C# 8.0中的模式匹配相对C# 7.0来说有了进一步的增强，对于如下类：
class Point { public int X { get; } public int Y { get; } public Point(int x, int y) => (X, Y) = (x, y); public void Deconstruct(out int x, out int y) => (x, y) = (X, Y); }
首先来看C# 7.0中一个经典的模式匹配示例：
static?string?Display(object?o) { ????switch?(o) ????{ ????????case?Point?p?when?p.X?==?0?&&?p.Y?==?0: ????????????return?"origin"; ????????case?Point?p: ????????????return?$"({p.X},?{p.Y})"; ????????default: ????????????return?"unknown"; ????} }
在C# 8.0中，它有更加精简的写法。
Switch表达式在C# 8.0中，可以利用新的switch方式成模式匹配：
static?string?Display(object?o)?=>?o?switch { ????Point?p?when?p.X?==?0?&&?p.Y?==?0?=>?"origin", ????Point?p???????????????????????????=>?$"({p.X},?{p.Y})", ????_?????????????????????????????????=>?"unknown" };
它利用一条switch语句完成了模式匹配，第一样看上去要简洁一些。不过，它还有更多更简单的写法。
Property patterns可以直接通过在属性上指定值作为判定条件，
static?string?Display(object?o)?=>?o?switch { ????Point?{?X:?0,?Y:?0?}?=>?"origin", ????Point?p???????????? =>?$"({p.X},?{p.Y})", ????_?????????????????? =>?"unknown" };
也可以将属性值传递出来。
static?string?Display(object?o)?=>?o?switch { ????Point?{?X:?0,?Y:?0?}?????????=>?"origin", ????Point?{?X:?var?x,?Y:?var?y?}?=>?$"({x},?{y})", ????_????????????????????????????=>?"unknown" };
Positional patterns利用 解构函数 ，可以写出更加精简的表达式。
static?string?Display(object?o)?=>?o?switch { ????Point(0,?0)?????????=>?"origin", ????Point(var?x,?var?y)?=>?$"({x},?{y})", ????_???????????????????=>?"unknown" };
如果没有类型转换，则可以写得更加简单了：
static?string?Display(Point?o)?=>?o?switch { ????(0,?0)?????????=>?"origin", ????(var?x,?var?y)?=>?$"({x},?{y})" };
非空判断如果只是判断空和非空，则有最简单的模式：
{?}??=>?o.ToString(), null?=>?"null"
Tuple patterns也支持直接对ValueTuple进行模式匹配，用起来非常灵活。
static?State?ChangeState(State?current,?Transition?transition,?bool?hasKey)?=> ????(current,?transition,?hasKey)?switch { ????(Opened,?Close,?_)?????=>?Closed, ????(Closed,?Open,??_)?????=>?Opened, ????(Closed,?Lock,?true)???=>?Locked, ????(Locked,?Unlock,?true)?=>?Closed, ????_?=>?throw?new?InvalidOperationException($"Invalid?transition") };
小结C# 8.0的模式匹配有了进一步的增强，虽然可以又有人要说是语法糖了，但用得好的话可以让你的代码逻辑简化不少，更加容易理解。
参考文章Do more with patterns in C# 8.0
到此这篇关于C#模式匹配的文章就介绍到这了。希望对大家的学习有所帮助，也希望大家多多支持。