using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;

namespace ConsoleApplication3
{
    class Program
    {
        private static List<int> calls = new List<int>();
        private static ConcurrentDictionary<int, ConcurrentQueue<Task>> Requests = new ConcurrentDictionary<int, ConcurrentQueue<Task>>();
        private static int queueTasks;

        static void Main(string[] args)
        {
            Task.Run(() => { Enqueue(new Task(() => { calls.Add(1); Thread.Sleep(1); }), 1); });
            Task.Run(() => { Enqueue(new Task(() => { calls.Add(2); Thread.Sleep(1); }), 1); });
            Task.Run(() => { Enqueue(new Task(() => { calls.Add(3); Thread.Sleep(1); }), 1); });
            Task.Run(() => { Enqueue(new Task(() => { calls.Add(4); Thread.Sleep(1); }), 1); });

            Thread.Sleep(500);

            foreach (var call in calls)
            {
                Console.WriteLine($"call: {call}");
            }

            Console.WriteLine($"queue tasks running: {queueTasks}");
        }

        private static void Enqueue(Task task, int siteId)
        {
            ConcurrentQueue<Task> queue;

            if (!Requests.ContainsKey(siteId))
            {
                Requests.GetOrAdd(siteId, new ConcurrentQueue<Task>());
            }

            Requests.TryGetValue(siteId, out queue);

            var isRunning = queue?.Count > 0;
            queue?.Enqueue(task);

            if (!isRunning)
            {
                Task.Run(() => CheckQueue(siteId));
                queueTasks++;
            }
        }

        private static void CheckQueue(int siteId)
        {
            ConcurrentQueue<Task> queue;
            Requests.TryGetValue(siteId, out queue);

            if (queue == null) return;

            while (queue.Count > 0)
            {
                Task task;
                queue.TryPeek(out task);

                if (task != null)
                {
                    task.Start();
                    task.Wait();
                    queue.TryDequeue(out task);
                }
            }
        }
    }
}