using System;

using System.Collections.Generic;

​

namespace ConsoleApp2

{

    class Program

    {

        static string Foo() => "Шарп так может";

        static string Foo2() => null;

        static string Foo3() => "Котлин ко ко ко";

        static string Foo4() => null;

​

        static void Main(string[] args)

        {

            var list = new List<string>();

​

            Foo().Let(s => list.Add(s));

            Foo2().Let(s => list.Add(s));

            Foo3().Let(s => list.Add(s));

            Foo4().Let(s => list.Add(s));

            Console.WriteLine(string.Join("\n", list));

        }

    }

​

    public static class Ext

    {

        public static void Let<T>(this T item, Action<T> callback)

            where T : class

        {

            if (item != null)

                callback(item);

        }

    }

}

​