Delphi unit test for a TThread with FreeOnTerminate = True

Question 1

Subclass the thread to make it more testable. TThread and TObject provide enough hooks that you can add sensing variables to observe that it reaches certain points with the states you want it to have.

I see three aspects to this particular class that you might wish to test:

It computes a value for its Id property based on the value sent to the constructor.
It computes the new Id property in the new thread, not the thread that calls the constructor.
It frees itself when it's finished.

All those things are testable from a subclass, but hard to test otherwise without making changes to the thread's interface. (All the other answers so far require changing the thread's interface, such as by adding more constructor arguments or by changing the way it starts itself. That can make the thread harder, or at least more cumbersome, to use in the real program.)

type
  PTestData = ^TTestData;
  TTestData = record
    Event: TEvent;
    OriginalId: Integer;
    FinalId: Integer;
  end;

  TTestableMyThread = class(TMyThread)
  private
    FData: PTestData;
  public
    constructor Create(AId: Integer; AData: PTestData);
    destructor Destroy; override;
    procedure AfterConstruction; override;
  end;

constructor TTestableMyThread.Create(AId: Integer; const AData: PTestData);
begin
  inherited Create(AId);
  FData := AData;
end;

destructor TestableMyThread.Destroy;
begin
  inherited;
  FData.FinalId := Id;
  // Tell the test that the thread has been freed
  FData.Event.SetEvent;
end;

procedure TTestableMyThread.AfterConstruction;
begin
  FData.OriginalId := Id;
  inherited; // Call this last because this is where the thread starts running
end;

Using that subclass, it's possible to write a test that checks the three qualities identified earlier:

procedure TestTMyThread.TestMyThread;
var
  Data: TTestData;
  WaitResult: TWaitResult;
begin
  Data.OriginalId := -1;
  Data.FinalId := -1;
  Data.Event := TSimpleEvent.Create;
  try
    TTestableMyThread.Create(1, @Data);

    // We don't free the thread, and the event is only set in the destructor,
    // so if the event is signaled, it means the thread freed itself: That
    // aspect of the test implicitly passes. We don't want to wait forever,
    // though, so we fail the test if we have to wait too long. Either the
    // Execute method is taking too long to do its computations, or the thread
    // isn't freeing itself.
    // Adjust the timeout based on expected performance of Execute.
    WaitResult := Data.Event.WaitFor(5000);
    case WaitResult of
      wrSignaled: ; // This is the expected result
      wrTimeOut: Fail('Timed out waiting for thread');
      wrAbandoned: Fail('Event was abandoned');
      wrError: RaiseLastOSError(Data.Event.LastError);
      else Fail('Unanticipated error waiting for thread');
    end;

    CheckNotEquals(2, Data.OriginalId,
      'Didn''t wait till Execute to calculate Id');
    CheckEquals(2, Data.FinalId,
      'Calculated wrong Id value');
  finally
    Data.Event.Free;
  end;
end;

Question 2

Because you made the thread free itself upon termination then you have asked it to destroy all traces of itself as soon as it is done. Since you cannot exert influence on when it finishes, it is wrong to refer to anything inside the thread after you start it.

The solutions proposed by other, namely asking the thread to signal you when it terminates, are good. I personally would probably elect to do it that way. If you use an event as a signal then you can wait on that event.

However, there is another way to do it.

Create the thread suspended.
Duplicate the thread handle.
Start the thread.
Wait on the duplicated handle.

Because you own the duplicated handle, rather than the thread, you are safe to wait on it. It seems a little more complicated, but I suppose it avoids creating an extra synchronization object where one is not needed. Note that I'm not advocating this approach over the approach of using an event to signal completion.

Anyway, here's a simple demonstration of the idea.

{$APPTYPE CONSOLE}

uses
  SysUtils, Windows, Classes;

type
  TMyThread = class(TThread)
  protected
    procedure Execute; override;
  public
    destructor Destroy; override;
  end;

destructor TMyThread.Destroy;
begin
  Writeln('I''m dead!');
  inherited;
end;

procedure TMyThread.Execute;
begin
end;

var
  DuplicatedHandle: THandle;

begin
  with TMyThread.Create(True) do // must create suspended
  begin
    FreeOnTerminate := True;
    Win32Check(DuplicateHandle(
      GetCurrentProcess,
      Handle,
      GetCurrentProcess,
      @DuplicatedHandle,
      0,
      False,
      DUPLICATE_SAME_ACCESS
    ));
    Start;
  end;

  Sleep(500);
  Writeln('I''m waiting');
  if WaitForSingleObject(DuplicatedHandle, INFINITE)=WAIT_OBJECT_0 then
    Writeln('Wait succeeded');
  CloseHandle(DuplicatedHandle);
  Readln;
end.

Question 3

Create the thread in a suspended state, then set the OnTerminate and finally Resume the thread.

In your test class, define a private boolean field FThreadDone which is initialized with false and set to true by the OnTerminate Eventhandler.

Also, your constructor logic is a bit dirty, as you should not initialize field prior to calling the inherited constructor.

So:

constructor TMyThread.Create(AId: Integer);
begin
  inherited Create(true);
  FreeOnTerminate := True;
  FId := AId;
end;
...
procedure TestTMyThread.TestMyThread;
begin
  FThreadDone := False;
  with TMyThread.Create(1) do begin // Note: Thread is suspended...
    OnTerminate := OnThreadTerminate;
    // Resume;                         // ... and finally started here!
    Start;

  end;
  While not FThreadDone do Application.ProcessMessages;
  CheckEquals(2, FMyId);
end;

procedure TestTMyThread.OnThreadTerminate(Sender: TObject);
begin
  FMyId := (Sender as TMyThread).Id;
  FThreadDone := True;
end;

This should do the job.

EDIT: Corrected stupid corrections, tested, works.

Question 4

Here is an example using an anonymous thread.

An event (TSimpleEvent) is created
An anonymous thread executes the test thread and
Waits for the event, which signals in the OnTerminate handler of the test thread
The anonymous thread is on hold until executed with a WaitFor
The result was picked up by the OnTerminate handler

The important thing here is that the event is waited for in a thread. No dead-lock situation.

Uses
  SyncObjs;

type

  TMyThread = class(TThread)
  private
    FId : Integer;
  protected
    procedure Execute; override;
  public
    constructor Create( anInt : Integer);
    property Id : Integer read FId;
  end;

  TestTMyThread = class
  strict private
    FMyId: Integer;
    FMyEvent : TSimpleEvent;
    procedure OnThreadTerminate(Sender: TObject);
  protected
  public
    procedure TestMyThread;
  end;

{ TMyThread }

constructor TMyThread.Create(anInt : Integer);
begin
  inherited Create(True);
  FreeOnTerminate := True;
  FId := anInt;
end;

procedure TMyThread.Execute;
begin
  Inc(FId);
end;

procedure TestTMyThread.TestMyThread;
var
  AnonThread : TThread;
begin
  FMyEvent := TSimpleEvent.Create(nil,true,false,'');
  try
    AnonThread :=
      TThread.CreateAnonymousThread(
        procedure
        begin
          With TMyThread.Create(1) do
          begin
            OnTerminate := Self.OnThreadTerminate;
            Start;
          end;
          FMyEvent.WaitFor; // Wait until TMyThread is ready
        end
      );
    AnonThread.FreeOnTerminate := False;
    AnonThread.Start;

    AnonThread.WaitFor; // Wait here until test is ready
    AnonThread.Free;

    Assert(FMyId = 2); // Check result
  finally
    FMyEvent.Free;
  end;
end;

procedure TestTMyThread.OnThreadTerminate(Sender: TObject);
begin
  FMyId := (Sender as TMyThread).Id;
  FMyEvent.SetEvent; // Signal TMyThread ready
end;

Update, since Delphi-2010 does not have an anonymous thread class, here is an alternative which you can implement:

Type
  TMyAnonymousThread = class(TThread)
    private
      FProc : TProc;
    protected
      procedure Execute; override;
    public
      constructor Create(CreateSuspended,SelfFree: Boolean; const aProc: TProc);
  end;

constructor TMyAnonymousThread.Create(CreateSuspended,SelfFree: Boolean; 
  const aProc: TProc);
begin
  Inherited Create(CreateSuspended);
  FreeOnTerminate := SelfFree;
  FProc := aProc;
end;

procedure TMyAnonymousThread.Execute;
begin
  FProc();
end;