c#中switch常用模式

声明模式

首先检查value的类型，然后根据类型输出相应的消息

public void ShowMessage(object value)
{switch (value){case int i: Console.WriteLine($"value is int:{i}"); break;case long l: Console.WriteLine($"value is long:{l}"); break;case bool b: Console.WriteLine($"value is bool:{b}"); break;case string s: Console.WriteLine($"value is string:{s}"); break;default: Console.WriteLine($"value is object"); break;}
}

类型模式

类型模式可以理解为在声明模式中使用了弃元：

public void ShowMessage(object value)
{switch (value){case int: Console.WriteLine($"value is int"); break;case long: Console.WriteLine($"value is long"); break;case bool: Console.WriteLine($"value is bool"); break;case string: Console.WriteLine($"value is string"); break;default: Console.WriteLine($"value is object"); break;}
}

常量模式

常量模式可以理解为原来C#6.0及之前的用法：

switch (score)
{case 10:case 9:Console.WriteLine("优秀");break;case 8:Console.WriteLine("良好");break;case 7:case 6:Console.WriteLine("及格");break;default:Console.WriteLine("不及格");break;
}

关系模式

switch(score)
{case >=80:Console.WriteLine("excellent");break;case >=60:Console.WriteLine("good");break;default:Console.WriteLine("poor");break;
}

逻辑模式

switch(value)
{case 0:Console.WriteLine("value is 0");break;case not 0 and(100 or - 100):Console.WriteLine("abs(value)==100");break;case not 0 and( > 0 and < 100):Console.WriteLine("value is positive and less than 100");break;case not 0 and > 0:Console.WriteLine("value is positive and greater than 100");break;case <-100 or( < 0 and > -100):Console.WriteLine("value is negative and not equals -100");break;
}

属性模式

switch(time)
{case {Year: 2020 or 2021,Month: <= 6,Day: 1}t:Console.WriteLine($ "the first day of every month in the first half of 2020 and 2021");break;case {Year: not 2022}:Console.WriteLine($ "not 2022");break;case {DayOfWeek: not DayOfWeek.Sunday and not DayOfWeek.Saturday}:Console.WriteLine($ "recursion");break;
}

位置模式

位置模式采用解构的特性来说明指定的模式是否匹配：

public record Point2D(int X, int Y); //记录可以解构
static void Print(Point2D point)
{switch(point){case( > 0, > 0):Console.WriteLine("first quadrant");break;case( < 0, > 0):Console.WriteLine("second quadrant");break;case( < 0, < 0):Console.WriteLine("third quadrant");break;case( > 0, < 0):Console.WriteLine("fourth quadrant");break;default:Console.WriteLine("coordinate axis");break;}
}

Var模式

Var模式往往和属性模式和位置模式结合，用于提取属性变量值：

switch(point)
{case(var x,var y,var z):Console.WriteLine($ "3D point:({x},{y},{z})");break; //在位置模式中使用case Point2D{X: var x, Y: var y}:Console.WriteLine($ "2D point:({x},{y})");break; //在属性模式中使用default:Console.WriteLine("others");break;
}

弃元模式

弃元模式在switch语句中用的不多，但是在switch表达式中使用的多：

var result = score
switch
{ >= 80 => "excellent", >= 60 => "good",_ => "poor" //弃元在switch表达式中就相当于default
};