C#------新特性之？？、？、？？= 使用

    public static void Main()
    {
        List<int>? numbers = null;
        int? a = null;

        Console.WriteLine((numbers is null)); //true

        // 如果numbers为空，则初始化numbers，同时添加一个5到numbers列表中
        (numbers ??= new List<int>()).Add(5);
        Console.WriteLine(string.Join(" ", numbers));  // output: 5
        Console.WriteLine((numbers is null)); //  false        


        Console.WriteLine((a is null)); //  true
        Console.WriteLine((a ?? 3)); //  3,若a不为空则返回a的值，否则返回3；此处a为空

        // 如果a为空，则赋值0；同时将a添加到numbers列表中
        numbers.Add(a ??= 0);
        Console.WriteLine((a is null)); // false        
        Console.WriteLine(string.Join(" ", numbers));  // output: 5 0
        Console.WriteLine(a);  // output: 0
    }

public string Name
{
    get => name;
    set => name = value ?? throw new ArgumentNullException(nameof(value), "Name cannot be null");    //若Name为空，将抛出异常
}

if (variable is null)
{
variable = expression;
}

和variable ??= expression;    是等效的