.NET中的模式匹配(Pattern Matching)是什么？如何用它简化复杂的if-else和switch？

if (obj is string) {
    string s = (string)obj;
    Console.WriteLine($"字符串长度：{s.Length}");
}
else if (obj is int) {
    int i = (int)obj;
    Console.WriteLine($"整数值的两倍：{i * 2}");
}

if (obj is string s) {
    Console.WriteLine($"字符串长度：{s.Length}");
}
else if (obj is int i) {
    Console.WriteLine($"整数值的两倍：{i * 2}");
}

var result = obj switch
{
    null => "空值",
    string s when s.Length == 0 => "空字符串",
    string s => $"字符串：{s}, 长度 {s.Length}",
    int i when i < 0 => $"负整数：{i}",
    int i => $"正整数或零：{i}",
    _ => "未知类型"
};

public class Person
{
    public string Name { get; set; }
    public int Age { get; set; }
}

var person = new Person { Name = "Alice", Age = 25 };
<p>var category = person switch
{
{ Age: < 13 } => "儿童",
{ Age: >= 13 and <= 19 } => "青少年",
{ Age: > 19 } and { Name: "Admin" } => "成年管理员",
{ Age: > 19 } => "成年人",
_ => "未知"
};</p>

(string name, int age) = ("Bob", 30);
<p>var desc = (name, age) switch
{
("Alice", 25) => "特定用户",
(<em>, 0) => "年龄为0",
(var n, var a) when a > 18 => $"成人：{n}",
</em> => "其他"
};</p>