How to catch unhandled exception that are raised in background thread?

I have for habit to execute anonymous thread like :

TThread.CreateAnonymousThread(
procedure
begin
.....
end).start;


But the problem is that if some unhandled exception will raise during the execution, then i will be not warned about it! For the main thread we have Application.OnException. Do we have something similar for background thread ?

Application.OnException

4 Answers
4

TThread has a public FatalException property:

TThread

FatalException

If the Execute method raises an exception that is not caught and handled within that method, the thread terminates and sets FatalException to the exception object for that exception. Applications can check FatalException from an OnTerminate event handler to determine whether the thread terminated due to an exception.

Execute

FatalException

FatalException

OnTerminate

For example:

procedure TMyForm.DoSomething;
begin
...
thread := TThread.CreateAnonymousThread(...);
thread.OnTerminate := ThreadTerminated;
thread.Start;
...
end;

procedure TMyForm.ThreadTerminated(Sender: TObject);
begin
if TThread(Sender).FatalException <> nil then
begin
...
end;
end;


TThread

CreateAnonymousThread

CreateAnonymousThread()

TThread

Start()

Execute

try..except

OnTerminate

N̶o̶,̶ ̶t̶h̶e̶r̶e̶ ̶i̶s̶ ̶n̶o̶t̶h̶i̶n̶g̶ ̶s̶i̶m̶i̶l̶a̶r̶ ̶f̶o̶r̶ ̶a̶ ̶b̶a̶c̶k̶g̶r̶o̶u̶n̶d̶ ̶t̶h̶r̶e̶a̶d̶.̶ (Thanks, Remy!)

The best solution is to always be absolutely certain to never let an exception escape from a thread. Ideally, your thread procedures should look something like this :

TThread.CreateAnonymousThread(
procedure
begin
try
your code
except
on E : Exception do
... handle it!
end;
end).start;


Ok, finally I got the best answer :

Assign the global ExceptionAcquired procedure to our own implementation (it is nil by default). This procedure gets called for unhandled exceptions that happen in other threads than the main thread.

ExceptionAcquired

ExceptionAcquired := MyGlobalExceptionAcquiredHandler;


The answer of J... and Remy Lebeau are good with what delphi offer, but i need a little more and I finally decide to modify a little the unit System.Classes

System.Classes

var
ApplicationHandleThreadException: procedure (Sender: TObject; E: Exception) of object = nil;

function ThreadProc(const Thread: TThread): Integer;
...
try
Thread.Execute;
except
Thread.FFatalException := AcquireExceptionObject;
if assigned(ApplicationHandleThreadException) and
assigned(Thread.FFatalException) and
(Thread.FFatalException is Exception) and
(not (Thread.FFatalException is EAbort)) then
ApplicationHandleThreadException(Thread, Exception(Thread.FFatalException));
end;


in this way you just need to assign ApplicationHandleThreadException to handle unhandled exception raise in any TThread. You don't need to be worry about the multi thread because global var like ExceptAddr are declared as threadvar so everything work fine, even to retrieve the stack trace !

ApplicationHandleThreadException

ExceptAddr

threadvar

https://quality.embarcadero.com/browse/RSP-21269

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