Question

I have three classes with common parent. Let’s say parent is Animal and children are Dog,Cat and Parrot.

And I have one observable collection which contains collection of animals user is working with. Collection contains all animals of same type – user is either working only with all dogs or all cats or all parrots.

So I declared ObservableCollection<Animal> animals and depending on user choices I want to change contents of property animals to ObservableCollection<Dog> or ObservableCollection<Cat> or ObservableCollection<PArrot>. So it doesn’t matter if user currently works with dogs or cats but he can choose all actions animals have in common.

But it doesn’t work. It seems that I can’t assign ObservableCollection<Cat> to Property of type ObservableCollection<animal>. I would think it should work, because animal is supertype of cat, so I can assign cat to animal variable as usual.

Why can’t I do this and how can I solve this problem?

Answer 1

One reason it doesn’t work: if it did work, you could have the nonsense scenario:

ObservableCollection<Animal> animals = new ObservableCollection<Dog>();
animals.Add(new Cat()); // cat is an animal, after all

Another reason is that it is simply not the case that ObservableCollection<Dog> inherits ObservableCollection<Animal> – they are just different (parallel) closed generic types of ObservableCollection<>.

What is permitted is that some interfaces with “out only” APIs (such as IEnumerable<T>) can be covariant, so if all you need is to iterate them, you can have:

IEnumerable<Animal> animals = new ObservableCollection<Dog>();

(presumably adding the dogs somewhere else)

Another approach would be to use a non-generic API:

IList animals = new ObservableCollection<Dog>();