C#索引器

C#索引器语法：

[访问修饰符] 数据类型 this [数据类型标识符]。

示例1：

using System;
using System.Collections.Generic;
using System.Text;

namespace PH6_2
{
    class Photo
    {
        string _title;
        public Photo(string title)
        {
            _title = title;
        }
        public string Title
        {
            get
            {
                return _title;
            }
            set
            {
                _title = value;
            }
        }
    }
    class Album
    {
        Photo[] photos;
        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 photo in photos)
                {
                    if (photo.Title==title)
                    {                       
                        return photo;
                    }
                }
                return null;
            }

        }
    }
    class TestIndex
    {
        static void Main(string[] args)
        {
            Photo first = new Photo("first");
            Photo second = new Photo("second");
            Photo third = new Photo("third");
            Album album = new Album(3);

            album[0] = first;
            album[1] = second;
            album[2] = third;

            Console.WriteLine("第2张照片:" + album[1].Title);
            Console.WriteLine(album["third"].Title);
        }
    } 
}

运行效果如下：

 

注意，不能定义两个相同数据类型的索引器，但是可以定义不同数据类型的索引器，其返回值可以相同。

例如下面的示例是不正确的：两个索引器，索引都是int，编译报错： 类型“PH6_2.TestIndex”已定义了一个名为“this”的具有相同参数类型的成员 。

    class TestIndex
    {
        int[] num1s ={ 1, 2, 3, 4, 5 };
        string[] s ={ "a", "b", "c" };

        public int this[int index]
        {
            get
            {
                return num1s[index];
            }
        }
        public string this[int index]
        {
            get
            {
                return s[index];
            }
        }

        static void Main(string[] args)
        {
            TestIndex testIndex = new TestIndex();
            uint i = 2;
            Console.WriteLine(testIndex[i]);
        }
    } 

可以将第2个索引器的索引改为uint，则编译通过。如下：

    class TestIndex
    {
        int[] num1s ={ 1, 2, 3, 4, 5 };
        string[] s ={ "a", "b", "c" };

        public int this[int index]
        {
            get
            {
                return num1s[index];
            }
        }
        public string this[uint index]
        {
            get
            {
                return s[index];
            }
        }

        static void Main(string[] args)
        {
            TestIndex testIndex = new TestIndex();
            uint i = 2;
            Console.WriteLine(testIndex[i]);
        }
    } 

  还需要注意：索引的索引类型不同，而返回值相同，也是正确的。如下示例是正确的：

    class TestIndex
    {
        int[] num1s ={ 1, 2, 3, 4, 5 };
        int[] num2s ={ 10, 11, 12, 13, 14, 15 };

        public int this[int index]
        {
            get
            {
                return num1s[index];
            }
        }
        public int this[uint index]
        {
            get
            {
                return num2s[index];
            }
        }

        static void Main(string[] args)
        {
            TestIndex testIndex = new TestIndex();
            uint i = 2;
            Console.WriteLine(testIndex[i]);
            Console.WriteLine(testIndex[2]);
        }
    }