using System;

using System.Collections.Generic;

​

namespace ConsoleApp1

{

    static class Program

    {

        private static string a = null;

        private static string b = "Нет, это котлин сосет";

        private static string c = null;

        private static string d = "Да, сосет";

        static void Main(string[] args)

        {

            var list = new List<string>();

            list.AddNotNull(a);

            list.AddNotNull(b);

            list.AddNotNull(c);

            list.AddNotNull(d);

            foreach (var elem in list)

               Console.WriteLine(elem); 

        }

​

        static void AddNotNull<T>(this List<T> l, T t)

        {

            if(t != null)

                l.Add(t);

        }

    }

}