TaskScheduler Class

Definition

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

public ref class TaskScheduler abstract
public abstract class TaskScheduler
type TaskScheduler = class
Public MustInherit Class TaskScheduler
Inheritance
TaskScheduler

Examples

The following example creates a custom task scheduler that limits the number of threads used by the app. It then launches two sets of tasks and displays information about the task and the thread on which the task is executing.

using System;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;

class Example
{
   static void Main()
   {
       // Create a scheduler that uses two threads.
       LimitedConcurrencyLevelTaskScheduler lcts = new LimitedConcurrencyLevelTaskScheduler(2);
       List<Task> tasks = new List<Task>();

       // Create a TaskFactory and pass it our custom scheduler.
       TaskFactory factory = new TaskFactory(lcts);
       CancellationTokenSource cts = new CancellationTokenSource();

       // Use our factory to run a set of tasks.
       Object lockObj = new Object();
       int outputItem = 0;

       for (int tCtr = 0; tCtr <= 4; tCtr++) {
          int iteration = tCtr;
          Task t = factory.StartNew(() => {
                                       for (int i = 0; i < 1000; i++) {
                                          lock (lockObj) {
                                             Console.Write("{0} in task t-{1} on thread {2}   ",
                                                           i, iteration, Thread.CurrentThread.ManagedThreadId);
                                             outputItem++;
                                             if (outputItem % 3 == 0)
                                                Console.WriteLine();
                                          }
                                       }
                                    }, cts.Token);
          tasks.Add(t);
      }
      // Use it to run a second set of tasks.
      for (int tCtr = 0; tCtr <= 4; tCtr++) {
         int iteration = tCtr;
         Task t1 = factory.StartNew(() => {
                                       for (int outer = 0; outer <= 10; outer++) {
                                          for (int i = 0x21; i <= 0x7E; i++) {
                                             lock (lockObj) {
                                                Console.Write("'{0}' in task t1-{1} on thread {2}   ",
                                                              Convert.ToChar(i), iteration, Thread.CurrentThread.ManagedThreadId);
                                                outputItem++;
                                                if (outputItem % 3 == 0)
                                                   Console.WriteLine();
                                             }
                                          }
                                       }
                                    }, cts.Token);
         tasks.Add(t1);
      }

      // Wait for the tasks to complete before displaying a completion message.
      Task.WaitAll(tasks.ToArray());
      cts.Dispose();
      Console.WriteLine("\n\nSuccessful completion.");
   }
}

// Provides a task scheduler that ensures a maximum concurrency level while
// running on top of the thread pool.
public class LimitedConcurrencyLevelTaskScheduler : TaskScheduler
{
   // Indicates whether the current thread is processing work items.
   [ThreadStatic]
   private static bool _currentThreadIsProcessingItems;

  // The list of tasks to be executed
   private readonly LinkedList<Task> _tasks = new LinkedList<Task>(); // protected by lock(_tasks)

   // The maximum concurrency level allowed by this scheduler.
   private readonly int _maxDegreeOfParallelism;

   // Indicates whether the scheduler is currently processing work items.
   private int _delegatesQueuedOrRunning = 0;

   // Creates a new instance with the specified degree of parallelism.
   public LimitedConcurrencyLevelTaskScheduler(int maxDegreeOfParallelism)
   {
       if (maxDegreeOfParallelism < 1) throw new ArgumentOutOfRangeException("maxDegreeOfParallelism");
       _maxDegreeOfParallelism = maxDegreeOfParallelism;
   }

   // Queues a task to the scheduler.
   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();
           }
       }
   }

   // Inform the ThreadPool that there's work to be executed for this scheduler.
   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);
   }

   // Attempts to execute the specified task on the current thread.
   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)
          // Try to run the task.
          if (TryDequeue(task))
            return base.TryExecuteTask(task);
          else
             return false;
       else
          return base.TryExecuteTask(task);
   }

   // Attempt to remove a previously scheduled task from the scheduler.
   protected sealed override bool TryDequeue(Task task)
   {
       lock (_tasks) return _tasks.Remove(task);
   }

   // Gets the maximum concurrency level supported by this scheduler.
   public sealed override int MaximumConcurrencyLevel { get { return _maxDegreeOfParallelism; } }

   // Gets an enumerable of the tasks currently scheduled on this scheduler.
   protected sealed override IEnumerable<Task> GetScheduledTasks()
   {
       bool lockTaken = false;
       try
       {
           Monitor.TryEnter(_tasks, ref lockTaken);
           if (lockTaken) return _tasks;
           else throw new NotSupportedException();
       }
       finally
       {
           if (lockTaken) Monitor.Exit(_tasks);
       }
   }
}
// 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
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())
      cts.Dispose()
      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

Remarks

For more information about this API, see Supplemental API remarks for TaskScheduler.

Constructors

TaskScheduler()

Initializes the TaskScheduler.

Properties

Current

Gets the TaskScheduler associated with the currently executing task.

Default

Gets the default TaskScheduler instance that is provided by .NET.

Id

Gets the unique ID for this TaskScheduler.

MaximumConcurrencyLevel

Indicates the maximum concurrency level this TaskScheduler is able to support.

Methods

Equals(Object)

Determines whether the specified object is equal to the current object.

(Inherited from Object)
Finalize()

Frees all resources associated with this scheduler.

FromCurrentSynchronizationContext()

Creates a TaskScheduler associated with the current SynchronizationContext.

GetHashCode()

Serves as the default hash function.

(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(Task)

Queues a Task to the scheduler.

ToString()

Returns a string that represents the current object.

(Inherited from Object)
TryDequeue(Task)

Attempts to dequeue a Task that was previously queued to this scheduler.

TryExecuteTask(Task)

Attempts to execute the provided Task on this scheduler.

TryExecuteTaskInline(Task, Boolean)

Determines whether the provided Task can be executed synchronously in this call, and if it can, executes it.

Events

UnobservedTaskException

Occurs when a faulted task's unobserved exception is about to trigger exception escalation policy, which, by default, would terminate the process.

Applies to

Thread Safety

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

See also