C # 9.0 的模式匹配

void M(object o1, object o2)
{
 var t = (o1, o2);
 if (t is (int, string)) {} // test if o1 is an int and o2 is a string
 switch (o1) {
 case int: break; // test if o1 is an int
 case System.String: break; // test if o1 is a string
 }
}

关系模式 与常数值相比，关系模式允许程序员表达输入值必须满足关系约束：

public static LifeStage LifeStageAtAge(int age) => age switch
 {
 < 0 => LifeStage.Prenatal,
 < 2 => LifeStage.Infant,
 < 4 => LifeStage.Toddler,
 < 6 => LifeStage.EarlyChild,
 < 12 => LifeStage.MiddleChild,
 < 20 => LifeStage.Adolescent,
 < 40 => LifeStage.EarlyAdult,
 < 65 => LifeStage.MiddleAdult,
 _ => LifeStage.LateAdult,
 };

关系模式支持 < 所有内置类型上的关系运算符<= 和， > >= 它们支持在表达式中 使用两个具有相同类型的操作数的内置类型。 具体而言，支持sbyte byte short ushort 、 int 、 uint 、 long ulong char float double decimal nint nuint 和的所有关系模式。

模式组合器

if (e is not null) ...
bool IsLetter(char c) => c is >= 'a' and <= 'z' or >= 'A' and <= 'Z';
bool IsLetter(char c) => c is (>= 'a' and <= 'z') or (>= 'A' and <= 'Z');

语义 (例如，类型) 关系运算符

bool IsValidPercentage(int x) => x is >= 0 and <= 100;
bool IsValidPercentage(object x) => x is >= 0 and <= 100;
bool IsValidPercentage(object x) => x is
 >= 0 and <= 100 or // integer tests
 >= 0F and <= 100F or // float tests
 >= 0D and <= 100D; // double tests

从左侧的左侧流向类型信息 and

bool isSmallByte(object o) => o is byte and < 100;

byte b = ...;
int x = b switch { <100 => 0, 100 => 1, 101 => 2, >101 => 3 };