c# 高级编程 10章223页 【集合】【Lookup<TKey, TElement>】

Lookup<TKey, TElement>

• 把键映射到，一个值的集合上
• 不能像字典那样创建，必须调用ToLookup()方法来创建Lookup<TKey, TElement>
• IEnumerable<T>类型，都可以调用ToLookup()
• ToLookup()接收一个参数Func<TSource, TKey>
  • 选择IEnumerable<T>的T的属性
  • 来作为Lookup<TKey, TElement>的TKey

    class Program
    {
        static void Main()
        {
            var racers = new List<Racer>();
            racers.Add(new Racer(26, "Jacques", "Villeneuve", "Canada", 11));
            racers.Add(new Racer(18, "Alan", "Jones", "Australia", 12));
            racers.Add(new Racer(11, "Jackie", "Stewart", "United Kingdom", 27));
            racers.Add(new Racer(15, "James", "Hunt", "United Kingdom", 10));
            racers.Add(new Racer(5, "Jack", "Brabham", "Australia", 14));

            var lookupRacers = racers.ToLookup(r => r.Country);

            foreach (Racer r in lookupRacers["Australia"])
            {
                Console.WriteLine(r);
            }
        }
    }

输出：

Alan Jones
Jack Brabham

其中：

    public class Racer : IComparable<Racer>, IFormattable
    {
        public Racer(int id, string firstName, string lastName, string country, int wins)
        {
            Id = id;
            FirstName = firstName;
            LastName = lastName;
            Country = country;
            Wins = wins;
        }

        public Racer(int id, string firstName, string lastName, string country)
          : this(id, firstName, lastName, country, wins: 0)
        { }

        public int Id { get; }
        public string FirstName { get; }
        public string LastName { get; }
        public string Country { get; }
        public int Wins { get; set; }

        public override string ToString() => $"{FirstName} {LastName}";


        public string ToString(string format, IFormatProvider formatProvider)
        {
            if (format == null) format = "N";
            switch (format.ToUpper())
            {
                case null:
                case "N": // name
                    return ToString();
                case "F": // first name
                    return FirstName;
                case "L": // last name
                    return LastName;
                case "W": // Wins
                    return $"{ToString()}, Wins: {Wins}";
                case "C": // Country
                    return $"{ToString()}, Country: {Country}";
                case "A": // All
                    return $"{ToString()}, {Country} Wins: {Wins}";
                default:
                    throw new FormatException(String.Format(formatProvider,
                          "Format {0} is not supported", format));
            }
        }

        public string ToString(string format) => ToString(format, null);

        public int CompareTo(Racer other)
        {
            if (other == null) return -1;
            int compare = string.Compare(LastName, other.LastName);
            if (compare == 0)
                return string.Compare(FirstName, other.FirstName);
            return compare;
        }
    }