Delphi: Pitfalls of Anonymous methods and Capture

I was writing a web request handler when I ran into a problem. I was plugging in multiple handlers, but only one of them actually worked.

Why?

To get to the essence of it, this doesn’t work

procedure TMyClass.Setup;
var
  Handler: THandlerClass;
  hType: THandlers;
begin
  Broker.Clear;
  for hType in [foo, bar]
  do begin
    case hType of
      foo: Handler := TFoo.Create;
      bar: Handler := TBar.Create;
    end;
    Broker.AddHandler(Handler.OnHandle);
  end;
end;

Zipped example source code: https://drive.google.com/open?id=1RxOxOlYOrjqPryW7vGNw40VTLav6IN_M

The handler will always be the “bar” handler, probably because it is last.

So, what works? Moving the creating and adding to the list to a separate method.

function TMyClass.WithProc<T>: T;
var
  Handler: T;
begin
  Handler := T.Create;
  Broker.AddHandler(Handler.OnHandle);
end;

procedure TMyClass.Setup;
var
  hType: THandlers;
begin
  Broker.Clear;
  for hType in [foo, bar]
  do begin
    case hType of
      foo: WithProc<TFoo>;
      bar: WithProc<TBar>;
    end;
  end;
end;


It behaves the same in both Tokyo and Rio.
Now – here is my question… Is this as expected or a bug ?

Update: The answer is – as expected. The reason is that the capture grabs a reference to the Variable, not to the value of the variable, hence the value of the Variable value will be the same for all captures, and the same as the last value set.

Personally, I would have liked to see the compiler issue a hint or a warning for captures in a loop.

4 thoughts on “Delphi: Pitfalls of Anonymous methods and Capture

  1. It is not clear that AddHandler contains a capture, ie. that anonymous methods play a role here. Like others have said, you could have constructed a better, simpler and more complete example to demonstrate your POV.

    Like

  2. In what way? The output is pretty conclusive.
    The reason there are so many units is that I modeled the SCCE after the original problem to ensure the behavior was not changed by scope/inheritance/etc.

    Like

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