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TaskScheduler Class

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


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

<PermissionSetAttribute(SecurityAction.InheritanceDemand, Unrestricted := True)> _
<HostProtectionAttribute(SecurityAction.LinkDemand, Synchronization := True,  _
	ExternalThreading := True)> _
Public MustInherit Class TaskScheduler

The TaskScheduler type exposes the following members.

Protected methodTaskSchedulerInitializes the TaskScheduler.

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.

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.

Public eventStatic memberUnobservedTaskExceptionOccurs when a faulted Task's unobserved exception is about to trigger exception escalation policy, which, by default, would terminate the process.

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.


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.

Imports System
Imports System.Collections.Generic
Imports System.Linq
Imports System.Threading
Imports System.Threading.Tasks
Module Module2

    Sub Main()

        ' Create a scheduler that uses only one thread.
        Dim lcts As New LimitedConcurrencyLevelTaskScheduler(1)

        ' Create a TaskFactory and pass it our custom scheduler.
        Dim factory As New TaskFactory(lcts)
        Dim cts As New CancellationTokenSource()

        ' Use our factory to run a task.
        Dim t As Task = factory.StartNew(Sub()
                                             For i As Integer = 1 To 50000
                                                 Console.Write("{0} on thread {1}.   ", i, Thread.CurrentThread.ManagedThreadId)
                                         End Sub,

        Console.WriteLine("Press any key to exit.")

    End Sub

    ''' <summary>
    ''' Provides a task scheduler that ensures a maximum concurrency level While
    ''' running on top of the ThreadPool.
    ''' </summary>
    Public Class LimitedConcurrencyLevelTaskScheduler
        Inherits TaskScheduler

        ''' <summary>Whether the current thread is processing work items.</summary>
        Private Shared _currentThreadIsProcessingItems As Boolean
        ''' <summary>The list of tasks to be executed.</summary>
        Private ReadOnly _tasks As LinkedList(Of Task) = New LinkedList(Of Task)() ' protected by lock(_tasks)
        ''' <summary>The maximum concurrency level allowed by this scheduler.</summary>
        Private ReadOnly _maxDegreeOfParallelism As Integer
        ''' <summary>Whether the scheduler is currently processing work items.</summary>
        Private _delegatesQueuedOrRunning As Integer = 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 Sub New(ByVal maxDegreeOfParallelism As Integer)

            If (maxDegreeOfParallelism < 1) Then
                Throw New ArgumentOutOfRangeException("maxDegreeOfParallelism")
            End If
            _maxDegreeOfParallelism = maxDegreeOfParallelism

        End Sub
        ''' <summary>Queues a task to the scheduler.</summary>
        ''' <param name="t">The task to be queued.</param>
        Protected Overrides Sub QueueTask(ByVal t As 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.
            SyncLock (_tasks)

                If (_delegatesQueuedOrRunning < _maxDegreeOfParallelism) Then

                    _delegatesQueuedOrRunning = _delegatesQueuedOrRunning + 1
                End If
            End SyncLock
        End Sub

        ''' <summary>
        ''' Informs the ThreadPool that there's work to be executed for this scheduler.
        ''' </summary>
        Private Sub NotifyThreadPoolOfPendingWork()

                                                   ' Note that the current thread is now processing work items.
                                                   ' This is necessary to enable inlining of tasks into this thread.
                                                   _currentThreadIsProcessingItems = True

                                                       ' Process all available items in the queue.
                                                       While (True)
                                                           Dim item As Task
                                                           SyncLock (_tasks)
                                                               ' When there are no more items to be processed,
                                                               ' note that we're done processing, and get out.
                                                               If (_tasks.Count = 0) Then
                                                                   _delegatesQueuedOrRunning = _delegatesQueuedOrRunning - 1
                                                                   Exit While
                                                               End If

                                                               ' Get the next item from the queue
                                                               item = _tasks.First.Value
                                                           End SyncLock

                                                           ' Execute the task we pulled out of the queue

                                                       End While
                                                       ' We're done processing items on the current thread
                                                       _currentThreadIsProcessingItems = False
                                                   End Try
                                               End Sub,
        End Sub

        ''' <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 Overrides Function TryExecuteTaskInline(ByVal t As Task, ByVal taskWasPreviouslyQueued As Boolean) As Boolean

            ' If this thread isn't already processing a task, we don't support inlining
            If (Not _currentThreadIsProcessingItems) Then
                Return False
            End If

            ' If the task was previously queued, remove it from the queue
            If (taskWasPreviouslyQueued) Then
            End If
            ' Try to run the task.
            Return MyBase.TryExecuteTask(t)
        End Function

        ''' <summary>Attempts to remove a previously scheduled task from the scheduler.</summary>
        ''' <param name="t">The task to be removed.</param>
        ''' <returns>Whether the task could be found and removed.</returns>
        Protected Overrides Function TryDequeue(ByVal t As Task) As Boolean

            SyncLock (_tasks)
                Return _tasks.Remove(t)
            End SyncLock

        End Function

        ''' <summary>Gets the maximum concurrency level supported by this scheduler.</summary>
        Public Overrides ReadOnly Property MaximumConcurrencyLevel As Integer

                Return _maxDegreeOfParallelism
            End Get
        End Property

        ''' <summary>Gets an enumerable of the tasks currently scheduled on this scheduler.</summary>
        ''' <returns>An enumerable of the tasks currently scheduled.</returns>
        Protected Overrides Function GetScheduledTasks() As IEnumerable(Of Task)

            Dim lockTaken As Boolean = False

                Monitor.TryEnter(_tasks, lockTaken)
                If (lockTaken) Then
                    Return _tasks.ToArray()
                    Throw New NotSupportedException()
                End If

                If (lockTaken) Then
                End If
            End Try
        End Function
    End Class
End Module

.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.