索引器

    索引器允许类或结构的实例按照与数组相同的方式进行索引。索引器类似于属性，不同之处在于它们的访问器采用参数。它可以使得像数组那样对对象使用下标。它提供了通过索引方式方便地访问类的数据信息的方法。

    个人理解：索引器是属性的增强。他提供一种索引方式，像下标或是字符串，使我们实现对数据的快速查找。

     实例：

class Program
    {
        static void Main(string[] args)
        {
            Album friends = new Album();
            Photo first = new Photo("Jean");
            Photo second = new Photo("Smith");
            friends[0] = first;
            friends[1] = second;
            //用下标索引
            Photo obj1Photo = friends[1];
            Console.WriteLine(obj1Photo.Title);
            //用字符串索引
            Photo obj2Photo = friends["Smith"];
            Console.WriteLine(obj2Photo.Title);

        }
    }
    public class Photo
    {
        private String _title;
        public string Title
        {
            get { return _title; }
        }
        public Photo()
        {
        }
        public Photo(string title)
        {
            _title = title;
        }
    }
    public class Album : Photo
    {
        Photo[] photos;
        public Album()
        {
            photos = new Photo[2];
        }
        public Album(int capacity)
        {
            photos = new Photo[capacity];
        }
        public Photo this[int index]
        {
            get
            {
                if (index < 0 || index > photos.Length)
                {
                    Console.WriteLine("索引无效");
                    return null;

                }
                return photos[index];
            }
            set
            {
                if (index < 0 || index >= photos.Length)
                {
                    Console.WriteLine("索引无效");
                    return;
                }
                photos[index] = value;

            }
        }
        public Photo this[string title]
        {
            get
            {
                foreach (Photo p in photos)
                {
                    if (p.Title == title)
                        return p;
                }
                Console.WriteLine("未找到");
                return null;

             }
         }
    }