C# 9 增强的模式匹配

C# 9 中进一步增强了模式匹配的用法，使得模式匹配更为强大

增加了 and/or/not 操作符，而且可以直接判断属性

IF

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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)
{

}

Switch

这不仅适用于 is 也可以在 switch 中使用

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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;
}