Thread-safe in delphi

Question

I have to modify and change some visual components in a thread and as you know it's not safe to doing this.

My question is how to write a completely thread-safe code? It is possible? if it is then can you please give me a simple example?

my code that is not threadsafe:

type
  tMyWorkerThread = class(TThread)
      public
         procedure Execute; override;
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure tMyWorkerThread.Execute;
begin
  //codes
  //working with visual components
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  TMyWorkerThread.Create(false);
end;

Thank you.

Answer 1

Writing a thread safe code in Delphi involves the basic care you would have in any other language, which means to deal with race conditions. A race condition happens when different threads access the same data. A good way to deal with that is to declare an instance of TCriticalSection and wrap the dangerous code in it.

The code below shows a getter and a setter of a property that, by hypotesis, has a race condition.

constructor TMyThread.Create;
begin
  CriticalX := TCriticalSection.Create;
end;

destructor TMyThread.Destroy; override;
begin
  FreeAndNil(CriticalX);
end;

function TMyThread.GetX: string;
begin
  CriticalX.Enter;
  try
    Result := FX;
  finally
    CriticalX.Leave;
  end;
end;

procedure TMyThread.SetX(const value: string);
begin
  CriticalX.Enter;
  try
    FX := Value;
  finally
    CriticalX.Leave;
  end;
end;

Notice the use of a single instance of TCriticalSection (CriticalX) to serialize the access to the data member FX.

However, with Delphi you have an aditional consideration! VCL is not thread safe, so in order to avoid VCL race conditions, any operation that results in screen changing must run in the main thread. You get that by calling such a code inside a Synchronize method. Considering the class above, you should do something like this:

procedure TMyThread.ShowX;
begin
  Synchronize(SyncShowX);
end;

procedure TMyThread.SyncShowX;
begin
  ShowMessage(IntToStr(FX));
end;

If you have Delphi 2010 or later, there is an easier way that makes use of anonymous methods:

procedure TMyThread.ShowX;
begin
  Synchronize(procedure begin
    ShowMessage(IntToStr(FX));
  end);
end;

I hope this helps!

Answer 2

You should only access VCL objects from main VCL thread.

Some reading methods (property getters) do work from other threads in practice - but you have to prove it in advance reading VCL sources for the specific Delphi build. Or not use it.

PS: Synchronize method runs given procedure in main VCL thread, pausing the caller thread, which may lead to a deadlock, if main thread was also blocked.

Read more: (actually making this answer to list some links)

• http://www.michael-puff.de/Programmierung/Delphi/Code-Snippets/VCLThreadDemo.shtml
• http://www.drbob42.com/uk-bug/hood-04.htm
• http://delphi.about.com/od/kbthread/a/thread-gui.htm
• Is it better to use TThread's "Synchronize" or use Window Messages for IPC between main and child thread?
• Delphi 6 : breakpoint triggered on non-VCL thread stops main thread repaints
• http://docs.embarcadero.com/products/rad_studio/delphiAndcpp2009/HelpUpdate2/EN/html/devwin32/win32_mthreadusemainthread_xml.html
• Simplifying VCL thread wrapper code
• Update a VCL component from CreateAnonymousThread
• http://thaddy.co.uk/threads/ - the mirror of "Multithreading - The Delphi Way" by Martin Harvey
• http://otl.17slon.com/ - new Delphi approach to threading