C# 可指定并行度任务调度器

可指定并行度的任务调度器

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/// <summary>
/// 指定最大并行度的任务调度器
/// </summary>
public class SpecifyDegreeOfParallelismTaskScheduler : TaskScheduler
{
    /// <summary>
    /// 信号量锁
    /// </summary>
    private static System.Threading.SemaphoreSlim _lock = new System.Threading.SemaphoreSlim(1);

    /// <summary>
    /// 当前线程是否正在处理任务
    /// </summary>
    [ThreadStatic]
    private static bool _currentThreadIsProcessingItems;

    /// <summary>
    /// 执行的任务队列
    /// </summary>
    private readonly LinkedList<Task> _tasks = new LinkedList<Task>();

    /// <summary>
    /// 指定的最大并行度
    /// </summary>
    private readonly int _maxDegressOfParallelism;

    /// <summary>
    ///当前调度器中正在执行的任务数
    /// </summary>
    private int _runingTasks = 0;

    /// <summary>
    /// 指示此调度器能够支持的最大并发级别。
    /// </summary>
    public override int MaximumConcurrencyLevel { get { return this._maxDegressOfParallelism; } }

    /// <summary>
    /// 初始化一个可指定最大并行度的任务调度器
    /// </summary>
    /// <param name="maxDegreeOfParallelism">最大并行度</param>
    public SpecifyDegreeOfParallelismTaskScheduler(int maxDegreeOfParallelism)
    {
        if (maxDegreeOfParallelism < 1)
            throw new ArgumentOutOfRangeException("maxDegreeOfParallelism至少为1");
        this._maxDegressOfParallelism = maxDegreeOfParallelism;
    }

    /// <summary>
    /// 将Task排队到调度器中
    /// </summary>
    /// <param name="task">要排队的任务</param>
    protected override void QueueTask(Task task)
    {
        _lock.Wait();
        try
        {
            this._tasks.AddLast(task);
            if (this._runingTasks < this._maxDegressOfParallelism)
            {
                ++this._runingTasks;
                ConsumeTaskOfPending();
            }
        }
        finally
        {
            _lock.Release();
        }
    }

    /// <summary>
    /// 尝试在当前线程上执行指定的任务
    /// </summary>
    /// <param name="task">被执行的任务</param>
    /// <param name="taskWasPreviouslyQueued">指定的任务之前是否已经排队</param>
    /// <returns>是否能在当前线程执行此任务</returns>
    protected override bool TryExecuteTaskInline(Task task, bool taskWasPreviouslyQueued)
    {
        //如果当前前程没有正在处理项目，无法内联
        if (!_currentThreadIsProcessingItems)
            return false;

        //如果任务之前已经被排队，将其从队列中删除
        if (taskWasPreviouslyQueued)
            TryDequeue(task);
        return base.TryExecuteTask(task);
    }

    /// <summary>
    /// 消费队列中等待的任务
    /// </summary>
    private void ConsumeTaskOfPending()
    {
        ThreadPool.UnsafeQueueUserWorkItem(p =>
        {
            _currentThreadIsProcessingItems = true;
            try
            {
                while (true)
                {
                    Task item;
                    _lock.Wait();
                    try
                    {
                        if (this._tasks.Count == 0)
                        {
                            --this._runingTasks;
                            break;
                        }
                        item = this._tasks.First.Value;
                        this._tasks.RemoveFirst();
                    }
                    finally
                    {
                        _lock.Release();
                    }
                    base.TryExecuteTask(item);
                }
            }
            finally
            {
                _currentThreadIsProcessingItems = false;
            }
        }, null);
    }

    /// <summary>
    /// 尝试将任务从队列移除
    /// </summary>
    /// <param name="task">要移除的任务</param>
    /// <returns>是否成功将任务从队列中移除</returns>
    protected override bool TryDequeue(Task task)
    {
        _lock.Wait();
        try
        {
            return this._tasks.Remove(task);
        }
        finally
        {
            _lock.Release();
        }
    }

    /// <summary>
    /// 获取当前调度器中已调度任务序列
    /// </summary>
    /// <returns>可遍历已调度任务序列</returns>
    protected override IEnumerable<Task> GetScheduledTasks()
    {
        _lock.Wait();
        try
        {
            return this._tasks.ToArray();
        }
        finally
        {
            _lock.Release();
        }
    }

}