using System;

using System.Collections.Generic;

using System.Linq;

public class Program 

{

    public static void Main()

    {

        List<Program> p = new List<Program>();

        // exception

        //p.First();

        // null but only implicitly

        p.FirstOrDefault();

        // null but explicitly

        var a = p.DefaultIfEmpty(null).First();

        // null, back to being implicitly as DefaultIfEmpty's value is ignored here

        var b = p.DefaultIfEmpty(new Program()).FirstOrDefault();

        Console.WriteLine(b);

    }

}