Generically, a covariant type parameter is one which is allowed to vary down as the class is subtyped (alternatively, vary with subtyping, hence the “co-” prefix). More concretely: trait List[+A] List[Int] is a subtype of List[AnyVal] because Int is a subtype of AnyVal. This means that you may provide an instance of List[Int] when a … Read more
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 … Read more
The question is “what is the difference between covariance and contravariance?” Covariance and contravariance are properties of a mapping function that associates one member of a set with another. More specifically, a mapping can be covariant or contravariant with respect to a relation on that set. Consider the following two subsets of the set of … Read more
Basically, variance applies when the CLR can ensure that it doesn’t need to make any representational change to the values. References all look the same – so you can use an IEnumerable<string> as an IEnumerable<object> without any change in representation; the native code itself doesn’t need to know what you’re doing with the values at … Read more