private static void Main() { var primesCalculatorDel = new Primes(FindPrimes); primesCalculatorDel.BeginInvoke(200000000, FindPrimesCallback, primesCalculatorDel); // Func<int, List<int>> ftn = FindPrimes; // ftn.BeginInvoke(200000000, FindPrimesCallback, ftn); for (var i = 0; i < 10; i++) { Console.Write("1"); Thread.Sleep(1000); } }