using System;

using System.Threading;

​

public class Program

{

    public static void Add(int a, int b, Action<int> callback) {

        new Thread(() => {

            Thread.Sleep(500); // block for 500ms

            callback(a + b);

        }).Start();

    }

​

    public static void Main()

    {

        Add(1, 2, result =>

        {

            Console.WriteLine(result);

        });

        Thread.Sleep(1000);

    }

}