typecast with class function - delphi

Question

i want typecast using with class functions.
i have base (TBase),derived (TDer) and typecasting (TMyType) class.
Ver : Delphi 7
TBase = class;
TDer = class;
TMyType = class;
TBase = class
function Say : String;
class function MYType:TMyType;
end;
TDer = class(TBase)
a: string;
b: string;
function Say2 : String;
end;
TMyType=class(TBase)
class function AsDer:TDer;
end;
{ TBase }
class function TBase.MYType: TMyType;
begin
Result:=TMyType(Self);
end;
function TBase.Say: String;
begin
Result:='TBase';
end;
{ TDer }
function TDer.Say2: String;
begin
Result:='TDer';
end;
{ TMyType }
class function TMyType.AsDer: TDer;
begin
Assert(Assigned(Self));
Result := TDer(Self) ;
end;
Sample usage is below, it's calls method but when set/get field's raise error.
procedure TForm1.Button1Click(Sender: TObject);
var
b,c:TBase;
begin
b:=TDer.Create;
c:=b.MYType.AsDer;
ShowMessage(b.MYType.AsDer.Say2); // OK. Running
if (#b<>#c) then ShowMessage('Not Equal'); // Shows message, Why ?
b.MYType.AsDer.a:='hey'; // Error
FreeAndNil(b);
end;
Do you have any idea?

Answer 1

The fundamental problem is here:
class function TBase.MYType: TMyType;
begin
Result:=TMyType(Self);
end;
This is a class method and so Self refers to a class and not an instance. Casting it to be an instance does not make it so. Exactly the same error is made in your AsDer class function.
Looking into the specifics, the call to
b.MYType.AsDer.Say2
is benign and appears to work fine because it does not refer to Self. You could equally write TDer(nil).Say2 and that code would also work without problem. Now, if the function Say2 referred to Self, that is referred to an instance, then there would be a runtime error.
#b<>#c
always evaluates to true because you are comparing the locations of two distinct local variables.
b.MYType.AsDer.a
is a runtime error because AsDer does not return an instance of TDer. So when you attempt to write to a you have a runtime error. This is because you are referring to Self and that's why this code fails, but the earlier call to Say2 does not.
I'm not really sure what you are trying to do here, but it looks all wrong. Even if you were working with instance methods rather than class methods, it would simply be wrong to case a base class instance to a derived class instance. If something is the wrong type, no amount of casting will turn it into the right type.
Furthermore, you should never write code that has a method of TBase assuming it is of type TDerived. The base class should know absolutely nothing of its derived classes. That is one of the very basic tenets of OOP design.

Answer 2

Here is the edited the new version :
TBase = class;
TDer = class;
TMyType = class;
TBase = class
MYType:TMyType;
constructor Create;
destructor Destroy;
function Say : String;
end;
TDer = class(TBase)
a: string;
b: string;
function Say2 : String;
end;
TMyType=class
public
T: TObject;
function AsDer:TDer;
end;
{ TBase }
constructor TBase.Create;
begin
MYType:=TMYType.Create;
MYType.T:=TObject(Self);
end;
destructor TBase.Destroy;
begin
MYType.Free;
end;
function TBase.Say: String;
begin
Result:='TBase';
end;
{ TDer }
function TDer.Say2: String;
begin
Result:='TDer';
end;
{ TMyType }
function TMyType.AsDer: TDer;
begin
Result := TDer(T) ;
end;
procedure TForm1.Button1Click(Sender: TObject);
var
b:TBase;
c:TDer;
begin
b:=TDer.Create;
TDer(b).a:='a';
c:=b.MYType.AsDer;
ShowMessage('b.MYType.AsDer='+b.MYType.AsDer.a+', c.a ='+ c.a); // OK. Running
FreeAndNil(b);
end;