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)

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

The TaskScheduler type exposes the following members.

  NameDescription
Protected methodSupported by Portable Class LibrarySupported in .NET for Windows Store appsTaskSchedulerInitializes the TaskScheduler.
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  NameDescription
Public propertyStatic memberSupported by Portable Class LibrarySupported in .NET for Windows Store appsCurrentGets the TaskScheduler associated with the currently executing task.
Public propertyStatic memberSupported by Portable Class LibrarySupported in .NET for Windows Store appsDefaultGets the default TaskScheduler instance that is provided by the .NET Framework.
Public propertySupported by Portable Class LibrarySupported in .NET for Windows Store appsIdGets the unique ID for this TaskScheduler.
Public propertySupported by Portable Class LibrarySupported in .NET for Windows Store appsMaximumConcurrencyLevelIndicates the maximum concurrency level this TaskScheduler is able to support.
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  NameDescription
Public methodSupported by Portable Class LibrarySupported in .NET for Windows Store appsEquals(Object)Determines whether the specified object is equal to the current object. (Inherited from Object.)
Protected methodSupported by Portable Class LibraryFinalizeAllows an object to try to free resources and perform other cleanup operations before it is reclaimed by garbage collection. (Inherited from Object.)

In .NET Framework Client Profile, this member is overridden by Finalize.


In Portable Class Library, this member is overridden by Finalize.
Public methodStatic memberSupported by Portable Class LibrarySupported in .NET for Windows Store appsFromCurrentSynchronizationContextCreates a TaskScheduler associated with the current System.Threading.SynchronizationContext.
Public methodSupported by Portable Class LibrarySupported in .NET for Windows Store appsGetHashCodeServes as the default hash function. (Inherited from Object.)
Protected methodSupported by Portable Class LibrarySupported in .NET for Windows Store appsGetScheduledTasksFor debugger support only, generates an enumerable of Task instances currently queued to the scheduler waiting to be executed.
Public methodSupported by Portable Class LibrarySupported in .NET for Windows Store appsGetTypeGets the Type of the current instance. (Inherited from Object.)
Protected methodSupported by Portable Class LibrarySupported in .NET for Windows Store appsMemberwiseCloneCreates a shallow copy of the current Object. (Inherited from Object.)
Protected methodSupported by Portable Class LibrarySupported in .NET for Windows Store appsQueueTaskQueues a Task to the scheduler.
Public methodSupported by Portable Class LibrarySupported in .NET for Windows Store appsToStringReturns a string that represents the current object. (Inherited from Object.)
Protected methodSupported by Portable Class LibrarySupported in .NET for Windows Store appsTryDequeueAttempts to dequeue a Task that was previously queued to this scheduler.
Protected methodSupported by Portable Class LibrarySupported in .NET for Windows Store appsTryExecuteTaskAttempts to execute the provided Task on this scheduler.
Protected methodSupported by Portable Class LibrarySupported in .NET for Windows Store appsTryExecuteTaskInlineDetermines whether the provided Task can be executed synchronously in this call, and if it can, executes it.
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  NameDescription
Public eventStatic memberSupported by Portable Class LibrarySupported in .NET for Windows Store appsUnobservedTaskExceptionOccurs 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 Samples for Parallel Programming with the .NET Framework 4 on the MSDN Code Gallery Web site.

Imports System.Collections.Generic
Imports System.Threading
Imports System.Threading.Tasks

Module Example
   Sub Main()
      ' Create a scheduler that uses two threads.  
      Dim lcts As New LimitedConcurrencyLevelTaskScheduler(2)
      Dim tasks As New List(Of Task)()

      ' 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 set of tasks.  
      Dim objLock As New Object()      
      Dim outputItem As Integer  
      For tCtr As Integer = 0 To 4
         Dim iteration As Integer = tCtr
         Dim t As Task = factory.StartNew(Sub()
                                             For i As Integer = 1 To 1000
                                                SyncLock objLock
                                                   Console.Write("{0} in task t-{1} on thread {2}   ", 
                                                   i, iteration, Thread.CurrentThread.ManagedThreadId)
                                                   outputItem += 1
                                                   If outputItem Mod 3 = 0 Then Console.WriteLine()
                                                End SyncLock 
                                             Next  
                                          End Sub,
                                cts.Token)
         tasks.Add(t)
      Next  
      ' Use it to run a second set of tasks.                        
      For tCtr As Integer = 0 To 4
         Dim iteration As Integer = tCtr
         Dim t1 As Task = factory.StartNew(Sub()
                                              For outer As Integer = 0 To 10
                                                 For i As Integer = &h21 To &h7E
                                                    SyncLock objLock
                                                       Console.Write("'{0}' in task t1-{1} on thread {2}   ", 
                                                                     Convert.ToChar(i), iteration, Thread.CurrentThread.ManagedThreadId)
                                                       outputItem += 1
                                                       If outputItem Mod 3 = 0 Then Console.WriteLine()
                                                    End SyncLock  
                                                 Next      
                                              Next                                            
                                           End Sub,
                                cts.Token)           
         tasks.Add(t1)
      Next 

      ' Wait for the tasks to complete before displaying a completion message.
      Task.WaitAll(tasks.ToArray())
      Console.WriteLine(vbCrLf + vbCrLf + "Successful completion.")
   End Sub  
End Module 

' Provides a task scheduler that ensures a maximum concurrency level while  
' running on top of the thread pool. 
Public Class LimitedConcurrencyLevelTaskScheduler : Inherits TaskScheduler
   ' Indicates whether the current thread is processing work items.
   <ThreadStatic()> Private Shared _currentThreadIsProcessingItems As Boolean  

   ' The list of tasks to be executed  
   Private ReadOnly _tasks As LinkedList(Of Task) = New LinkedList(Of Task)() 

   'The maximum concurrency level allowed by this scheduler.  
   Private ReadOnly _maxDegreeOfParallelism As Integer  

   ' Indicates whether the scheduler is currently processing work items.  
   Private _delegatesQueuedOrRunning As Integer = 0 ' protected by lock(_tasks)

   ' Creates a new instance with the specified degree of parallelism.  
   Public Sub New(ByVal maxDegreeOfParallelism As Integer)
      If (maxDegreeOfParallelism < 1) Then  
         Throw New ArgumentOutOfRangeException("maxDegreeOfParallelism")
      End If
         _maxDegreeOfParallelism = maxDegreeOfParallelism
   End Sub  

   ' Queues a task to the scheduler.  
   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  

   ' Inform the ThreadPool that there's work to be executed for this scheduler.  
   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  

   ' Attempts to execute the specified task on the current thread.  
   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 
         ' Try to run the task.  
         If TryDequeue(t) Then  
            Return MyBase.TryExecuteTask(t)
         Else 
            Return False  
         End If      
      Else  
         Return MyBase.TryExecuteTask(t)
      End If    
   End Function  

   ' Attempt to remove a previously scheduled task from the scheduler.  
   Protected Overrides Function TryDequeue(ByVal t As Task) As Boolean  
      SyncLock (_tasks)
         Return _tasks.Remove(t)
      End SyncLock  
   End Function  

   ' Gets the maximum concurrency level supported by this scheduler.  
   Public Overrides ReadOnly Property MaximumConcurrencyLevel As Integer  
      Get  
         Return _maxDegreeOfParallelism
      End Get  
   End Property  

   ' Gets an enumerable of the tasks currently scheduled on this scheduler.  
   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  
' The following is a portion of the output from a single run of the example: 
'    'T' in task t1-4 on thread 3   'U' in task t1-4 on thread 3   'V' in task t1-4 on thread 3    
'    'W' in task t1-4 on thread 3   'X' in task t1-4 on thread 3   'Y' in task t1-4 on thread 3    
'    'Z' in task t1-4 on thread 3   '[' in task t1-4 on thread 3   '\' in task t1-4 on thread 3    
'    ']' in task t1-4 on thread 3   '^' in task t1-4 on thread 3   '_' in task t1-4 on thread 3    
'    '`' in task t1-4 on thread 3   'a' in task t1-4 on thread 3   'b' in task t1-4 on thread 3    
'    'c' in task t1-4 on thread 3   'd' in task t1-4 on thread 3   'e' in task t1-4 on thread 3    
'    'f' in task t1-4 on thread 3   'g' in task t1-4 on thread 3   'h' in task t1-4 on thread 3    
'    'i' in task t1-4 on thread 3   'j' in task t1-4 on thread 3   'k' in task t1-4 on thread 3    
'    'l' in task t1-4 on thread 3   'm' in task t1-4 on thread 3   'n' in task t1-4 on thread 3    
'    'o' in task t1-4 on thread 3   'p' in task t1-4 on thread 3   ']' in task t1-2 on thread 4    
'    '^' in task t1-2 on thread 4   '_' in task t1-2 on thread 4   '`' in task t1-2 on thread 4    
'    'a' in task t1-2 on thread 4   'b' in task t1-2 on thread 4   'c' in task t1-2 on thread 4    
'    'd' in task t1-2 on thread 4   'e' in task t1-2 on thread 4   'f' in task t1-2 on thread 4    
'    'g' in task t1-2 on thread 4   'h' in task t1-2 on thread 4   'i' in task t1-2 on thread 4    
'    'j' in task t1-2 on thread 4   'k' in task t1-2 on thread 4   'l' in task t1-2 on thread 4    
'    'm' in task t1-2 on thread 4   'n' in task t1-2 on thread 4   'o' in task t1-2 on thread 4    
'    'p' in task t1-2 on thread 4   'q' in task t1-2 on thread 4   'r' in task t1-2 on thread 4    
'    's' in task t1-2 on thread 4   't' in task t1-2 on thread 4   'u' in task t1-2 on thread 4    
'    'v' in task t1-2 on thread 4   'w' in task t1-2 on thread 4   'x' in task t1-2 on thread 4    
'    'y' in task t1-2 on thread 4   'z' in task t1-2 on thread 4   '{' in task t1-2 on thread 4    
'    '|' in task t1-2 on thread 4   '}' in task t1-2 on thread 4   '~' in task t1-2 on thread 4    
'    'q' in task t1-4 on thread 3   'r' in task t1-4 on thread 3   's' in task t1-4 on thread 3    
'    't' in task t1-4 on thread 3   'u' in task t1-4 on thread 3   'v' in task t1-4 on thread 3    
'    'w' in task t1-4 on thread 3   'x' in task t1-4 on thread 3   'y' in task t1-4 on thread 3    
'    'z' in task t1-4 on thread 3   '{' in task t1-4 on thread 3   '|' in task t1-4 on thread 3  

.NET Framework

Supported in: 4.5.2, 4.5.1, 4.5, 4

.NET Framework Client Profile

Supported in: 4

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone 8, Silverlight 8.1

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

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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