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;
    }
}

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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;
    }
}

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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;
}
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关系模式

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