C# 9 新特性 —— 增強(qiáng)的模式匹配

Intro

C# 9 中進(jìn)一步增強(qiáng)了模式匹配的用法，使得模式匹配更為強(qiáng)大，我們一起來了解一下吧

Sample

C# 9 中增強(qiáng)了模式匹配的用法，增加了 and/or/not 操作符，而且可以直接判斷屬性，來看一下下面的這個(gè)示例：

var?person?=?new?Person();

//?or
//?string.IsNullOrEmpty(person.Description)
if?(person.Description?is?null?or?{?Length:?0?})
{
????Console.WriteLine($"{nameof(person.Description)}?is?IsNullOrEmpty");
}

//?and
//?!string.IsNullOrEmpty(person.Name)
if?(person.Name?is?not?null?and?{?Length:?>?0?})
{
????if?(person.Name[0]?is?(>=?'a'?and?<=?'z')?or?(>=?'A'?and?<=?'Z')?or?'.')
????{
????}
}

//?not
if?(person.Name?is?not?null)
{
}

這里的代碼使用 DnSpy 反編譯之后的代碼是下面這樣的：

Person?person?=?new?Person();
string?text?=?person.Description;
bool?flag?=?text?==?null?||?text.Length?==?0;
if?(flag)
{
????Console.WriteLine("Description?is?IsNullOrEmpty");
}
text?=?person.Name;
bool?flag2?=?text?!=?null?&&?text.Length?>?0;
if?(flag2)
{
????char?c?=?person.Name[0];
????if?(c?>=?'a')
????{
????????if?(c?>?'z')
????????{
????????????goto?IL_8B;
????????}
????}
????else?if?(c?>=?'A')
????{
????????if?(c?>?'Z')
????????{
????????????goto?IL_8B;
????????}
????}
????else?if?(c?!=?','?&&?c?!=?'.')
????{
????????goto?IL_8B;
????}
????bool?flag3?=?true;
????goto?IL_8E;
????IL_8B:
????flag3?=?false;
????IL_8E:
????bool?flag4?=?flag3;
????if?(flag4)
????{
????}
}
bool?flag5?=?person.Name?!=?null;
if?(flag5)
{
}

Switch

這不僅適用于 is 也可以在 switch 中使用

switch?(person.Age)
{
????case?>=?0?and?<=?3:
????????Console.WriteLine("baby");
????????break;

????case?>?3?and?14:
????????Console.WriteLine("child");
????????break;

????case?>?14?and?22:
????????Console.WriteLine("youth");
????????break;

????case?>?22?and?60:
????????Console.WriteLine("Adult");
????????break;

????case?>=?60?and?<=?500:
????????Console.WriteLine("Old?man");
????????break;

????case?>?500:
????????Console.WriteLine("monster");
????????break;
}

反編譯后的代碼：

int?age?=?person.Age;
int?num?=?age;
if?(num?22)
{
????if?(num?14)
????{
????????if?(num?>=?0)
????????{
????????????if?(num?>?3)
????????????{
????????????????Console.WriteLine("child");
????????????}
????????????else
????????????{
????????????????Console.WriteLine("baby");
????????????}
????????}
????}
????else?if?(num?>?14)
????{
????????Console.WriteLine("youth");
????}
}
else?if?(num?60)
{
????if?(num?>?22)
????{
????????Console.WriteLine("Adult");
????}
}
else?if?(num?>?500)
{
????Console.WriteLine("monster");
}
else
{
????Console.WriteLine("Old?man");
}

More

可以看到有些情況下可以簡(jiǎn)化不少代碼，尤其是 if 分支比較多的情況下使用上面 switch 這樣的寫法會(huì)清晰很多

但是如果只是 string.IsNullOrEmpty 這種代碼最好還是不要寫得這么騷了，小心要被同事吐槽了

炫技需謹(jǐn)慎，小心被 ...

Reference

• https://docs.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-9
• https://github.com/WeihanLi/SamplesInPractice/tree/master/CSharp9Sample
• https://github.com/WeihanLi/SamplesInPractice/blob/master/CSharp9Sample/PatternMatchingSample.cs