索引器是C#是一个比较特殊的功能,
它一般用来提供数组/集合被外部访问的接口(其实就是对集合或数组成员的方便的操作 eg:获取OR设置数组/集合元素…)
C#的索引器定义在类中,可以重载(索引可以不是整形,也就是可以快速访问Dictionary[即map]),
需要注意的点
它固定要有get,set的方法,get需要返回要取的值
set方法由于没有参数,看似不可以赋值,但其提供了一个默认的value关键字

public class Kingdom
    {
        private List<string> dux;
        public Kingdom()
        {

        }
        public Kingdom(List<string> dux)
        {
            this.dux = dux;
        }

        public string this[int num]
        {
            get
            {
                if (dux == null)
                    return "未初始化";
                if (num < 0 || num > dux.Count)
                    return "內容為空";
                return dux[num];
            }
            set
            {
                if (dux == null)
                    Console.WriteLine("未初始化");
                if(num<0||num>dux.Count)
                    Console.WriteLine("錯誤的放入位置");
                dux[num] = value;
            }
        }
    }

测试：

public static void Main(string[] args)
{
   
    List<string> dux = new List<string>() { "奥尔良","勃艮第","阿尔玛尼克","阿基坦","布列塔尼","诺曼底"};

    Kingdom france = new Kingdom(dux);

    france[1] = "图卢兹";

    for(int i = 0; i < dux.Count; i++)
    {
        Console.WriteLine(france[i]);
    }

}

结果：

奥尔良
图卢兹
阿尔玛尼克
阿基坦
布列塔尼
诺曼底

对Dictionary也是如此

public class Kingdom
    {
        private Dictionary<string,string> dux;
        public Kingdom()
        {

        }
        public Kingdom(Dictionary<string, string> dux)
        {
            this.dux = dux;
        }
		//索引的值可以不为整形
        public string this[string num]
        {
            get
            {
                if (dux == null)
                    return "未初始化";
                if (dux[num]==null)
                    return "內容為空";
                return dux[num];
            }
            set
            {
                if (dux == null)
                    Console.WriteLine("未初始化");
                if (dux[num]==null)
                    Console.WriteLine("錯誤的放入位置");
                dux[num] = value;
            }
        }
    }

public static void Main(string[] args)
{
    
     Dictionary<string, string> duxes = new Dictionary<string, string>();
     duxes.Add("a", "b");
     duxes.Add("c","d");
     duxes.Add("e", "f");
     Kingdom france = new Kingdom(duxes);

     france["c"] = "abcdefg";

     var keys = duxes.Keys;

     foreach(string str in keys)
     {
         Console.WriteLine(france[str]);
     }

 }

b
abcdefg
f