contravariance

I am trying to figure out the exact meaning of the words Covariance and Contravariance from several articles online and questions on StackOverflow, and from what I can understand, it's only another word for polymorphism . Am I correct with the above statement? Or have I got it wrong ? Jon Skeet It's certainly related to polymorphism. I wouldn't say they're just "another word" for polymorphism though - they're about very specific situations, where you can treat one type as if it were another type in a certain context . For instance, with normal polymorphism you can treat any reference to a

Given the following types: public interface IMyClass { } public class MyClass : IMyClass { } I wonder how can I convert a List<MyClass> to a List<IMyClass> ? I am not completely clear on the covariance/contravariance topics, but I understand that I cannot just plainly cast the List because of that. I could come up with this trivial solution only; lacking any elegance, wasting resources: ... public List<IMyClass> ConvertItems(List<MyClass> input) { var result = new List<IMyClass>(input.Count); foreach (var item in input) { result.Add(item); } return result; } .... How can you solve it in a more

I am just trying to understand the extends keyword in Java Generics. List<? extends Animal> means we can stuff any object in the List which IS A Animal then won't the following also mean the same thing: List<Animal> Can someone help me know the difference between the above two? To me extends just sound redundant here. Thanks! List<Dog> is a subtype of List<? extends Animal> , but not a subtype of List<Animal> . Why is List<Dog> not a subtype of List<Animal> ? Consider the following example: void mySub(List<Animal> myList) { myList.add(new Cat()); } If you were allowed to pass a List<Dog> to

I have a struct that has inner mutability. use std::cell::RefCell; struct MutableInterior { hide_me: i32, vec: Vec<i32>, } struct Foo { //although not used in this particular snippet, //the motivating problem uses interior mutability //via RefCell. interior: RefCell<MutableInterior>, } impl Foo { pub fn get_items(&self) -> &Vec<i32> { &self.interior.borrow().vec } } fn main() { let f = Foo { interior: RefCell::new(MutableInterior { vec: Vec::new(), hide_me: 2, }), }; let borrowed_f = &f; let items = borrowed_f.get_items(); } Produces the error: error[E0597]: borrowed value does not live long