using System;

using System.Collections.Generic;

​

static class Program

{

    static string? calc1() => null;

    static string? calc2() => "hi";

    static string? calc3() => null;

    static string? calc4() => "everyone";

    static void Main()

    {

        var list = new List<string>();

        calc1()?.let(list.Add);

        calc2()?.let(list.Add);

        calc3()?.let(list.Add);

        calc4()?.let(list.Add);

        list.ForEach(Console.WriteLine);

    }

​

    static void let<T>(this T s, Action<T> a) => a(s);

}