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:

obj := SomeClass.Create(nil);

The unit this is in does not contain a form and even TForm requires a TComponent be passed to it when you call its constructor. What should I pass in place of nil if anything exists or is there a way to get it to accept nil.

Thank you.

Also, I don't have the source code which calls the method this is in, or I would just have it pass the form it has access to.

EDIT: Fixed the code example.

EDIT2: Fixed the code example because I had a second brain fart when I first wrote it.

EDIT3: I don't have the code for the constructor either.

Tippet

That's not a constructor, that's a cast. Did you mean SomeClass.Create(nil)? – Bark

That's not even Delphi code. ;-) – Amarillis

You have not provided an accurate picture of your problem. That exception is not caused by the code you've posted. What line of code inside the constructor is executing when the exception is raised? What you've described does not happen in general, so the specific value of SomeClass is important. Please include it. – Joggle

I don't have that code either. – Tippet

@mnuzzo, try passing a Form or a DataModule as owner. Just put a button on a form and do SomeClass.Create(Self) from there, see if it works, and we'll take it from there. If you don't have the source for SomeClass and it doesn't accept either TForm or TDataModule as owner, you can't use it. ie: need to ask the developer what it wants as Owner. – Wainscot

This might be an answer for you #5420760. It's my cross reference :) The object you'll pass as the Owner will destroy your object. If you give there nil, you need to Free it by yourself. – Birddog

@Tippet Do you have documentation about SomeClass, what's required to be passed as the parameter to the constructor will be stated there!! – Sumner

Nope. No documentation, no source code, first one to go where I'm going at this company. – Tippet

@Rob, upvote because it's an interesting puzzle: Why passing nil to a constructor causes a EIntfCasterror exception. Why the downvote? – Wainscot

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;

Huey

nitpicking: I think EIntfCastError can also happen when casting an instance to an interface. – Amarillis

@TOndrej, technically, you're always casting an interface to an interface; When you call TInterfacedObject as ISomething you're actually invoking the IUnknown.QueryInterface method. – Wainscot

@Cosmin, you beat me to it. :) – Huey

Agreed but the cast to IInterface (or IUnknown) is not obvious, happening "behind the scenes" so I thought it's worth mentioning. – Amarillis

+1. A well behaved TComponent descendant should work fine with Owner=nil, but I don't think that's a requirement. For what it's worth, do we know for sure SomeClass is actually a TComponent? – Wainscot