExtractValue('AppMinor[', ']'));
      AppRelease    := StrToInt(FileGrabber.ExtractValue('AppRelease[[', ']'));
    except on E:Exception do
    begin
      HandleException;
    end;
    end;

    if (APP_VER_MAJOR < AppMajor) or (APP_VER_MINOR < AppMinor) or (APP_VER_RELEASE < AppRelease) then
    begin
      VersionStr := Format('%d.%d.%d', [AppMajor, AppMinor, AppRelease]);
      UpdateText := 'Downloading Version ' + VersionStr;
      Synchronize(SyncUpdateLbl);
    end;

  finally
    FileGrabber.Free;
    HTTP.Free;
  end;
  Synchronize(SyncFinalize);
end;

procedure TUpdaterThread.SyncFinalize;
begin
  DoTransition(frmMain.TransSearcher3, frmMain.gbLogin, True, 500);
end;

procedure TUpdaterThread.SyncUpdateLbl;
begin
  frmMain.lblCheckingForUpdates.Caption := UpdateText;
end;

procedure TUpdaterThread.HandleException;
begin
  FException := Exception(ExceptObject);
  try
    Synchronize(DoHandleException);
  finally
    FException := nil;
  end;
end;

procedure TUpdaterThread.DoHandleException;
begin
  Application.ShowException(FException);
end;

end.

If you need more info just let me know.

Again: The IDE catches all the exceptions, but my program does not show them.

EDIT: It was Cosmin's solution that worked in the end - and the reason it didn't at first, was because I didn't add the ErrMsg variable, instead I just placed whatever the variable would contain into the Synchronize, which would NOT work, however I have NO idea why. I realized it when I had no other ideas, and I just messed around with the solutions.

As always, the joke's on me. =P

Answer 1

Something very important you need to understand about multi-theraded development:

Each thread has its own call-stack, almost as if they're separate programs. This includes the main-thread of your program.

Threads can only interact with each other in specific ways:

• They can operate on shared data or objects. This can lead to concurrency issues 'race conditions', and therefore you need to be able to help them 'share data nicely'. Which brings us to the next point.
• They can "signal each other" using a variety of OS support routines. These include things like:
  • Mutexes
  • Critical Sections
  • Events
• And finally you can send messages to other threads. Provided the thread has in some way been written to be a message receiver.

NB: Note that threads cannot strictly speaking call other threads directly. If for example Thread A tried to call Thread B directly, that would be a step on Thread A's call-stack!

This brings us to the topic of the question: "exceptions are not being raised in my threads"

The reason for this is that all an exception does is:

• Record the error
• And unwind the call-stack. <-- NB: Your TThread instance can't unwind the main thread's call-stack, and cannot arbitrarily interrupt the main threads execution.

So TThread will not automatically report exceptions to your main application.

You have to make the explicit decision as to how you wish to handle errors in threads, and implement accordingly.

Solution

• The first step is the same as within a single threaded application. You need to decide what the error means and how the thread should react.
  • Should the thread continue processing?
  • Should the thread abort?
  • Should the error be logged/reported?
  • Does the error need a user decision? <-- This is by far the most difficult to implement, so we'll skip it for now.
• Once this has been decided, implement the appropriate excpetion handler.
• TIP: Make sure the exception doesn't escape the thread. The OS won't like you if it does.
• If you need the main program (thread) to report the error to the user, you have a few options.
  • If the thread was written to return a result object, then it's easy: Make a change so that it can return the error in that object if something went wrong.
  • Send a message to the main thread to report the error. Note, the main thread already implements a message loop, so your application will report the error as soon as it processes that message.

---

EDIT: Code Sample for indicated requirement.

If all you want to do is notify the user, then Cosmind Prund's answer should work perfectly for Delphi 2010. Older versions of Delphi need a little more work. The following is conceptually similar to Jeff's own answer, but without the mistakes:

procedure TUpdaterThread.ShowException;
begin
  MessageDlg(FExceptionMessage, mtError, [mbOk], 0);
end;

procedure TUpdaterThread.Execute;
begin
  try

    raise Exception.Create('Test Exception');
    //The code for your thread goes here
    //
    //

  except
    //Based on your requirement, the except block should be the outer-most block of your code
    on E: Exception do
    begin
      FExceptionMessage := 'Exception: '+E.ClassName+'. '+E.Message;
      Synchronize(ShowException);
    end;
  end;
end;

---

Some important corrections on Jeff's own answer, including the implementation shown within his question:

The call to Terminate is only relevant if your thread is implemented within a while not Terminated do ... loop. Take a look at what the Terminate method actually does.

The call to Exit is an unnecessary waste, but you probably did this because of your next mistake.

In your question, you're wrapping each step in its own try...except to handle the exception. This is an absolute no-no! By doing this you pretend that even though an exception occurred, everything is ok. Your thread tries the next step, but is actually guaranteed to fail! This is not the way to handle exceptions!

Answer 2

Here's my very, very short "take" on the issue. It only works on Delphi 2010+ (because that version introduced Anonymous methods). Unlike the more sophisticated methods already posted mine only shows the error message, nothing more, nothing less.

procedure TErrThread.Execute;
var ErrMsg: string;
begin
  try
    raise Exception.Create('Demonstration purposes exception');
  except on E:Exception do
    begin
      ErrMsg := E.ClassName + ' with message ' + E.Message;
      // The following could be all written on a single line to be more copy-paste friendly  
      Synchronize(
        procedure
        begin
          ShowMessage(ErrMsg);
        end
      );
    end;
  end;
end;