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);
}

首先來看C# 7.0中一個(gè)經(jīng)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，它有更加精簡(jiǎn)的寫法。

Switch表達(dá)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"
};

它利用一條switch語句完成了模式匹配，第一樣看上去要簡(jiǎn)潔一些。不過，它還有更多更簡(jiǎn)單的寫法。

Property patterns

可以直接通過在屬性上指定值作為判定條件，

static?string?Display(object?o)?=>?o?switch
{
????Point?{?X:?0,?Y:?0?}?=>?"origin",
????Point?p????????????  =>?$"({p.X},?{p.Y})",
????_??????????????????  =>?"unknown"
};

也可以將屬性值傳遞出來。

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

利用解構(gòu)函數(shù)，可以寫出更加精簡(jiǎn)的表達(dá)式。

static?string?Display(object?o)?=>?o?switch
{
????Point(0,?0)?????????=>?"origin",
????Point(var?x,?var?y)?=>?$"({x},?{y})",
????_???????????????????=>?"unknown"
};

如果沒有類型轉(zhuǎn)換，則可以寫得更加簡(jiǎn)單了：

static?string?Display(Point?o)?=>?o?switch
{
????(0,?0)?????????=>?"origin",
????(var?x,?var?y)?=>?$"({x},?{y})"
};

非空判斷

如果只是判斷空和非空，則有最簡(jiǎn)單的模式：

{?}??=>?o.ToString(),
null?=>?"null"

Tuple patterns

也支持直接對(duì)ValueTuple進(jìn)行模式匹配，用起來非常靈活。

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")
};