C# – Covariance and IList


I would like a Covariant collection whose items can be retrieved by index. IEnumerable is the only .net collection that I'm aware of that is Covariant, but it does not have this index support.

Specifically, I'd like to do this:

List<Dog> dogs = new List<Dog>();

IEnumerable<Animal> animals = dogs;
IList<Animal> animalList = dogs; // This line does not compile

Now, I'm aware of why this is a problem. List implements ICollection that has an Add method. By up casting to IList of Animals, it would allow subsequent code to add any type of animal which is not allowed in the "real" List<Dog> collection.

So is anyone aware of a collection that supports index lookups that is also covariant? I would like to not create my own.

Best Solution

Update: from .NET 4.5 onwards there is IReadOnlyList<out T> and IReadOnlyCollection<out T> which are both covariant; The latter is basically IEnumerable<out T> plus Count; the former adds T this[int index] {get;}. It should also be noted that IEnumerable<out T> is covariant from .NET 4.0 onwards.

Both List<T> and ReadOnlyCollection<T> (via List<T>.AsReadOnly()) implement both of these.

It can only be covariant if it only has a get indexer, i.e.

public T this[int index] { get; }

But all main collections have {get;set;}, which makes that awkward. I'm not aware of any that would suffice there, but you could wrap it, i.e. write an extension method:

var covariant = list.AsCovariant();

which is a wrapper around an IList<T> that only exposes the IEnumerable<T> and the get indexer...? should be only a few minutes work...

public static class Covariance
    public static IIndexedEnumerable<T> AsCovariant<T>(this IList<T> tail)
        return new CovariantList<T>(tail);
    private class CovariantList<T> : IIndexedEnumerable<T>
        private readonly IList<T> tail;
        public CovariantList(IList<T> tail)
            this.tail = tail;
        public T this[int index] { get { return tail[index]; } }
        public IEnumerator<T> GetEnumerator() { return tail.GetEnumerator();}
        IEnumerator IEnumerable.GetEnumerator() { return tail.GetEnumerator(); }
        public int Count { get { return tail.Count; } }
public interface IIndexedEnumerable<out T> : IEnumerable<T>
    T this[int index] { get; }
    int Count { get; }