using System;

using System.Reactive.Linq;

using System.Reactive.Threading.Tasks;

using System.Threading;

using System.Threading.Tasks;

​

namespace ConsoleApp1

{

    class Program

    {

        static readonly SemaphoreSlim semaphore = new SemaphoreSlim(1, 1);

​

        static async Task Main(string[] args)

        {

            Console.WriteLine("Completely out of order ...");

​

            var outOfOrder = Observable.Range(1, 4)

                .Select(x => LongProcessAsync(x).ToObservable())

                .Merge()

                .Do(x => Console.WriteLine($"{x} concatenated"));

​

            await outOfOrder.RunAsync(CancellationToken.None);

​

            Console.WriteLine();

            Console.WriteLine("Kinda in order ...");

​

            var kindaInOrder = Observable.Range(1, 4)

                .Select(x => LongProcessAsync(x).ToObservable())

                .Concat()

                .Do(x => Console.WriteLine($"{x} concatenated"));

​

            await kindaInOrder.RunAsync(CancellationToken.None);

​

            Console.WriteLine();

            Console.WriteLine("Completely in order ...");

​

            var completelyInOrder = Observable.Range(1, 4)

                .Select(x => OrderedLongProcessAsync(x).ToObservable())

                .Concat()

                .Do(x => Console.WriteLine($"{x} concatenated"));

​

            await completelyInOrder.RunAsync(CancellationToken.None);

        }

​

        static async Task<int> LongProcessAsync(int n)

        {

            Console.WriteLine($"Job {n} started");

            await Task.Delay(TimeSpan.FromSeconds(4 - n));

            Console.WriteLine($"Job {n} done");

​

            return n;

        }

​

        static async Task<int> OrderedLongProcessAsync(int n)

        {

            await semaphore.WaitAsync();

            try

            {

                return await LongProcessAsync(n);

            }

            finally

            {

                semaphore.Release();

            }

        }

    }

}