using System;

​

public abstract class AbsCl {}

​

public class ImplCl : AbsCl {}

​

public class Program

{

    public static void Main()

    {

        PrintAbs(default(ImplCl));

        PrintT(default(ImplCl));

    }

    public static void PrintAbs(AbsCl? abs) { Console.WriteLine(abs?.GetType());}

    public static void PrintT<T>(T? typ) where T : AbsCl { Console.WriteLine(typeof(T));}

}