C#.Rihter.ContinueWith

        static void Main(string[] args)
        {
            Task<int> t = new Task<int>(Work);
            t.Start();
            Thread.Sleep(3000); // doing smth in main thread

            t.ContinueWith(t => Console.WriteLine("job is done " + t.Result));
        }

        static int Work()
        {
            Console.WriteLine("work thread");
            Thread.Sleep(1000); // doing smth in other thread
            return 123;
        }

output

work thread
job is done 123