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The as operator

__alex
__alex asked
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Last Modified: 2010-04-05
I frequently use the is operator but never the as operator and wonder what it is good for. Everything can be done with a simple cast...

procedure Foo(o: TObject);
begin
  if o is TMyObject then
    TMyObject(o).DoStuff;
  ...
end;

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Top Expert 2004
Commented:
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Top Expert 2004

Commented:
from delphi help:

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. If the declared type of object is unrelated to class
--that is, if the types are distinct and one is not an ancestor of the other--a compilation error results.

meikl ;-)
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Author

Commented:
@kretzschmar
Why check things twice:
if o is TMyObject then             // First check
  (o as TMyObject).DoStuff;     // Second check ???

@mokule
Good point but...
if you know b is a memo, why do you want to set its caption?
If you don't know what b is it's better to check it or an exception might be raised:
  if b is TLabel then TLabel(b).Caption := foo;

Still can't think of any (real) use of the as

Author

Commented:
@geobul -> @kretzschmar
Top Expert 2004

Commented:
:-)) see my first comment,

this is the difference,
the compiler developers cannot know,
if you check it before with the is-operater,
thats why the as-operator is introduced

Author

Commented:
@kretzschmar
Ok, now I get your point, thanks!
Top Expert 2004

Commented:
maybe this illustrates the difference,
watch about the different exceptions u get

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TForm1 = class(TForm)
    Button1: TButton;
    Button2: TButton;
    ComboBox1: TComboBox;
    ComboBox2: TComboBox;
    procedure AsTypeCastClick(Sender: TObject);
    procedure HardTypeCastClick(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure ShowItems(AComboBox : TComboBox);
begin
  if assigned(AComboBox) then
    ShowMessage(AComboBox.Items.Text);
end;

//is assigned to Button1 and ComboBox1 onClick-event
procedure TForm1.AsTypeCastClick(Sender: TObject);
begin
  ShowItems(sender as TComboBox);
end;

//is assigned to Button2 and ComboBox2 onClick-event
procedure TForm1.HardTypeCastClick(Sender: TObject);
begin
  ShowItems(TComboBox(sender));
end;

end.

meikl ;-)

Commented:
>(o as TMyObject).DoStuff;     // Second check ???

It's not a check but typecast
Software Engineer, Advisory
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Author

Commented:
Very nice! It makes the code look pretty...
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