using System;

using System.Threading.Tasks;

​

public class Program

{

    public static void Main()

    {

        Console.WriteLine("press 'e' key to email everybody");

        while (Console.ReadLine().ToLower() == "e")

        {

            Console.WriteLine("... e pressed for email to say 'Hi Bob'");

            Email.startEmail();

        }

    }

​

}

 class Email

    {

        public async static void startEmail()

        {

            Console.WriteLine("... started email");

            await EmailAsync("Hi Bob", EmailCompleted); // <-- pass the callback method here

            Jack();

        }

​

        private static async Task EmailAsync(string message, Action<string> callback)

        {

            Console.WriteLine("... sending email, message is " + message);

            await Task.Delay(1000);

            callback(message);

            EmailCompleted(message);

        }

​

        public static void EmailCompleted(string message)

        {

            // This method will be called whenever a file is processed.

            Console.WriteLine(message + "sent as email");

        }

    public static void Jack()

    {

        Console.WriteLine("Jack!");

    }

 }