I didn't attend PDC 2008, but I heard some news that C# 4.0 is announced to support Generic covariance and contra-variance. That is, List<string> can be assigned to List<object>. How could that be?
In Jon Skeet's book C# in Depth, it is explained why C# generics doesn't support covariance and contra-variance. It is mainly for writing secure code. Now, C# 4.0 changed to support them. Would it bring chaos?
Anybody know the details about C# 4.0 can give some explanation?
Here is a good article that covers the upcoming covariance and contra-variance implementations on delegates and interfaces in C# 4.0:
Variance will only be supported in a safe way - in fact, using the abilities that the CLR already has. So the examples I give in the book of trying to use a List<Banana> as a List<Fruit> (or whatever it was) still won't work - but a few other scenarios will.
Firstly, it will only be supported for interfaces and delegates.
Secondly, it requires the author of the interface/delegate to decorate the type parameters as in (for contravariance) or out (for covariance). The most obvious example is IEnumerable<T> which only ever lets you take values "out" of it - it doesn't let you add new ones. That will become IEnumerable<out T>. That doesn't hurt type safety at all, but lets you return an IEnumerable<string> from a method declared to return IEnumerable<object> for instance.
Contravariance is harder to give concrete examples for using interfaces, but it's easy with a delegate. Consider Action<T> - that just represents a method which takes a T parameter. It would be nice to be able to convert seamlessly use an Action<object> as an Action<string> - any method which takes an object parameter is going to be fine when it's presented with a string instead. Of course, C# 2 already has covariance and contravariance of delegates to some extent, but via an actual conversion from one delegate type to another (creating a new instance) - see P141-144 for examples. C# 4 will make this more generic, and (I believe) will avoid creating a new instance for the conversion. (It'll be a reference conversion instead.)
Hope this clears it up a bit - please let me know if it doesn't make sense!
Anders Noråse explains in C# 4.0 - Covariance and contra-variance the concept and shows, that it's already supported today in IL since .NET 2.0.
Not that Jon hasn't already covered it, but here are some links to blogs and videos from Eric Lippert. He does a nice job of explaining it with examples.
http://blogs.msdn.com/ericlippert/archive/tags/Covariance+and+Contravariance/default.aspx
The videos:
http://blogs.msdn.com/ericlippert/archive/2009/10/26/some-new-videos.aspx