Passing nil as a parameter in place of a TComponent

Question

I\'ve come across some code that\'s throwing an exception (EIntfCasterror Cast not supported) when it passes nil to a constructor expecting a TComponent, like so:

Answer 1

EIntfCastError has nothing to do with the Owner passed in the constructor. It's because you try to cast an interface to another interface you think it supports when it doesn't in fact support it.

MyNewInterface := MyInterface as IADifferentInterface;

You're never ever required to pass in Owner, even when creating a TForm. The following code is perfectly legal:

var
  MyForm: TForm1;
begin
  MyForm := TForm1.Create(nil);
  try
    MyForm.ShowModal;
  finally
    MyForm.Free;
  end
end;

So is this (although it's pretty dumb - it illustrates the point, though):

implementation

var
  Button: TButton;

procedure TForm1.FormCreate(Sender: TObject);
begin
  Button := TButton.Create(nil);
  Button.Parent := Form1;
  Button.Left := 10;
  Button.Top := 10;
  Button.Caption := 'Button';
  Button.Name := 'MyDumbButton';
  Button.OnClick := TheButtonClick;
end;

procedure TForm1.TheButtonClick(Sender: TObject);
begin
  ShowMessage(TButton(Sender).Name + ' clicked');
end;

procedure TForm1.FormDestroy(Sender: TObject);
begin
  Button.Free;
end;