System.Object 作为字典的键

把 类实例 作为key，而放入字典的value时， 类实例的引用 和value建立了一一对应的关系， 若想检索value , 就必须保存着类实例的引用， 这不太方便 ！ 要想方便 ，这个类需要重写 Equals() 和 GetHashCode()

System.String 作为字典的键

Int32 作为字典的键

如果 键类型 没有实现 IEquatable 接口，也没有重写 GetHashCode() 方法

    public class Employee
        private readonly string _name;
        private readonly decimal _salary;
        private readonly EmployeeId _id;
        public Employee(EmployeeId id, string name, decimal salary)
            _id = id;
            _name = name;
            _salary = salary;
        public override string ToString() => $"{_id.ToString()}: {_name, -20} {_salary :C}";
    public class EmployeeIdException : Exception
        public EmployeeIdException(string message) : base(message) { }
    public struct EmployeeId : IEquatable<EmployeeId>
        private readonly char _prefix;
        private readonly int _number;
        public EmployeeId(string id)
            if (id == null) throw new ArgumentNullException(nameof(id));
            _prefix = (id.ToUpper())[0];
            int numLength = id.Length - 1;
                _number = int.Parse(id.Substring(1, numLength > 6 ? 6 : numLength));
            catch (FormatException)
                throw new EmployeeIdException("Invalid EmployeeId format");
        public override string ToString() => _prefix.ToString() + $"{_number,6:000000}";
        public override int GetHashCode() => (_number ^ _number << 16) * 0x1505_1505;
        public bool Equals(EmployeeId other) => _prefix == other._prefix && _number == other._number;
        public override bool Equals(object obj) => Equals((EmployeeId)obj);
        public static bool operator ==(EmployeeId left, EmployeeId right) => left.Equals(right);
        public static bool operator !=(EmployeeId left, EmployeeId right) => !(left == right);
    class Program
        static void Main()
            var idJimmie = new EmployeeId("C48");
            var jimmie = new Employee(idJimmie, "Jimmie Johnson", 150926.00m);
            var idJoey = new EmployeeId("F22");
            var joey = new Employee(idJoey, "Joey Logano", 45125.00m);
            var idKyle = new EmployeeId("T18");
            var kyle = new Employee(idKyle, "Kyle Bush", 78728.00m);
            var idCarl = new EmployeeId("T19");
            var carl = new Employee(idCarl, "Carl Edwards", 80473.00m);
            var idMatt = new EmployeeId("T20");
            var matt = new Employee(idMatt, "Matt Kenseth", 113970.00m);
            var employees = new Dictionary<EmployeeId, Employee>(31)
                [idJimmie] = jimmie,
                [idJoey] = joey,
                [idKyle] = kyle,
                [idCarl] = carl,
                [idMatt] = matt
            foreach (var employee in employees.Values)
                Console.WriteLine(employee);
            while (true)
                Console.Write("Enter employee id (X to exit)> ");
                var userInput = Console.ReadLine();
                userInput = userInput.ToUpper();
                if (userInput == "X") break;
                EmployeeId id;
                    id = new EmployeeId(userInput);
                    if (!employees.TryGetValue(id, out Employee employee))
                        Console.WriteLine($"Employee with id {id} does not exist");
                        Console.WriteLine(employee);
                catch (EmployeeIdException ex)
                    Console.WriteLine(ex.Message);
输出：（存入字典的数据项都能用键给检索出来）
C000048: Jimmie Johnson       ￥150,926.00
F000022: Joey Logano          ￥45,125.00
T000018: Kyle Bush            ￥78,728.00
T000019: Carl Edwards         ￥80,473.00
T000020: Matt Kenseth         ￥113,970.00
Enter employee id (X to exit)> T19
T000019: Carl Edwards         ￥80,473.00
Enter employee id (X to exit)> 148
Employee with id 1000048 does not exist
Enter employee id (X to exit)> T48
Employee with id T000048 does not exist
Enter employee id (X to exit)> C48
C000048: Jimmie Johnson       ￥150,926.00
Enter employee id (X to exit)> F22
F000022: Joey Logano          ￥45,125.00
Enter employee id (X to exit)>
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