class Program    {        public  delegate  T myDel<U,T>(U u);        static void Main(string[] args)        {            List<Person> personList = new List<Person>(){            new Person{  Age=12,Name="孙俩岁" },            new Person{  Age=13,Name="dfsdfs" },            new Person{  Age=42,Name="dsf" },            new Person{  Age=22,Name="ffhtr" }            };            myDel<Person, bool> myd = new myDel<Person, bool>(getPs);           // Func<Person, bool> myd = getPs;            var lists = personList.Where(myd);            foreach (var l in lists)            {                Console.WriteLine(l.Age);            }            Console.ReadKey();        }        public static bool getPs(Person p)        {            return p.Age > 20;        }    }  public   class Person    {        public string Name { get; set; }        public int Age { get; set; }    } 为什么编译不通过