class DayCollection//声明一个存储星期几的类
    {
        //定义字符串数组days用以存储星期信息
        string[] days = { "Sun", "Mon", "Tues", "Wed", "Thurs", "Fri", "Sat" };
        //方法GetDay用于根据字符串查找星期几，找不到返回-1
        private int GetDay(string testDay)
        {
            int i = 0;
            foreach (string day in days)
            {//遍历查询testDay在数组中的索引
                if (day == testDay)
                {
                    return i;
                }
                i++;
            }
            return -1;
        }
        public int this[string day]
        {
            get
            { return (GetDay(day)); }//调用GetDay方法得到day在数组中的索引并返回
        }
    }

    class Program
    {

        static void Main(string[] args)
        {
            //创建DayCollection累对象week
            DayCollection week = new DayCollection();
      
            //利用索引器查询“Fri”在days数组中对应的索引
            System.Console.WriteLine(week["Fri"]);
        //无效的查询返回-1
            System.Console.WriteLine(week["InvalidDay"]);
            Console.ReadKey();
        }
       
    }

testDay的参数来源是什么？
     if (day == testDay)
                {
                    return i;
                }
                i++;
            }
            return -1;
索引器中也没有testDay啊
搞不懂- -麻烦帮忙解释一下