模式匹配

模式匹配性能更好，因为 C# 编译器会根据你的模式编译出最优的匹配路径。

switch 表达式

public static RGBColor FromRainbow(Rainbow colorBand) =>
    colorBand switch
    {
        Rainbow.Red    => new RGBColor(0xFF, 0x00, 0x00),
        Rainbow.Orange => new RGBColor(0xFF, 0x7F, 0x00),
        Rainbow.Yellow => new RGBColor(0xFF, 0xFF, 0x00),
        Rainbow.Green  => new RGBColor(0x00, 0xFF, 0x00),
        Rainbow.Blue   => new RGBColor(0x00, 0x00, 0xFF),
        Rainbow.Indigo => new RGBColor(0x4B, 0x00, 0x82),
        Rainbow.Violet => new RGBColor(0x94, 0x00, 0xD3),
        _              => throw new ArgumentException(message: "invalid enum value", paramName: nameof(colorBand)),
    };

属性模式

public static decimal ComputeSalesTax(Address location, decimal salePrice) =>
    location switch
    {
        { State: "WA" } => salePrice * 0.06M,
        { State: "MN" } => salePrice * 0.075M,
        { State: "MI" } => salePrice * 0.05M,
        // other cases removed for brevity...
        _ => 0M
    };

元组模式

public static string RockPaperScissors(string first, string second)
    => (first, second) switch
    {
        ("rock", "paper") => "rock is covered by paper. Paper wins.",
        ("rock", "scissors") => "rock breaks scissors. Rock wins.",
        ("paper", "rock") => "paper covers rock. Paper wins.",
        ("paper", "scissors") => "paper is cut by scissors. Scissors wins.",
        ("scissors", "rock") => "scissors is broken by rock. Rock wins.",
        ("scissors", "paper") => "scissors cuts paper. Scissors wins.",
        (_, _) => "tie"
    };

位置模式

某些类型包含 Deconstruct 方法，该方法将其属性解构为离散变量。

static Quadrant GetQuadrant(Point point) => point switch
{
    (0, 0) => Quadrant.Origin,
    var (x, y) when x > 0 && y > 0 => Quadrant.One,
    var (x, y) when x < 0 && y > 0 => Quadrant.Two,
    var (x, y) when x < 0 && y < 0 => Quadrant.Three,
    var (x, y) when x > 0 && y < 0 => Quadrant.Four,
    var (_, _) => Quadrant.OnBorder,
    _ => Quadrant.Unknown
};

 模式匹配支持递归模式

string QueryMessage(Entry entry)
{
    return entry switch
    {
        UserEntry u => dbContext1.User.FirstOrDefault(i => i.Id == u.UserId) switch
        {
            null => dbContext2.User.FirstOrDefault(i => i.Id == u.UserId)?.Content ?? "",
            var found => found.Content
        },
        DataEntry d => dbContext1.Data.FirstOrDefault(i => i.Id == d.DataId) switch
        {
            null => dbContext2.Data.FirstOrDefault(i => i.Id == u.DataId)?.Content ?? "",
            var found => found.Content
        },
        EventEntry { EventId: var eventId, CanRead: true } => dbContext1.Event.FirstOrDefault(i => i.Id == eventId) switch
        {
            null => dbContext2.Event.FirstOrDefault(i => i.Id == eventId)?.Content ?? "",
            var found => found.Content
        },
        EventEntry { CanRead: false } => "",
        _ => throw new InvalidArgumentException("无效的参数")
    };
}