Delphi multiThreading - delphi

Question

I'm new at SO, so forgive me if my question isn't in the right place or been answered before.
The questions is about multi-threading with Delphi 10.4.
I'm getting Access Violation error on my app, here is a very simple example:
type
myThread = class(TThread)
protected
procedure Execute; override;
end;
TForm1 = class(TForm)
Button1: TButton;
Memo1: TMemo;
procedure Button1Click(Sender: TObject);
procedure FormClose(Sender: TObject; var Action: TCloseAction);
private
{ Private declarations }
mySideTask : myThread;
public
{ Public declarations }
end;
var
Form1: TForm1;
implementation
{$R *.dfm}
procedure TForm1.Button1Click(Sender: TObject);
begin
with mySideTask.Create do
FreeOnTerminate:=True
end;
procedure TForm1.FormClose(Sender: TObject; var Action: TCloseAction);
begin
if mySideTask<>nil then
begin
mySideTask.Terminate;
mySideTask.WaitFor;
FreeAndNil(mySideTask);
end;
end;
{ myThread }
procedure myThread.Execute;
begin
Synchronize(
procedure
begin
Form1.Memo1.Lines.Add('running my side task')
end);
end;
No error if I don't create an instance of the thread (which is confusing me):
procedure TForm1.Button1Click(Sender: TObject);
begin
myThread.Create
end;
Can you please let me know what am I missing.

Answer 1

The code in Button1Click() is wrong. You are calling Create() as an instance method on your mySideTask variable, but it is not pointing at a valid object instance. You need to instead call Create() as a constructor on the class type itself.
Try this instead:
procedure TForm1.Button1Click(Sender: TObject);
begin
mySideTask := myThread.Create(False{True});
//mySideTask.FreeOnTerminate := True;
//mySideTask.Start;
end;
Notice I commented out the handling of FreeOnTerminate=True. The reason for that is because that setting is meant for create-and-forget type of threads. The thread will destroy itself after its Execute() method exits. So it is not safe to call WaitFor() or Free() on a thread that could destroy itself at any moment.
If you want to use FreeOnTerminate=True, then the code should look more like this instead:
type
myThread = class(TThread)
protected
procedure Execute; override;
end;
TForm1 = class(TForm)
Button1: TButton;
Memo1: TMemo;
procedure Button1Click(Sender: TObject);
procedure FormClose(Sender: TObject; var Action: TCloseAction);
private
{ Private declarations }
mySideTask : myThread;
procedure SideTaskTerminated(Sender: TObject);
public
{ Public declarations }
end;
var
Form1: TForm1;
implementation
{$R *.dfm}
procedure TForm1.Button1Click(Sender: TObject);
begin
mySideTask := myThread.Create(True);
mySideTask.FreeOnTerminate := True;
mySideTask.OnTerminated := SideTaskTerminated;
mySideTask.Start;
end;
procedure TForm1.SideTaskTerminated(Sender: TObject);
begin
mySideTask := nil;
end;
procedure TForm1.FormClose(Sender: TObject; var Action: TCloseAction);
begin
if mySideTask <> nil then
begin
mySideTask.FreeOnTerminate := False;
mySideTask.Terminate;
mySideTask.WaitFor;
FreeAndNil(mySideTask);
end;
end;
{ myThread }
procedure myThread.Execute;
begin
Synchronize(
procedure
begin
Form1.Memo1.Lines.Add('running my side task')
end);
end;