Can I read properties of a VCL control in a non-GUI thread?

Question

Is thread safe if a thread asynchronously read the information from VCL Controls in Delphi?

eg.

procedure TMyThread.Execute;
var bOK:Boolean; 
    iOK:Integer;
begin
   while not terminated do
   begin
      bOk:=MyForm.cbCheckBox.Checked;
      iOK:=MyForm.Left;
      sleep(20);
   end;
end;

If it is not thread safe how should I do to catch the event when the checkbox has changed its property.

Answer 1

No it is not safe. Your code is liable to lead to the window handle being created with affinity to the wrong thread.

Don't use the GUI to store your applications state. Use the GUI to show a view onto that state. Once you separate the state from the view you are home and dry. Your worker threads can use the underlying state state without touching GUI.