LimitedConcurrencyLevelTaskScheduler.cs

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using System.Collections.Generic;
using System.Linq;

namespace System.Threading.Tasks.Schedulers
{
    public class LimitedConcurrencyLevelTaskScheduler : TaskScheduler
    {
        [ThreadStatic]
        private static bool _currentThreadIsProcessingItems;
        private readonly LinkedList<Task> _tasks = new LinkedList<Task>(); // protected by lock(_tasks)
        private readonly int _maxDegreeOfParallelism;
        private int _delegatesQueuedOrRunning = 0; // protected by lock(_tasks)

        public LimitedConcurrencyLevelTaskScheduler(int maxDegreeOfParallelism)
        {
            if (maxDegreeOfParallelism < 1) throw new ArgumentOutOfRangeException("maxDegreeOfParallelism");
            _maxDegreeOfParallelism = maxDegreeOfParallelism;
        }

        protected sealed override void QueueTask(Task task)
        {
            // Add the task to the list of tasks to be processed.  If there aren't enough
            // delegates currently queued or running to process tasks, schedule another.
            lock (_tasks)
            {
                _tasks.AddLast(task);
                if (_delegatesQueuedOrRunning < _maxDegreeOfParallelism)
                {
                    ++_delegatesQueuedOrRunning;
                    NotifyThreadPoolOfPendingWork();
                }
            }
        }

        private void NotifyThreadPoolOfPendingWork()
        {
            ThreadPool.UnsafeQueueUserWorkItem(_ =>
            {
                // Note that the current thread is now processing work items.
                // This is necessary to enable inlining of tasks into this thread.
                _currentThreadIsProcessingItems = true;
                try
                {
                    // Process all available items in the queue.
                    while (true)
                    {
                        Task item;
                        lock (_tasks)
                        {
                            // When there are no more items to be processed,
                            // note that we're done processing, and get out.
                            if (_tasks.Count == 0)
                            {
                                --_delegatesQueuedOrRunning;
                                break;
                            }

                            // Get the next item from the queue
                            item = _tasks.First.Value;
                            _tasks.RemoveFirst();
                        }

                        // Execute the task we pulled out of the queue
                        base.TryExecuteTask(item);
                    }
                }
                // We're done processing items on the current thread
                finally { _currentThreadIsProcessingItems = false; }
            }, null);
        }

        protected sealed override bool TryExecuteTaskInline(Task task, bool taskWasPreviouslyQueued)
        {
            // If this thread isn't already processing a task, we don't support inlining
            if (!_currentThreadIsProcessingItems) return false;

            // If the task was previously queued, remove it from the queue
            if (taskWasPreviouslyQueued) TryDequeue(task);

            // Try to run the task.
            return base.TryExecuteTask(task);
        }

        protected sealed override bool TryDequeue(Task task)
        {
            lock (_tasks) return _tasks.Remove(task);
        }

        public sealed override int MaximumConcurrencyLevel { get { return _maxDegreeOfParallelism; } }

        protected sealed override IEnumerable<Task> GetScheduledTasks()
        {
            bool lockTaken = false;
            try
            {
                Monitor.TryEnter(_tasks, ref lockTaken);
                if (lockTaken) return _tasks.ToArray();
                else throw new NotSupportedException();
            }
            finally
            {
                if (lockTaken) Monitor.Exit(_tasks);
            }
        }
    }
}