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TaskScheduler Class

Represents an object that handles the low-level work of queuing tasks onto threads.

System.Object
  System.Threading.Tasks.TaskScheduler

Namespace:  System.Threading.Tasks
Assembly:  mscorlib (in mscorlib.dll)

[PermissionSetAttribute(SecurityAction.InheritanceDemand, Unrestricted = true)]
[HostProtectionAttribute(SecurityAction.LinkDemand, Synchronization = true, 
	ExternalThreading = true)]
public abstract class TaskScheduler

The TaskScheduler type exposes the following members.

  NameDescription
Protected methodTaskSchedulerInitializes the TaskScheduler.
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  NameDescription
Public propertyStatic memberCurrentGets the TaskScheduler associated with the currently executing task.
Public propertyStatic memberDefaultGets the default TaskScheduler instance that is provided by the .NET Framework.
Public propertyIdGets the unique ID for this TaskScheduler.
Public propertyMaximumConcurrencyLevelIndicates the maximum concurrency level this TaskScheduler is able to support.
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  NameDescription
Public methodEquals(Object)Determines whether the specified Object is equal to the current Object. (Inherited from Object.)
Protected methodFinalizeFrees all resources associated with this scheduler. (Overrides Object.Finalize().)
Public methodStatic memberFromCurrentSynchronizationContextCreates a TaskScheduler associated with the current System.Threading.SynchronizationContext.
Public methodGetHashCodeServes as a hash function for a particular type. (Inherited from Object.)
Protected methodGetScheduledTasksFor debugger support only, generates an enumerable of Task instances currently queued to the scheduler waiting to be executed.
Public methodGetTypeGets the Type of the current instance. (Inherited from Object.)
Protected methodMemberwiseCloneCreates a shallow copy of the current Object. (Inherited from Object.)
Protected methodQueueTaskQueues a Task to the scheduler.
Public methodToStringReturns a string that represents the current object. (Inherited from Object.)
Protected methodTryDequeueAttempts to dequeue a Task that was previously queued to this scheduler.
Protected methodTryExecuteTaskAttempts to execute the provided Task on this scheduler.
Protected methodTryExecuteTaskInlineDetermines whether the provided Task can be executed synchronously in this call, and if it can, executes it.
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  NameDescription
Public eventStatic memberUnobservedTaskExceptionOccurs when a faulted Task's unobserved exception is about to trigger exception escalation policy, which, by default, would terminate the process.
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TaskScheduler acts as the extension point for all pluggable scheduling logic. This includes mechanisms such as how to schedule a task for execution, and how scheduled tasks should be exposed to debuggers.

For more information and code examples, see Task Schedulers.

Several sample task schedulers are available on Code Gallery: Samples for Parallel Programming with the .NET Framework 4.

NoteNote

The HostProtectionAttribute attribute applied to this type or member has the following Resources property value: Synchronization | ExternalThreading. The HostProtectionAttribute does not affect desktop applications (which are typically started by double-clicking an icon, typing a command, or entering a URL in a browser). For more information, see the HostProtectionAttribute class or SQL Server Programming and Host Protection Attributes.

The following example is taken from the Parallel Extensions Samples on the MSDN Code Gallery Web site.


namespace System.Threading.Tasks.Schedulers
{

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

    class Program
    {
        static void Main()
        {
            LimitedConcurrencyLevelTaskScheduler lcts = new LimitedConcurrencyLevelTaskScheduler(1);
            TaskFactory factory = new TaskFactory(lcts);

            factory.StartNew(()=> 
                {
                    for (int i = 0; i < 500; i++)
                    {
                        Console.Write("{0} on thread {1}", i, Thread.CurrentThread.ManagedThreadId);
                    }
                }
            );

            Console.ReadKey();
        }
    }

    /// <summary>
    /// Provides a task scheduler that ensures a maximum concurrency level while
    /// running on top of the ThreadPool.
    /// </summary>
    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>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)
            {
                _tasks.AddLast(task);
                if (_delegatesQueuedOrRunning < _maxDegreeOfParallelism)
                {
                    ++_delegatesQueuedOrRunning;
                    NotifyThreadPoolOfPendingWork();
                }
            }
        }

        /// <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);
            }
        }
    }
}


.NET Framework

Supported in: 4

.NET Framework Client Profile

Supported in: 4

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

All members of the abstract TaskScheduler type are thread-safe and may be used from multiple threads concurrently.

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