How to do generic typecasting in delphi?

Question

I have a base class test define below

type
Tinfo = procedure of object;

Test = class(TObject)
public
  procedure Add    ( const a : Tinfo ); reintroduce ;
end;


procedure Test.Add(const a: Tinfo);
begin
  Writeln('base class add function');
  // dosomething more  
end;

and I have a derived generic class from this base class

TTesting<T> = class(Test)
   public
     procedure Add    ( const a : T  ); reintroduce ;
   end;

and I am typecasting T to Tinfo but it gives me the error

procedure TTesting<T>.Add(const a : T );
begin
  inherited Add(Tinfo(a) );  // gives me error here 
end;

is there any way I can implement this?

Answer 1

First your cast is wrong, you obviously want to cast a and not T.

However if you want to type cast on a procedure of object which is a type that cannot be polymorphic in any way it makes no sense to put that into a generic type at all.

What should T be? It only can be a TInfo in your code.

If you however want T to be any event/method type you should store a TMethod in your base class and then work with that in your generic class. But remember that you cannot have a constraint that limits T to be an event type. So you might check that in your constructor.

type
  PMethod = ^TMethod;

  Test = class(TObject)
  public
    procedure Add(const a: TMethod ); reintroduce ;
  end;

procedure Test.Add(const a: TMethod);
begin
  Writeln('base class add function');
  // dosomething more
end;

type
  TTesting<T> = class(Test)
  public
    constructor Create;
    procedure Add(const a: T); reintroduce ;
  end;

constructor TTesting<T>.Create;
begin
  Assert(PTypeInfo(TypeInfo(T)).Kind = tkMethod);
  inherited Create;
end;

procedure TTesting<T>.Add(const a: T);
begin
  inherited Add(PMethod(@a)^);
end;