DELPHI: Generics and polymorphism

Question

This has been asked several different ways already - but I haven\'t found my answer yet.

Can someone clarify a few things for me please. Using : Delphi XE2

I

Answer 1

Delphi generics do not support covariance and contravariance so what you are attempting to do is not possible with the language's current syntax. I suggest you have a read of the following blog articles that cover the matter in more detail.

• Craig Stuntz: Comparing C#, C++, and Delphi (Win32) Generics
• Mason Wheeler: Generics and the Covariance Problem

Fundamentally what you are attempting to do is this:

type
  TBase = class;
  TDerived = class(TBase);
  TBaseList = TList<TBase>;
  TDerivedList = TList<TDerived>;
var
  BaseList: TBaseList;
  DerivedList: TDerivedList;
...
BaseList := TDerivedList;//does not compile

The designers have not stopped you doing this out of spite. There is a good reason. Consider the following standard example:

type
  TAnimal = class;
  TCat = class(TAnimal);
  TPenguin = class(TAnimal);

var
  AnimalList: TList<TAnimal>;
  CatList: TList<TCat>;
  Penguin: TPenguin;
...
AnimalList := CatList;//does not compile because...
AnimalList.Add(Penguin);//...of the danger of this

Whilst it is reasonable to add a TPenguin to a TList<TAnimal>, the actual list that AnimalList refers to is a TList<TCat> and a penguin is not a cat.

And, if you want to think of it in the context of your example hierarchy, here's an illustration of code that justifies the language design.

aList := aCustomerList;//does not compile
aList.Add(aCustomPerson);
//this would add a TCustomPerson instance to a list containing 
//TCustomer instances, but a TCustomPerson is not a TCustomer