Mega Code Archive
Hacking readonly properties
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Title: Hacking readonly properties.
Question: Delphi compiler is smart to know if in your code you wrote an assignment to a read only property
and does not compile your code.
We know that read only properties can be modified directly if the class that is modifying it
is declared in the same unit that the class that owns the property.
But if it is not the case?
Answer:
Suppose you have a TMyClass declared in unit2 and use it in Unit1:
* Note that this works only to properties that are NOT read/written by a method (Get/Set methods).
==============================================
unit Unit2;
interface
type
TMyClass = class
private
FMyReadOnlyProperty: string;
public
constructor Create;
property MyReadOnlyProperty: string read FMyReadOnlyProperty;
end;
implementation
constructor TMyClass.Create;
begin
FMyReadOnlyProperty:= 'Default Value';
end;
end.
==============================================
unit Unit1;
interface
uses
Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
StdCtrls, unit2;
type
TForm1 = class(TForm)
Button1: TButton;
procedure Button1Click(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;
var
Form1: TForm1;
implementation
{$R *.DFM}
procedure TForm1.Button1Click(Sender: TObject);
var
MyClass: TMyClass;
PString: ^string;
begin
MyClass:= TMyClass.Create;
{This shows the default value.}
ShowMessage(MyClass.MyReadOnlyProperty);
{The compiler does not allow this to be done.}
//MyClass.MyReadOnlyProperty:= 'A Value';
{Get the address of MyClass.MyReadOnlyProperty}
PString:= Addr(MyClass.MyReadOnlyProperty);
{But in this case the compiler doesn't know what is being done with the
property, because this is a pointer operation}
PString^:= 'Wrote to a read only property !!!';
{And this shows the new value of the "Read Only" property.}
ShowMessage(MyClass.MyReadOnlyProperty);
MyClass.Free;
end;
end.
{This is NOT a bug. Do you think this is? Please, send me your suggestions. rribas@facilities.com.br}
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