namespace TaskQueue
{
    internal struct TaskInfo
    {
        public string Id { get; set; }
        public Task Task { get; set; }

        public TaskInfo(string id, Task task)
        {
            Id = id;
            Task = task;
        }
    }

    internal class TaskQueue
    {
        private readonly object _theLock = new object();
        private readonly int _maxTaskCount;
        private readonly ICollection<TaskInfo> _runningTaskList;
        private readonly ICollection<TaskInfo> _waitingTaskList;

        public TaskQueue(int maxTaskCount)
        {
            _maxTaskCount = maxTaskCount;
            _runningTaskList = new List<TaskInfo>();
            _waitingTaskList = new List<TaskInfo>();
        }

        public void AddTask(TaskInfo taskInfo)
        {
            lock (_theLock)
            {
                if (_runningTaskList.Count < _maxTaskCount)
                {
                    if (_waitingTaskList.Contains(taskInfo))
                        _waitingTaskList.Remove(taskInfo);

                    _runningTaskList.Add(taskInfo);
                    taskInfo.Task.ContinueWith(ContinueWith);
                    taskInfo.Task.Start();
                }
                else
                {
                    if (!_waitingTaskList.Contains(taskInfo))
                    {
                        _waitingTaskList.Add(taskInfo);
                        Console.WriteLine($"Task {taskInfo.Id} waiting....");
                    }
                }
            }
        }

        private void ContinueWith(Task task)
        {
            lock (_theLock)
            {
                _runningTaskList.Remove(_runningTaskList.FirstOrDefault(t => t.Task == task));
                if(_waitingTaskList.Count > 0)
                    AddTask(_waitingTaskList.FirstOrDefault());
            }
        }
    }
}

namespace TaskQueue
{
    public class Program
    {
        static void Main()
        {
            var queue = new TaskQueue(5);

            for (var i = 0; i < 10; i++)
            {
                var guid = Guid.NewGuid().ToString().Substring(0, 4);
                Console.WriteLine($"Creating task {guid}");

                var task = new Task(() => Method(guid));
                queue.AddTask(new TaskInfo(guid, task));
            }
            
            Console.WriteLine("Completed, press any key to quit.");
            Console.ReadLine();
        }

        private static void Method(string id)
        {
            Console.WriteLine($"Execute task {id}");
            Thread.Sleep(5000);
        }
    }
}