public interface IFoo
    {

    }

    public interface IFooCollection
    {
        IEnumerable<IFoo> Items { get; }
    }

    public class FooCollection<T> : IFooCollection where T : IFoo
    {
        public IEnumerable<T> Items { get; set; }

        IEnumerable<IFoo> IFooCollection.Items
        {
            get { return Items; }
        }
    }