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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
Assemblies:   mscorlib (in mscorlib.dll)
  System.Threading.Tasks (in System.Threading.Tasks.dll)

[DebuggerDisplayAttribute("Id={Id}")]
[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 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.

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  

.NET Framework

Supported in: 4.5.3, 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

Supported in: Windows Phone 8.1

Supported in: Windows Phone Silverlight 8.1

Supported in: Windows Phone Silverlight 8

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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