covariance in c#

Question

Is it possible to cast a List<Subclass> to List<Superclass> in C# 4.0?

Something along these lines:

class joe : human {}

List<joe> joes = GetJoes();

List<human> humanJoes = joes;

Isn't this what covariance is for?

if you can do:

human h = joe1 as human;

why shouldn't you be able to do

List<human> humans = joes as List<human>; 

than it wouldn't be legal to do (joe)humans[0] because that item has been down casted.. and everyone would be happy. Now the only alternative is to create a new List

Answer 1

No. The co/contravariance features of C# 4.0 only support interfaces and delegates. The do not support concrete types like List<T>.

Answer 2

Instantiate a new human-list that takes the joes as input:

List<human> humanJoes = new List<human>(joes);

Answer 3

You can't do this, because it wouldn't be safe. Consider:

List<Joe> joes = GetJoes();    
List<Human> humanJoes = joes;
humanJoes.Clear();
humanJoes.Add(new Fred());
Joe joe = joes[0];

Clearly the last line (if not an earlier one) has to fail - as a Fred isn't a Joe. The invariance of List<T> prevents this mistake at compile time instead of execution time.

Answer 4

No. As Jared said, the co/contravariance features of C# 4.0 only support interfaces and delegates. However it doesn't work with IList<T> either, and the reason is that IList<T> contains methods to add and change items in the list -- as Jon Skeet's new answer says.

The only way to be able to cast a list of "joe" to "human" is if the interface is purely read-only by design, something like this:

public interface IListReader<out T> : IEnumerable<T>
{
    T this[int index] { get; }
    int Count { get; }
}

Even a Contains(T item) method would not be allowed, because when you cast IListReader<joe> to IListReader<human>, there is no Contains(human item) method in IListReader<joe>.

You could "force" a cast from IList<joe> to IListReader<joe>, IListReader<human> or even IList<human> using a GoInterface. But if the list is small enough to copy, a simpler solution is to just copy it into a new List<human>, as Paw pointed out.

Answer 5

If I allow your List<Joe> joes to be generalized as ...

  List<Human> humans = joes;

... the two references humans and joes are, now onward, pointing to the exact same list. The code following the above assignment has no way of preventing an insertion/addition of an instance of another type of human , say a Plumber, into the list. Given that class Plumber: Human {}

humans.Add(new Plumber()); // Add() now accepts any Human not just a Joe 

the list that humans refers to now contains both joes and plumbers. Note that the same list object is still referred to by the reference joes. Now if I use the reference joes to read from the list object I might pop out a plumber instead of a joe. Plumber and Joe are not known to be implicitly interconvertable... so my getting of a plumber instead of a joe from the list breaks down type safety. A plumber is certainly not welcome through a reference to a list of joes.

However in the recent versions of C# , its kind of possible to work around this limitation for a generic class/collection by implementing a generic interface whose type parameter has an out modifier on it. Say we now have ABag<T> : ICovariable<out T>. The out modifier restricts the T to ouput positions only (e.g. method return types). You cannot enter any T into the bag. You can only read them out of it. This allows us to generalize joes to an ICovariable<Human> without worrying about inserting a Plumber into it as the interface doesnt allow that. We can now write ...

ICovariable<Human> humans = joes ; // now its good !
humans.Add(new Plumber()); // error