private void OnEndOfShift(WorkShift endingShift)
        {
            ShiftFinished?.Invoke(endingShift);

            WorkShift nextShift = null;

            while (true)
            {
                // Shift has ended but all work may not yet be done
                while (_topLevelWorkItem.State != WorkItemState.Complete)
                {
                    // This will return null if all queues are empty.
                    nextShift = SelectNextShift();
                    if (nextShift != null)
                    {
                        ShiftStarting?.Invoke(nextShift);
                        nextShift.Start();
                        return;
                    }
                }

                // If the shift has ended for an isolated queue, restore
                // the queues and keep trying. Otherwise, we are done.
                if (_isolationLevel > 0)
                {
                    RestoreQueues();
                }
                else
                {
                    break;
                }
            }

            // All done - shutdown all shifts
            foreach (var shift in Shifts)
            {
                shift.ShutDown();
            }
        }