逆变，协变

 class Fax
    {

    }
    class LaserPrinter : Fax
    {

    }
     delegate Tresult Func<in T,out Tresult>(T arg);//out 关键字指定该类型参数是协变的,in 关键字指定该类型参数是逆变的

    class Program
    {
        private static T Min<T>(T o, T b)where T:class,IComparable<T>
        {
            Func<Fax, LaserPrinter> l = teee;
            Func<LaserPrinter, Fax> f = l;//逆变 
            //协变 
            if (o.CompareTo(b) < 0) return o;
            return b;
        }
        static LaserPrinter teee(Fax i)
        {
            return new LaserPrinter();
        }
}