问题
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?
回答1:
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!
回答2:
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.
https://blogs.msdn.microsoft.com/ericlippert/2007/10/16/covariance-and-contravariance-in-c-part-one/
The videos:
https://www.youtube.com/watch?v=3MQDrKbzvqU
https://www.youtube.com/watch?v=XRIadQaBYlI
https://www.youtube.com/watch?v=St9d2EDZfrg
来源：https://stackoverflow.com/questions/245607/how-is-generic-covariance-contra-variance-implemented-in-c-sharp-4-0