TaskScheduler Class
Represents an object that handles the low-level work of queuing tasks onto threads.
Assembly: mscorlib (in mscorlib.dll)
The TaskScheduler type exposes the following members.
| Name | Description | |
|---|---|---|
![]() ![]() | Current | Gets the TaskScheduler associated with the currently executing task. |
![]() ![]() | Default | Gets the default TaskScheduler instance that is provided by the .NET Framework. |
![]() | Id | Gets the unique ID for this TaskScheduler. |
![]() | MaximumConcurrencyLevel | Indicates the maximum concurrency level this TaskScheduler is able to support. |
| Name | Description | |
|---|---|---|
![]() | Equals(Object) | Determines whether the specified Object is equal to the current Object. (Inherited from Object.) |
![]() | Finalize | Frees all resources associated with this scheduler. (Overrides Object.Finalize.) |
![]() ![]() | FromCurrentSynchronizationContext | Creates a TaskScheduler associated with the current System.Threading.SynchronizationContext. |
![]() | GetHashCode | Serves as a hash function for a particular type. (Inherited from Object.) |
![]() | GetScheduledTasks | For debugger support only, generates an enumerable of Task instances currently queued to the scheduler waiting to be executed. |
![]() | GetType | Gets the Type of the current instance. (Inherited from Object.) |
![]() | MemberwiseClone | Creates a shallow copy of the current Object. (Inherited from Object.) |
![]() | QueueTask | Queues a Task to the scheduler. |
![]() | ToString | Returns a string that represents the current object. (Inherited from Object.) |
![]() | TryDequeue | Attempts to dequeue a Task that was previously queued to this scheduler. |
![]() | TryExecuteTask | Attempts to execute the provided Task on this scheduler. |
![]() | TryExecuteTaskInline | Determines whether the provided Task can be executed synchronously in this call, and if it can, executes it. |
| Name | Description | |
|---|---|---|
![]() ![]() | UnobservedTaskException | Occurs 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.
Note |
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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) Next End Sub, cts.Token) Console.WriteLine("Press any key to exit.") Console.ReadKey() 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> <ThreadStatic()> 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) _tasks.AddLast(t) If (_delegatesQueuedOrRunning < _maxDegreeOfParallelism) Then _delegatesQueuedOrRunning = _delegatesQueuedOrRunning + 1 NotifyThreadPoolOfPendingWork() End If End SyncLock End Sub ''' <summary> ''' Informs the ThreadPool that there's work to be executed for this scheduler. ''' </summary> Private Sub NotifyThreadPoolOfPendingWork() ThreadPool.UnsafeQueueUserWorkItem(Sub() ' 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) 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 _tasks.RemoveFirst() End SyncLock ' Execute the task we pulled out of the queue MyBase.TryExecuteTask(item) End While ' We're done processing items on the current thread Finally _currentThreadIsProcessingItems = False End Try End Sub, Nothing) 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 TryDequeue(t) 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 Get 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 Try Monitor.TryEnter(_tasks, lockTaken) If (lockTaken) Then Return _tasks.ToArray() Else Throw New NotSupportedException() End If Finally If (lockTaken) Then Monitor.Exit(_tasks) End If End Try End Function End Class End Module
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.





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