C#8.0中的模式匹配

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

来源：https://www.cnblogs.com/TianFang/p/10322434.html