int? int??

单问号---为泛型 Nullable<int> 的简写方式.

双问号---用于判断前一个操作数是否为null,如为null则"返回"后一个操作数,否则"返回"前一个操作数.

publicint? para =null;
//public int para;如果不注释掉此行,而把上一行注释掉的话,下面会报错!
publicint par()
{
    returnthis.para ??0;
}

执行下面程序，返回值为 8

publicint? para =8;
//public int para;如果不注释掉此行,而把上一行注释掉的话,下面会报错!
publicint par()
{
    returnthis.para ??0;
}

执行下面程序，返回值为 12

class Program
    {
        publicint? paral =12;

        staticvoid Main(string[] args)
        {
            Program program =new Program();
            Console.WriteLine(program.Para());
        }

        publicint Para()
        {
            returnthis.paral ??8;
        }
    }

执行下面程序 返回值为 0

class Program
    {
        publicint? paral =0;

        staticvoid Main(string[] args)
        {
            Program program =new Program();
            Console.WriteLine(program.Para());
        }

        publicint Para()
        {
            returnthis.paral ??8;
        }
    }