Belov_LW_CS4

using System.Collections.Generic;

namespace Classwork
{
    class BookList
    {
        HashSet<string> books = new HashSet<string>();
        public static BookList operator +(BookList bookList, string name)
        {
            BookList result = (BookList)bookList.MemberwiseClone();
            result.books.Add(name);
            return result;
        }
        public static BookList operator -(BookList bookList, string name)
        {
            BookList result = (BookList)bookList.MemberwiseClone();
            result.books.Remove(name);
            return result;
        }
        public static bool operator ==(BookList bookList, string name)
        {
            return bookList.books.Contains(name);
        }
        public static bool operator !=(BookList bookList, string name)
        {
            return !bookList.books.Contains(name);
        }
        public string this[int index]
        {
            get
            {
                var enumerator = books.GetEnumerator();
                bool isValue = false;
                for (int i = 0; i <= index; i++)
                    isValue = enumerator.MoveNext();
                if (isValue)
                    return enumerator.Current;
                else
                    return null;
            }
        }
    }
}