Delphi generics invalid cast

Question

I'm using Delphi XE3 and I don't understand why the first cast below does not compile :(

type
  TBase = class
  end;

  TDerived = class(TBase)
  end;

 procedure cast();
 var
   bases : TObjectList<TBase>;
   deriveds : TObjectList<TDerived>;

 begin
   bases := deriveds; // specific to generid, Here !
   deriveds := bases; // generic to specific, ok should not compile as is !
 end;

is `TDerived` class supposed to extend `TBase` like `TDerived = class(TBase)` ? — fantaghirocco, Sep 17 '15 at 12:07
@fantaghirocco The same would happen if that were the case too. — Jerry Dodge, Sep 17 '15 at 12:11
FWIW read my article on the subject: http://delphisorcery.blogspot.de/2014/10/generics-and-variance.html — Stefan Glienke, Sep 17 '15 at 12:24
I fixed your code to derive `TDerived` from `TBase`. It's clear from the naming that you intended to do so. The reason your code does not compile is that Delphi does not support covariance of generics. As explained in the duplicate. — David Heffernan, Sep 17 '15 at 12:24

score 0 · Answer 1 · answered Sep 17 '15 at 12:06

0

It's simple: These are two different types, and are incompatible. The whole idea of Generics is to make typecasting strict, and the compiler is doing exactly what it's designed to do - prevent you from mixing types up.

answered Sep 17 '15 at 12:06

Jerry Dodge

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Delphi generics invalid cast

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