using System;

public class Program

{

    public static void Main()

    {

        Do<int>();

        Do<MyClass>();

​

    }

​

    public static void Do<U>(U p = default) where U : class

    {

        Console.WriteLine(p is null ? "null" : p);

    }

    public static void Do<U>(U? p = default) where U : struct

    {

        Console.WriteLine(p is null ? "null" : p);

    }

}

​

public class MyClass{}