索引

    class Program
    {

        static void Main()
        {
            var pair = new Pair<int>(10,12);

            Console.WriteLine($"this is {pair.First} and {pair[PairItem.Second]} other {pair[0]}");
            Console.ReadKey();
        }
    }

    interface IPair<T>
    {
        T First { get; }
        T Second { get; }
        T this[PairItem index] { get; }
    }

    internal enum PairItem
    {
        First,
        Second
    }

    internal struct Pair<T> : IPair<T>
    {
        public Pair(T first,T second)
        {
            First = first;
            Second = second;
        }

        public T this[PairItem index]
        {
            get
            {
                switch (index)
                {
                    case PairItem.First:
                        return First;
                    case PairItem.Second:
                        return Second;
                    default:
                        throw new NotImplementedException();
                }
            }
            set
            {
                switch (index)
                {
                    case PairItem.First:
                        First = value;
                        break;
                    case PairItem.Second:
                        Second = value;
                        break;
                    default:
                        throw new NotImplementedException();
                }
            }
        }

        public T First{ get; private set; }

        public T Second { get; private set; }
    }

重点在于这个