6

public class Box : IEnumerable
    {
        private List<PrintEdition> books = new List<PrintEdition>();

        public Box()
        {
            books.Add(new Book("Биба", 15, 2, 3, true));
            books.Add(new Book("Боба", 40, 22, 5, true));
            books.Add(new Journal("Вообще не книга", 3, 20, 4, 5));
            books.Add(new CopyBook("Какой-то мусор", 60, 66, 9, 6));
        }
        #region IEnumenator Members
        public IEnumerator GetEnumerator()
        {
            return new BoxEnumerator(books);
        }
        #endregion
        public void DeleteAt(int position)
        {
            books.Remove(books.ElementAt(position));
        }
        public void Add(PrintEdition item)
        {
            books.Add(item);
        }
    }
    public class BoxEnumerator : IEnumerator
    {
        private List<PrintEdition> books;
        public BoxEnumerator(List<PrintEdition> books)
        {
            this.books = books;
        }
        int position = -1;
        public object Current
        {
            get
            {
                if (position == -1 || position > books.Count)
                {
                    throw new InvalidOperationException();
                }
                return books.ElementAt(position);
            }
        }

        public bool MoveNext()
        {
            if (position < books.Count - 1)
            {
                position++;
                return true;
            }
            return false;
        }

        public void Reset()
        {
            position = -1;
        }
    }
    class NameComparer : IComparer<PrintEdition>
    {
        public int Compare(PrintEdition x, PrintEdition y)
        {
            PrintEdition t1 = (PrintEdition)x;
            PrintEdition t2 = (PrintEdition)y;
            return String.Compare(t1.name, t2.name);
        }
    }