IDictionary, injection and covariance (followup)

Jon Skeet suggests an alternative which I like, as it avoids polluting the generics with information about the underlying/internal data structure: inject a delegate to handle list instantiation when it’s needed:

public class Lookup<A, B>
{
    public Lookup(
            IDictionary<A, IList<B>> underlyingDict,
            Func<IList<B>> listCreator)
    {
        // client passes in a delegate that Lookup can use
        // to instantiate the desired list type ...
    }

    // ... remaining code
}