Coroutine.cs

public class Coroutine
    {
        private readonly Queue<IEnumerator> _queuedCoroutines = new Queue<IEnumerator>();

        public bool Finished => _queuedCoroutines.Count == 0;

        public WaitForSeconds Awaiting { get; protected set; } = null;

        public string Name { get; set; } = string.Empty;

        public Coroutine(IEnumerator routine, string name)
        {
            Name = name;
            _queuedCoroutines.Enqueue(routine);
        }

        public Coroutine(Func<IEnumerator> routine, string name)
        {
            Name = name;
            _queuedCoroutines.Enqueue(routine.Invoke());
        }

        public Coroutine(Action action, string name)
        {
            Name = name;
            _queuedCoroutines.Enqueue(ActionCoroutine(action));
        }

        public Coroutine ContinueWith(IEnumerator routine)
        {
            _queuedCoroutines.Enqueue(routine);
            return this;
        }

        public Coroutine ContinueWith(Action action)
        {
            _queuedCoroutines.Enqueue(ActionCoroutine(action));
            return this;
        }

        public Coroutine ContinueWith(Func<IEnumerator> routine)
        {
            _queuedCoroutines.Enqueue(routine.Invoke());
            return this;
        }

        private IEnumerator ActionCoroutine(Action action)
        {
            action.Invoke();
            yield return null;
        }

        public void Update(GameTime gametime)
        {
            if(Finished)
                return;

            double finishTimeError = 0;

            if (Awaiting != null)
            {
                Awaiting.Update(gametime);

                if (!Awaiting.Finished)
                    return;

                finishTimeError = Awaiting.ElapsedSeconds - Awaiting.Seconds;

                Awaiting = null;
            }

            bool timeCorrectionRequired = false;

            do
            {
                if(_queuedCoroutines.Count == 0)
                    break;

                if (_queuedCoroutines.Peek().MoveNext())
                {
                    WaitForSeconds seconds = _queuedCoroutines.Peek().Current as WaitForSeconds;

                    if (seconds != null)
                    {
                        Awaiting = seconds;
                        Awaiting.ElapsedSeconds += finishTimeError;

                        if (Awaiting.Finished)
                        {
                            finishTimeError = Awaiting.ElapsedSeconds - Awaiting.Seconds;
                            timeCorrectionRequired = true;
                        }
                        else
                        {
                            timeCorrectionRequired = false;
                        }
                    }
                    else
                    {
                        timeCorrectionRequired = false;
                    }
                }
                else
                {
                    _queuedCoroutines.Dequeue();
                    timeCorrectionRequired = false;
                }

            } while (timeCorrectionRequired);
        }
    }