using System;

using System.Collections.Generic;

using System.Collections.Immutable;

using System.Linq;

​

public static class Program

{

    private static readonly ImmutableDictionary<string, Action> CommandMap

        = new Dictionary<string, Action>()

        {

            ["help"] = () => PrintHelp(),

            ["quit"] = () => ExitProgram(),

            ["shutdown"] = () => ShutdownSystem(),

        }.ToImmutableDictionary();

​

    private static void PrintHelp() => Console.WriteLine($"{nameof(PrintHelp)}");

    private static void ExitProgram() => Console.WriteLine($"{nameof(ExitProgram)}");

    private static void ShutdownSystem() => Console.WriteLine($"{nameof(ShutdownSystem)}");

​

    private static IEnumerable<string> CommandList { get; }

        = new[] { "help", "reboot", "shutdown" };

​

    private static void FixMe()

    {

        var list = CommandList

            // Converts the element to 'null' when 'TryGetValue(TKey, out TVaue)'

            // returns 'false'

            .Select(n => CommandMap.TryGetValue(n, out var action)

                    ? action

                    : null)

            .Where(a => a is {});

        foreach (var action in list)

        {

            action();

        }

    }

​

    public static void Main(string[] args)

    {

        FixMe();

    }

}

​