Quick sanity check: covariance, contravariance, lack of variance inC#

[raylopez99]

What are these terms in my header anyway?

But more importantly for me, look at this code:

Lack of variance:
why a List<Banana> isn’t a List<Fruit>

Many people initially expect the following code to compile:

List<Banana> bananas = new List<Banana>();
List<Fruit> fruit = bananas;

//however, it fails to compile (from Jon Skeet's cheat sheet)

However, this should work, right??:

List <Fruit> mybaseFruit = new List <Fruit>();

class Banana: Fruit {}

Banana myBanana = new Banana(); //note, this is NOT a list, but an
instantiation of a derived class Banana having base class Fruit

mybaseFruit.Add(myBanana); //should work, right? Otherwise what's the
point of inheritance?

RL

 

[Marc Gravell]

However, this should work, right??:

And it does. Was there a question?

Marc

 

[Stefan Hoffmann]

hi Ray,

Many people initially expect the following code to compile:

List<Banana> bananas = new List<Banana>();
List<Fruit> fruit = bananas;

This should work:

List<Fruit> fruit = (Fruit)bananas;


mfG
--> stefan <--

 

[Marc Gravell]

This should work:

You cannot cast between the two lists (assuming you meant (List<Fruit>),
not (Fruit)).

The closest you can get is a generic method:

static void AddFruit<T>(IList<T> list, T item) where T : Fruit
{
list.Add(item);
}

(Add isn't a good choice because the caller could do that moer
conveniently directly, but searching etc would work in the above, using
properties from the base Fruit class)

Marc

 

[Stefan Hoffmann]

hi Marc,

You cannot cast between the two lists (assuming you meant (List<Fruit>),
not (Fruit)).

Indeed I meant (Fruit), I have not read carefully enough.

The closest you can get is a generic method:

static void AddFruit<T>(IList<T> list, T item) where T : Fruit
{
list.Add(item);
}

Maybe an Adapter is a solution, like ArrayList.Adapter(). Which is in
fact a kind of your supposed generic method.


mfG
--> stefan <--