using System;

​

public class Program

{

    public static void Main()

    {

        string s1 = null; 

        string s2 = string.Empty; 

        Console.WriteLine(s1 ?? "s1 null"); 

        Console.WriteLine(string.IsNullOrEmpty(s1) ? "s1 null or empty": "s1 not null"); 

        Console.WriteLine(s2 ?? "s2 null"); 

        Console.WriteLine(string.IsNullOrEmpty(s2) ? "s2 null or empty": "s2 not null"); 

        decimal? d = null; 

        Console.WriteLine(Math.Abs(d ?? 123)); 

        d = 1; 

        Console.WriteLine(Math.Abs(d ?? 123)); 

​

    }

}