AsyncObservables.Services.VariableTimePrimeCheckService.IsPrimeAsync C# (CSharp) Method

        public override async Task<bool> IsPrimeAsync(int number)
        {
            if (number == _numberToDelay)
            {
                await Task.Delay(2000);
            }
            return await base.IsPrimeAsync(number);
        }
    }

        private static void ContrlingOrderOfAsyncCode()
        {
            Demo.DisplayHeader("Contrling the order of async code with Concat");

            var resetEvent = new AutoResetEvent(false);

            Console.WriteLine("Using SelectMany wont maintain items order");
            var svc = new VariableTimePrimeCheckService(numberToDelay: 3);
            IObservable<int> primes =
                from number in Observable.Range(1, 10)
                from isPrime in svc.IsPrimeAsync(number)
                where isPrime
                select number;

            primes
                .DoLast(() => resetEvent.Set(), delay: TimeSpan.FromSeconds(1))
                .SubscribeConsole("primes - unorderd");

            // Waiting for the previous example to finish
            resetEvent.WaitOne();

            Console.WriteLine("Using concat does enforce order");
            primes =
                Observable.Range(1, 10)
                    .Select(async (number) => new { number, IsPrime = await svc.IsPrimeAsync(number) })
                    .Concat()
                    .Where(x => x.IsPrime)
                    .Select(x => x.number);

            primes.SubscribeConsole("primes");

            // Waiting for the previous example to finish
            resetEvent.WaitOne();
        }