C#  Task  TaskFactory  设置最大并行线程数的方法

public class LimitedConcurrencyLevelTaskScheduler : TaskScheduler
{
    /// <summary>Whether the current thread is processing work items.</summary> 
    [ThreadStatic]
    private static bool _currentThreadIsProcessingItems;
    /// <summary>The list of tasks to be executed.</summary> 
    private readonly LinkedList<Task> _tasks = new LinkedList<Task>(); // protected by lock(_tasks) 
                                                                       /// <summary>The maximum concurrency level allowed by this scheduler.</summary> 
    private readonly int _maxDegreeOfParallelism;
    /// <summary>Whether the scheduler is currently processing work items.</summary> 
    private int _delegatesQueuedOrRunning = 0; // protected by lock(_tasks) 
    /// <summary> 
    /// Initializes an instance of the LimitedConcurrencyLevelTaskScheduler class with the 
    /// specified degree of parallelism. 
    /// </summary> 
    /// <param name="maxDegreeOfParallelism">The maximum degree of parallelism provided by this scheduler.</param> 
    public LimitedConcurrencyLevelTaskScheduler(int maxDegreeOfParallelism)
    {
        if (maxDegreeOfParallelism < 1) throw new ArgumentOutOfRangeException("maxDegreeOfParallelism");
        _maxDegreeOfParallelism = maxDegreeOfParallelism;
    }
    /// <summary>
    /// current executing number;
    /// </summary>
    public int CurrentCount { get; set; }
    /// <summary>Queues a task to the scheduler.</summary> 
    /// <param name="task">The task to be queued.</param> 
    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)
        {
            Console.WriteLine("Task Count : {0} ", _tasks.Count);
            _tasks.AddLast(task);
            if (_delegatesQueuedOrRunning < _maxDegreeOfParallelism)
            {
                ++_delegatesQueuedOrRunning;
                NotifyThreadPoolOfPendingWork();
            }
        }
    }
    int executingCount = 0;
    private static object executeLock = new object();
    /// <summary> 
    /// Informs the ThreadPool that there's work to be executed for this scheduler. 
    /// </summary> 
    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);
    }
    /// <summary>Attempts to execute the specified task on the current thread.</summary> 
    /// <param name="task">The task to be executed.</param> 
    /// <param name="taskWasPreviouslyQueued"></param> 
    /// <returns>Whether the task could be executed on the current thread.</returns> 
    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);
    }
    /// <summary>Attempts to remove a previously scheduled task from the scheduler.</summary> 
    /// <param name="task">The task to be removed.</param> 
    /// <returns>Whether the task could be found and removed.</returns> 
    protected sealed override bool TryDequeue(Task task)
    {
        lock (_tasks) return _tasks.Remove(task);
    }
    /// <summary>Gets the maximum concurrency level supported by this scheduler.</summary> 
    public sealed override int MaximumConcurrencyLevel { get { return _maxDegreeOfParallelism; } }
    /// <summary>Gets an enumerable of the tasks currently scheduled on this scheduler.</summary> 
    /// <returns>An enumerable of the tasks currently scheduled.</returns> 
    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);
        }
    }
}

static void Main(string[] args)
{
        TaskFactory fac = new TaskFactory(new LimitedConcurrencyLevelTaskScheduler(5));
        //TaskFactory fac = new TaskFactory();
        for (int i = 0; i < 1000; i++)
        {
            fac.StartNew(s => {
                Thread.Sleep(1000);
                Console.WriteLine("Current Index {0}, ThreadId {1}",s,Thread.CurrentThread.ManagedThreadId);
            }, i);
        }
        Console.ReadKey();
}

/// <summary>Queues a task to the scheduler.</summary> 
    /// <param name="task">The task to be queued.</param> 
    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)
        {
            Console.WriteLine("Task Count : {0} ", _tasks.Count);
            _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);
    }