How Can I Achieve Covariant Collections with Index Lookup in C#?

Covariance and IList: Extending Collections for Index Lookups

In C#, covariant collections allow retrieving items from a parent collection using a child collection. However, the standard IEnumerable interface, which supports covariance, lacks direct index lookup capabilities.

Since List implements ICollection with an Add method, directly casting it to IList would allow unrestricted addition of any animal type, violating the original List's constraints.

To address this issue, .NET 4.5 and later introduce IReadOnlyList and IReadOnlyCollection, which are both covariant. IReadOnlyList extends IReadOnlyCollection with a get indexer, allowing indexed retrieval while maintaining covariance.

Both List and ReadOnlyCollection (obtained via List.AsReadOnly()) implement these interfaces, providing a covariant collection with index support.

However, if you need a covariant collection with both get and set indexers, options are limited. A potential solution is to wrap the existing IList into a CovariantList, which only exposes the get indexer while providing the other necessary properties:

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 T this[int index] => tail[index];
        public IEnumerator<T> GetEnumerator() => tail.GetEnumerator();
        public int Count => tail.Count;
    }
}
public interface IIndexedEnumerable<out T> : IEnumerable<T>
{
    T this[int index] { get; }
    int Count { get; }
}

This approach provides a covariant collection that supports both IEnumerable and indexed retrieval, while preventing unintended modifications to the original collection.

The above is the detailed content of How Can I Achieve Covariant Collections with Index Lookup in C#?. For more information, please follow other related articles on the PHP Chinese website!