enum Weekday
{
    Monday,
    Tuesday,
    Wednesday,
    Thursday,
    Friday,
    Saturday,
    Sunday
}

class Program
{
    static void Main(string[] args)
    {
        Weekday today = Weekday.Tuesday;
        Console.WriteLine("今天是" + today.ToString() + "。");
        Console.WriteLine("明天是" + (Weekday)(((int)today + 1) % 7) + "。");
        Console.WriteLine("昨天是" + (Weekday)(((int)today + 6) % 7) + "。");
    }
}

今天是 Tuesday。
明天是 Wednesday。
昨天是 Monday。

public abstract class Enumeration<TEnum> : IEquatable<Enumeration<TEnum>> where TEnum : Enumeration<TEnum>
{
    private static readonly Dictionary<int, TEnum?> Enumerations = GetEnumerations();

    /// <summary>
    /// 枚舉類型
    /// </summary>
    private static readonly Type EnumType = typeof(TEnum);

    protected Enumeration(int value, string name)
    {
        Value = value;
        Name = name;
    }

    /// <summary>
    /// 名稱
    /// </summary>
    public string Name { get; protected init; }

    /// <summary>
    /// 值
    /// </summary>
    public int Value { get; protected init; }

    public static TEnum? FromName(string name) => Enumerations.Values.SingleOrDefault(x => x.Name == name);

    public static TEnum? FromValue(int value) => Enumerations.TryGetValue(value, out var enumeration) ? enumeration : default;

    public bool Equals(Enumeration<TEnum>? other) => other is not null && GetType() == other.GetType() && Value == other.Value;

    public override bool Equals(object? obj) => obj is Enumeration<TEnum> other && Equals(other);

    public override int GetHashCode() => HashCode.Combine(Value, Name);

    private static Dictionary<int, TEnum?> GetEnumerations() => EnumType
        .GetFields(BindingFlags.Public | BindingFlags.Static | BindingFlags.FlattenHierarchy)
        .Where(x => EnumType.IsAssignableFrom(x.FieldType))
        .Select(x => (TEnum?)x.GetValue(default)).ToDictionary(x => x?.Value ?? 0);
}

/// <summary>
/// 信用卡類型
/// </summary>
public enum CreditCardType
{
    None = 0,
    Standard = 1,
    Silver = 2,
    Gold = 3,
}

/// <summary>
/// 信用卡
/// </summary>
public abstract class CreditCard : Enumeration<CreditCard>
{
    public static readonly CreditCard Gold = new GoldCreditCard();
    public static readonly CreditCard Silver = new SilverCreditCard();
    public static readonly CreditCard Standard = new StandardCreditCard();
    public static readonly CreditCard None = new NoneCreditCard();
    
    private CreditCard(int value, string name) : base(value, name)
    {
    }

    /// <summary>
    /// 折扣
    /// </summary>
    public abstract double Discount { get; }

    /// <summary>
    /// 金卡
    /// </summary>
    private sealed class GoldCreditCard : CreditCard
    {
        public GoldCreditCard() : base(3, nameof(Gold))
        {
        }

        public override double Discount => 0.2;
    }

    private sealed class SilverCreditCard : CreditCard
    {
        public SilverCreditCard() : base(2, nameof(Silver))
        {
        }

        public override double Discount => 0.1;
    }

    /// <summary>
    /// 標(biāo)準(zhǔn)
    /// </summary>
    private sealed class StandardCreditCard : CreditCard
    {
        public StandardCreditCard() : base(1, nameof(Standard))
        {
        }

        public override double Discount => 0.05;
    }

    /// <summary>
    /// 無
    /// </summary>
    private sealed class NoneCreditCard : CreditCard
    {
        public NoneCreditCard() : base(0, nameof(None))
        {
        }

        public override double Discount => 0;
    }
}

public class SmartEnumsTests
{
    private readonly ITestOutputHelper _testOutputHelper;

    public SmartEnumsTests(ITestOutputHelper testOutputHelper)
    {
        _testOutputHelper = testOutputHelper;
    }

    /// <summary>
    /// 基本用法
    /// </summary>
    [Fact]
    public void Use()
    {
        var standardCard = CreditCard.FromName("Standard");
        var standardCard2 = CreditCard.FromValue(1);

        Assert.Equal(standardCard, standardCard2);
        _testOutputHelper.WriteLine(standardCard!.ToJson());
    }
}