How can I use TTask and TThreadPool in Delphi to run multiple tasks and ensure that its start and completion are reported accurately?

I am trying to implement multithreading in Delphi 10.4 (Sydney). I am using TTask to run tasks concurrently and TThreadPool to limit the simultaneously running threads.

Here's my code:

var
  MyThreadPool: TThreadPool;
  I: Integer;
  Tasks: TArray<ITask>;

function ProcessTask(const TaskId: Integer): TProc;
begin
  Result := procedure
  Begin
    WriteLn(Format('Started TaskId: %d ThreadId: %d',[TaskId,TThread.Current.ThreadID]));
    Sleep(2000); // Simulate work
    WriteLn(format('TaskId %d completed. ThreadId: %d',[TaskId, TThread.Current.ThreadID]));
  End;
end;

begin
  try

    Writeln('Creating TThreadPool');
    MyThreadPool := TThreadPool.Create;

    try
      MyThreadPool.SetMinWorkerThreads(1);
      MyThreadPool.SetMaxWorkerThreads(4);

      // Hold 10 tasks
      SetLength(Tasks,10);

      for I := 0 to High(Tasks) do
      Begin
        Tasks[I] := TTask.Create(
          ProcessTask(I+1),
          MyThreadPool
        );

        // Start the task
        Tasks[I].Start;
      End;

      // Wait for all tasks to complete
      for I := 0 to High(Tasks) do
        Tasks[I].Wait;

      WriteLn('All tasks completed.');
      ReadLn;

    finally
      MyThreadPool.Free;
    end;

  except
    on E: Exception do
      Writeln(E.ClassName, ': ', E.Message);
  end;
end.

The output of the above code is:

Creating TThreadPool
Started TaskId: 1 ThreadId: 17292
Started TaskId: 1 ThreadId: 17292
Started TaskId: 2 ThreadId: 19256
Started TaskId: 3 ThreadId: 25268
Started TaskId: 3 ThreadId: 25268
Started TaskId: 4 ThreadId: 21112
TaskId 1 completed. ThreadId: 17292
Started TaskId: 5 ThreadId: 17292
TaskId 2 completed. ThreadId: 19256
Started TaskId: 6 ThreadId: 19256
TaskId 3 completed. ThreadId: 25268
TaskId 3 completed. ThreadId: 25268
TaskId 4 completed. ThreadId: 21112
Started TaskId: 7 ThreadId: 21112Started TaskId: 8 ThreadId: 25268Started TaskId: 7 ThreadId: 21112Started TaskId: 8 ThreadId: 25268
TaskId 5 completed. ThreadId: 17292
Started TaskId: 9 ThreadId: 17292
TaskId 6 completed. ThreadId: 19256
Started TaskId: 10 ThreadId: 19256
TaskId 8 completed. ThreadId: 25268
TaskId 8 completed. ThreadId: 25268
TaskId 7 completed. ThreadId: 21112
TaskId 9 completed. ThreadId: 17292
TaskId 10 completed. ThreadId: 19256
All tasks completed.

The problem with the code is that the Start and Completion reporting is duplicated in certain cases:

1. The Started TaskId appears twice for TaskId 1,3,7,8.
2. The TaskId completed appears twice for TaskId 3,8.

Without looking too deeply, the problem is likely that you are using WriteLn in multi-threaded code which can be prone to problems. Add some sort of synchronization to your output to ensure only one WriteLn call is executed at a time.

var
  ConsoleLock: TCriticalSection;

procedure SafeWriteLn(const Msg: string);
begin
  ConsoleLock.Enter;
  try
    WriteLn(Msg);
  finally
    ConsoleLock.Leave;
  end;
end;

ConsoleLock := TCriticalSection.Create;
try
  //your stuff
finally
  ConsoleLock.Free;
end;