switch-声明和类型模式匹配

1.声明和类型模式：类型为 T 的声明模式在表达式结果为非 NULL 且满足以下任一条件时与表达式匹配

var numbers = new int[] { 10, 20, 30 };
Console.WriteLine(GetSourceLabel(numbers)); // output: 1
var letters = new List<char> { 'a', 'b', 'c', 'd' };
Console.WriteLine(GetSourceLabel(letters)); // output: 2
static int GetSourceLabel<T>(IEnumerable<T> source) => source switch
{
Array array => 1,
ICollection<T> collection => 2,
_ => 3,
};

2.如果只想检查表达式类型，可使用弃元 _ 代替变量名，如以下示例所示：

public abstract class Vehicle {}
public class Car : Vehicle {}
public class Truck : Vehicle {}
public static class TollCalculator
{
public static decimal CalculateToll(this Vehicle vehicle) => vehicle switch
{
Car _ => 2.00m,
Truck _ => 7.50m,
null => throw new ArgumentNullException(nameof(vehicle)),
_ => throw new ArgumentException("Unknown type of a vehicle", nameof(vehicle)),
};
}

从 C# 9.0 开始，可对此使用类型模式，如以下示例所示：

public static decimal CalculateToll(this Vehicle vehicle) => vehicle switch
{
Car => 2.00m,
Truck => 7.50m,
null => throw new ArgumentNullException(nameof(vehicle)),
_ => throw new ArgumentException("Unknown type of a vehicle", nameof(vehicle)),
};

常量模式，从 C# 7.0 开始，可使用常量模式来测试表达式结果是否等于指定的常量，如以下示例所示：

public static decimal GetGroupTicketPrice(int visitorCount) => visitorCount switch
{
1 => 12.0m,
2 => 20.0m,
3 => 27.0m,
4 => 32.0m,
0 => 0.0m,
_ => throw new ArgumentException($"Not supported number of visitors: {visitorCount}",
nameof(visitorCount)),
};

在常量模式中，可使用任何常量表达式，例如： integer 或 floating-point 数值文本 char 或 string 文本 布尔值 true 或 false enum 值 声明常量字段或本地的名称 null

常量模式用于检查 null ，如以下示例所示：

if (input is null)
{
   return;
}

Console.WriteLine(Classify(13)); // output: Too high
Console.WriteLine(Classify(double.NaN)); // output: Unknown
Console.WriteLine(Classify(2.4)); // output: Acceptable
static string Classify(double measurement) => measurement switch
{
< -4.0 => "Too low",
> 10.0 => "Too high",
double.NaN => "Unknown",
_ => "Acceptable",
};

Console.WriteLine(GetCalendarSeason(new DateTime(2021, 3, 14))); // output: spring
Console.WriteLine(GetCalendarSeason(new DateTime(2021, 7, 19))); // output: summer
Console.WriteLine(GetCalendarSeason(new DateTime(2021, 2, 17))); // output: winter
static string GetCalendarSeason(DateTime date) => date.Month switch
{
>= 3 and < 6 => "spring",
>= 6 and < 9 => "summer",
>= 9 and < 12 => "autumn",
12 or (>= 1 and < 3) => "winter",
_ => throw new ArgumentOutOfRangeException(nameof(date), $"Date with unexpected month: {date.Month}."),
};

逻辑模式-从 C# 9.0 开始，可使用 not 、 and 和 or 模式连结符来创建以下逻辑模式：

Console.WriteLine(Classify(13)); // output: High
Console.WriteLine(Classify(-100)); // output: Too low
Console.WriteLine(Classify(5.7)); // output: Acceptable
static string Classify(double measurement) => measurement switch
{
< -40.0 => "Too low",
>= -40.0 and < 0 => "Low",
>= 0 and < 10.0 => "Acceptable",
>= 10.0 and < 20.0 => "High",
>= 20.0 => "Too high",
double.NaN => "Unknown",
};

析取 or 模式在任一模式与表达式匹配时与表达式匹配，如以下示例所示：

Console.WriteLine(GetCalendarSeason(new DateTime(2021, 1, 19))); // output: winter
Console.WriteLine(GetCalendarSeason(new DateTime(2021, 10, 9))); // output: autumn
Console.WriteLine(GetCalendarSeason(new DateTime(2021, 5, 11))); // output: spring
static string GetCalendarSeason(DateTime date) => date.Month switch
{
3 or 4 or 5 => "spring",
6 or 7 or 8 => "summer",
9 or 10 or 11 => "autumn",
12 or 1 or 2 => "winter",
_ => throw new ArgumentOutOfRangeException(nameof(date), $"Date with unexpected month:
{date.Month}."),
};

and 模式连结符的优先级高于 or 。 要显式指定优先级，请使用括号，如以下示例所示：

static bool IsLetter(char c) => c is (>= 'a' and <= 'z') or (>= 'A' and <= 'Z');

属性模式 从 C# 8.0 开始，可使用属性模式来将表达式的属性或字段与嵌套模式进行匹配，如以下示例所示：

static bool IsConferenceDay(DateTime date) => date is { Year: 2020, Month: 5, Day: 19 or 20 or 21 };

Console.WriteLine(TakeFive("Hello, world!")); // output: Hello
Console.WriteLine(TakeFive("Hi!")); // output: Hi!
Console.WriteLine(TakeFive(new[] { '1', '2', '3', '4', '5', '6', '7' })); // output: 12345
Console.WriteLine(TakeFive(new[] { 'a', 'b', 'c' })); // output: abc
static string TakeFive(object input) => input switch
{
string { Length: >= 5 } s => s.Substring(0, 5),
string s => s,
ICollection<char> { Count: >= 5 } symbols => new string(symbols.Take(5).ToArray()),
ICollection<char> symbols => new string(symbols.ToArray()),
null => throw new ArgumentNullException(nameof(input)),
_ => throw new ArgumentException("Not supported input type."),
};

属性模式是一种递归模式。 也就是说，可以将任何模式用作嵌套模式。 使用属性模式将部分数据与嵌套模式进 行匹配，如以下示例所示：

public record Point(int X, int Y);
public record Segment(Point Start, Point End);
static bool IsAnyEndOnXAxis(Segment segment) =>
segment is { Start: { Y: 0 } } or { End: { Y: 0 } };

static bool IsAnyEndOnXAxis(Segment segment) => segment is { Start.Y: 0 } or { End.Y: 0 };