Type casts

[donvittorio]

Hello all,

Can anybody explain to me the difference between the two methods of type casting? For example:

(Sender as TButton).Caption := 'Button1'
or
TButton(Sender).Caption := 'Button1'

I know that there must be a difference because sometimes Delphi does not like the second way of type casting, but I've never figured out why.

Steve

 

[Nordlund]

Hi.
This is from the Delphi help file:

It is sometimes useful to treat an expression as if it belonged to different type. A typecast allows you to do this by, in effect, temporarily changing an expression's type. For example, Integer('A') casts the character A as an integer.

The syntax for a typecast is

typeIdentifier(expression)

-----------

The as operator performs checked typecasts. The expression

object as class

returns a reference to the same object as object, but with the type given by class. At runtime, object must be an instance of the class denoted by class or one of its descendants, or be nil; otherwise an exception is raised.

----------- Explanation.

When (Sender AS TButton), the Sender has to be a TButton,
and are therefore no typecast.

The real typecase is TButton(Sender) and in this case Sender can be of another type than TButton...

...Anm I right, comments please...


//Nordlund

 

[Nordlund]

Oopps.. Some vital spelling mistakes, but you may figure it out anyway...

//Nordlund

 

[sggaunt]

Norlund
Of course the typecast will only work if the sender object has the required properies (i.e it is a parent object of the TButton(with a Caption property) or a Tbutton or a decendent of Tbutton.
The difference is that you will only find this out at Runtime when an exception may occur, using the AS construct is safer. But even this should be tested with an
if (sender is TButton) then etc.



Steve
Two things are infinite
The Universe and Human stupidity: Einstein.
(He wasn't too sure about the Universe)