Task Class

 

Represents an asynchronous operation.

To browse the .NET Framework source code for this type, see the Reference Source.

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


[HostProtectionAttribute(SecurityAction.LinkDemand, Synchronization = true, 
	ExternalThreading = true)]
public class Task : IAsyncResult, IDisposable

NameDescription
System_CAPS_pubmethodTask(Action)

Initializes a new Task with the specified action.

System_CAPS_pubmethodTask(Action, CancellationToken)

Initializes a new Task with the specified action and CancellationToken.

System_CAPS_pubmethodTask(Action, CancellationToken, TaskCreationOptions)

Initializes a new Task with the specified action and creation options.

System_CAPS_pubmethodTask(Action, TaskCreationOptions)

Initializes a new Task with the specified action and creation options.

System_CAPS_pubmethodTask(Action<Object>, Object)

Initializes a new Task with the specified action and state.

System_CAPS_pubmethodTask(Action<Object>, Object, CancellationToken)

Initializes a new Task with the specified action, state, and options.

System_CAPS_pubmethodTask(Action<Object>, Object, CancellationToken, TaskCreationOptions)

Initializes a new Task with the specified action, state, and options.

System_CAPS_pubmethodTask(Action<Object>, Object, TaskCreationOptions)

Initializes a new Task with the specified action, state, and options.

NameDescription
System_CAPS_pubpropertyAsyncState

Gets the state object supplied when the Task was created, or null if none was supplied.

System_CAPS_pubpropertySystem_CAPS_staticCompletedTask

Gets a task that has already completed successfully.

System_CAPS_pubpropertyCreationOptions

Gets the TaskCreationOptions used to create this task.

System_CAPS_pubpropertySystem_CAPS_staticCurrentId

Returns the ID of the currently executing Task.

System_CAPS_pubpropertyException

Gets the AggregateException that caused the Task to end prematurely. If the Task completed successfully or has not yet thrown any exceptions, this will return null.

System_CAPS_pubpropertySystem_CAPS_staticFactory

Provides access to factory methods for creating and configuring Task and Task<TResult> instances.

System_CAPS_pubpropertyId

Gets an ID for this Task instance.

System_CAPS_pubpropertyIsCanceled

Gets whether this Task instance has completed execution due to being canceled.

System_CAPS_pubpropertyIsCompleted

Gets whether this Task has completed.

System_CAPS_pubpropertyIsFaulted

Gets whether the Task completed due to an unhandled exception.

System_CAPS_pubpropertyStatus

Gets the TaskStatus of this task.

NameDescription
System_CAPS_pubmethodConfigureAwait(Boolean)

Configures an awaiter used to await this Task.

System_CAPS_pubmethodContinueWith(Action<Task, Object>, Object)

Creates a continuation that receives caller-supplied state information and executes when the target Task completes.

System_CAPS_pubmethodContinueWith(Action<Task, Object>, Object, CancellationToken)

Creates a continuation that receives caller-supplied state information and a cancellation token and that executes asynchronously when the target Task completes.

System_CAPS_pubmethodContinueWith(Action<Task, Object>, Object, CancellationToken, TaskContinuationOptions, TaskScheduler)

Creates a continuation that receives caller-supplied state information and a cancellation token and that executes when the target Task completes. The continuation executes based on a set of specified conditions and uses a specified scheduler.

System_CAPS_pubmethodContinueWith(Action<Task, Object>, Object, TaskContinuationOptions)

Creates a continuation that receives caller-supplied state information and executes when the target Task completes. The continuation executes based on a set of specified conditions.

System_CAPS_pubmethodContinueWith(Action<Task, Object>, Object, TaskScheduler)

Creates a continuation that receives caller-supplied state information and executes asynchronously when the target Task completes. The continuation uses a specified scheduler.

System_CAPS_pubmethodContinueWith(Action<Task>)

Creates a continuation that executes asynchronously when the target Task completes.

System_CAPS_pubmethodContinueWith(Action<Task>, CancellationToken)

Creates a continuation that receives a cancellation token and executes asynchronously when the target Task completes.

System_CAPS_pubmethodContinueWith(Action<Task>, CancellationToken, TaskContinuationOptions, TaskScheduler)

Creates a continuation that executes when the target task competes according to the specified TaskContinuationOptions. The continuation receives a cancellation token and uses a specified scheduler.

System_CAPS_pubmethodContinueWith(Action<Task>, TaskContinuationOptions)

Creates a continuation that executes when the target task completes according to the specified TaskContinuationOptions.

System_CAPS_pubmethodContinueWith(Action<Task>, TaskScheduler)

Creates a continuation that executes asynchronously when the target Task completes. The continuation uses a specified scheduler.

System_CAPS_pubmethodContinueWith<TResult>(Func<Task, TResult>)

Creates a continuation that executes asynchronously when the target Task<TResult> completes and returns a value.

System_CAPS_pubmethodContinueWith<TResult>(Func<Task, TResult>, CancellationToken)

Creates a continuation that executes asynchronously when the target Task completes and returns a value. The continuation receives a cancellation token.

System_CAPS_pubmethodContinueWith<TResult>(Func<Task, TResult>, CancellationToken, TaskContinuationOptions, TaskScheduler)

Creates a continuation that executes according to the specified continuation options and returns a value. The continuation is passed a cancellation token and uses a specified scheduler.

System_CAPS_pubmethodContinueWith<TResult>(Func<Task, TResult>, TaskContinuationOptions)

Creates a continuation that executes according to the specified continuation options and returns a value.

System_CAPS_pubmethodContinueWith<TResult>(Func<Task, TResult>, TaskScheduler)

Creates a continuation that executes asynchronously when the target Task completes and returns a value. The continuation uses a specified scheduler.

System_CAPS_pubmethodContinueWith<TResult>(Func<Task, Object, TResult>, Object)

Creates a continuation that receives caller-supplied state information and executes asynchronously when the target Task completes and returns a value.

System_CAPS_pubmethodContinueWith<TResult>(Func<Task, Object, TResult>, Object, CancellationToken)

Creates a continuation that executes asynchronously when the target Task completes and returns a value. The continuation receives caller-supplied state information and a cancellation token.

System_CAPS_pubmethodContinueWith<TResult>(Func<Task, Object, TResult>, Object, CancellationToken, TaskContinuationOptions, TaskScheduler)

Creates a continuation that executes based on the specified task continuation options when the target Task completes and returns a value. The continuation receives caller-supplied state information and a cancellation token and uses the specified scheduler.

System_CAPS_pubmethodContinueWith<TResult>(Func<Task, Object, TResult>, Object, TaskContinuationOptions)

Creates a continuation that executes based on the specified task continuation options when the target Task completes. The continuation receives caller-supplied state information.

System_CAPS_pubmethodContinueWith<TResult>(Func<Task, Object, TResult>, Object, TaskScheduler)

Creates a continuation that executes asynchronously when the target Task completes. The continuation receives caller-supplied state information and uses a specified scheduler.

System_CAPS_pubmethodSystem_CAPS_staticDelay(Int32)

Creates a task that completes after a time delay.

System_CAPS_pubmethodSystem_CAPS_staticDelay(Int32, CancellationToken)

Creates a cancellable task that completes after a time delay.

System_CAPS_pubmethodSystem_CAPS_staticDelay(TimeSpan)

Creates a task that completes after a specified time interval.

System_CAPS_pubmethodSystem_CAPS_staticDelay(TimeSpan, CancellationToken)

Creates a cancellable task that completes after a specified time interval.

System_CAPS_pubmethodDispose()

Releases all resources used by the current instance of the Task class.

System_CAPS_protmethodDispose(Boolean)

Disposes the Task, releasing all of its unmanaged resources.

System_CAPS_pubmethodEquals(Object)

Determines whether the specified object is equal to the current object.(Inherited from Object.)

System_CAPS_protmethodFinalize()

Allows an object to try to free resources and perform other cleanup operations before it is reclaimed by garbage collection.(Inherited from Object.)

System_CAPS_pubmethodSystem_CAPS_staticFromCanceled(CancellationToken)

Creates a Task that's completed due to cancellation with a specified cancellation token.

System_CAPS_pubmethodSystem_CAPS_staticFromCanceled<TResult>(CancellationToken)

Creates a Task<TResult> that's completed due to cancellation with a specified cancellation token.

System_CAPS_pubmethodSystem_CAPS_staticFromException(Exception)

Creates a Task that has completed with a specified exception.

System_CAPS_pubmethodSystem_CAPS_staticFromException<TResult>(Exception)

Creates a Task<TResult> that's completed with a specified exception.

System_CAPS_pubmethodSystem_CAPS_staticFromResult<TResult>(TResult)

Creates a Task<TResult> that's completed successfully with the specified result.

System_CAPS_pubmethodGetAwaiter()

Gets an awaiter used to await this Task.

System_CAPS_pubmethodGetHashCode()

Serves as the default hash function. (Inherited from Object.)

System_CAPS_pubmethodGetType()

Gets the Type of the current instance.(Inherited from Object.)

System_CAPS_protmethodMemberwiseClone()

Creates a shallow copy of the current Object.(Inherited from Object.)

System_CAPS_pubmethodSystem_CAPS_staticRun(Action)

Queues the specified work to run on the thread pool and returns a Task object that represents that work.

System_CAPS_pubmethodSystem_CAPS_staticRun(Action, CancellationToken)

Queues the specified work to run on the thread pool and returns a Task object that represents that work. A cancellation token allows the work to be cancelled.

System_CAPS_pubmethodSystem_CAPS_staticRun(Func<Task>)

Queues the specified work to run on the thread pool and returns a proxy for the task returned by function.

System_CAPS_pubmethodSystem_CAPS_staticRun(Func<Task>, CancellationToken)

Queues the specified work to run on the thread pool and returns a proxy for the task returned by function.

System_CAPS_pubmethodSystem_CAPS_staticRun<TResult>(Func<TResult>)

Queues the specified work to run on the thread pool and returns a Task<TResult> object that represents that work.

System_CAPS_pubmethodSystem_CAPS_staticRun<TResult>(Func<TResult>, CancellationToken)

Queues the specified work to run on the thread pool and returns a Task(TResult) object that represents that work. A cancellation token allows the work to be cancelled.

System_CAPS_pubmethodSystem_CAPS_staticRun<TResult>(Func<Task<TResult>>)

Queues the specified work to run on the thread pool and returns a proxy for the Task(TResult) returned by function.

System_CAPS_pubmethodSystem_CAPS_staticRun<TResult>(Func<Task<TResult>>, CancellationToken)

Queues the specified work to run on the thread pool and returns a proxy for the Task(TResult) returned by function.

System_CAPS_pubmethodRunSynchronously()

Runs the Task synchronously on the current TaskScheduler.

System_CAPS_pubmethodRunSynchronously(TaskScheduler)

Runs the Task synchronously on the TaskScheduler provided.

System_CAPS_pubmethodStart()

Starts the Task, scheduling it for execution to the current TaskScheduler.

System_CAPS_pubmethodStart(TaskScheduler)

Starts the Task, scheduling it for execution to the specified TaskScheduler.

System_CAPS_pubmethodToString()

Returns a string that represents the current object.(Inherited from Object.)

System_CAPS_pubmethodWait()

Waits for the Task to complete execution.

System_CAPS_pubmethodWait(CancellationToken)

Waits for the Task to complete execution. The wait terminates if a cancellation token is canceled before the task completes.

System_CAPS_pubmethodWait(Int32)

Waits for the Task to complete execution within a specified number of milliseconds.

System_CAPS_pubmethodWait(Int32, CancellationToken)

Waits for the Task to complete execution. The wait terminates if a timeout interval elapses or a cancellation token is canceled before the task completes.

System_CAPS_pubmethodWait(TimeSpan)

Waits for the Task to complete execution within a specified time interval.

System_CAPS_pubmethodSystem_CAPS_staticWaitAll(Task[])

Waits for all of the provided Task objects to complete execution.

System_CAPS_pubmethodSystem_CAPS_staticWaitAll(Task[], CancellationToken)

Waits for all of the provided Task objects to complete execution unless the wait is cancelled.

System_CAPS_pubmethodSystem_CAPS_staticWaitAll(Task[], Int32)

Waits for all of the provided Task objects to complete execution within a specified number of milliseconds.

System_CAPS_pubmethodSystem_CAPS_staticWaitAll(Task[], Int32, CancellationToken)

Waits for all of the provided Task objects to complete execution within a specified number of milliseconds or until the wait is cancelled.

System_CAPS_pubmethodSystem_CAPS_staticWaitAll(Task[], TimeSpan)

Waits for all of the provided cancellable Task objects to complete execution within a specified time interval.

System_CAPS_pubmethodSystem_CAPS_staticWaitAny(Task[])

Waits for any of the provided Task objects to complete execution.

System_CAPS_pubmethodSystem_CAPS_staticWaitAny(Task[], CancellationToken)

Waits for any of the provided Task objects to complete execution unless the wait is cancelled.

System_CAPS_pubmethodSystem_CAPS_staticWaitAny(Task[], Int32)

Waits for any of the provided Task objects to complete execution within a specified number of milliseconds.

System_CAPS_pubmethodSystem_CAPS_staticWaitAny(Task[], Int32, CancellationToken)

Waits for any of the provided Task objects to complete execution within a specified number of milliseconds or until a cancellation token is cancelled.

System_CAPS_pubmethodSystem_CAPS_staticWaitAny(Task[], TimeSpan)

Waits for any of the provided Task objects to complete execution within a specified time interval.

System_CAPS_pubmethodSystem_CAPS_staticWhenAll(IEnumerable<Task>)

Creates a task that will complete when all of the Task objects in an enumerable collection have completed.

System_CAPS_pubmethodSystem_CAPS_staticWhenAll(Task[])

Creates a task that will complete when all of the Task objects in an array have completed.

System_CAPS_pubmethodSystem_CAPS_staticWhenAll<TResult>(IEnumerable<Task<TResult>>)

Creates a task that will complete when all of the Task<TResult> objects in an enumerable collection have completed.

System_CAPS_pubmethodSystem_CAPS_staticWhenAll<TResult>(Task<TResult>[])

Creates a task that will complete when all of the Task<TResult> objects in an array have completed.

System_CAPS_pubmethodSystem_CAPS_staticWhenAny(IEnumerable<Task>)

Creates a task that will complete when any of the supplied tasks have completed.

System_CAPS_pubmethodSystem_CAPS_staticWhenAny(Task[])

Creates a task that will complete when any of the supplied tasks have completed.

System_CAPS_pubmethodSystem_CAPS_staticWhenAny<TResult>(IEnumerable<Task<TResult>>)

Creates a task that will complete when any of the supplied tasks have completed.

System_CAPS_pubmethodSystem_CAPS_staticWhenAny<TResult>(Task<TResult>[])

Creates a task that will complete when any of the supplied tasks have completed.

System_CAPS_pubmethodSystem_CAPS_staticYield()

Creates an awaitable task that asynchronously yields back to the current context when awaited.

NameDescription
System_CAPS_pubinterfaceSystem_CAPS_privpropertyIAsyncResult.AsyncWaitHandle

Gets a WaitHandle that can be used to wait for the task to complete.

System_CAPS_pubinterfaceSystem_CAPS_privpropertyIAsyncResult.CompletedSynchronously

Gets an indication of whether the operation completed synchronously.

NameDescription
System_CAPS_pubmethodDispatcherOperationWait()

Overloaded. Waits indefinitely for the underlying DispatcherOperation to complete.(Defined by TaskExtensions.)

System_CAPS_pubmethodDispatcherOperationWait(TimeSpan)

Overloaded. Waits for the specified amount of time for the underlying DispatcherOperation to complete.(Defined by TaskExtensions.)

System_CAPS_pubmethodIsDispatcherOperationTask()

Returns a value that indicates whether this Task is associated with a DispatcherOperation.(Defined by TaskExtensions.)

System_CAPS_noteNote

To view the .NET Framework source code for this type, see the Reference Source. You can browse through the source code online, download the reference for offline viewing, and step through the sources (including patches and updates) during debugging; see instructions.

The Task class represents a single operation that does not return a value and that usually executes asynchronously. Task objects are one of the central components of the task-based asynchronous pattern first introduced in the .NET Framework 4. Because the work performed by a Task object typically executes asynchronously on a thread pool thread rather than synchronously on the main application thread, you can use the Status property, as well as the IsCanceled, IsCompleted, and IsFaulted properties, to determine the state of a task. Most commonly, a lambda expression is used to specify the work that the task is to perform.

For operations that return values, you use the Task<TResult> class.

In this section:

Creating and executing a task
Separating task creation and execution
Waiting for one or more tasks to complete
Tasks and culture
For debugger developers

Task instances may be created in a variety of ways. The most common approach, which is available starting with the .NET Framework 4.5, is to call the static Run method. The Run method provides a simple way to start a task using default values and without requiring additional parameters. The following example uses the Run(Action) method to start a task that loops and then displays the number of loop iterations:

using System;
using System.Threading.Tasks;

public class Example
{
   public static void Main()
   {
      Task t = Task.Run( () => {
                                  // Just loop.
                                  int ctr = 0;
                                  for (ctr = 0; ctr <= 1000000; ctr++)
                                  {}
                                  Console.WriteLine("Finished {0} loop iterations",
                                                    ctr);
                               } );
      t.Wait();
   }
}
// The example displays the following output:
//        Finished 1000001 loop iterations

An alternative, and the most common method to start a task in .NET Framework 4, is the static TaskFactory.StartNew method. The Task.Factory property returns a TaskFactory object. Overloads of the TaskFactory.StartNew method let you specify parameters to pass to the task creation options and a task scheduler. The following example uses the TaskFactory.StartNew method to start a task. It is functionally equivalent to the code in the previous example.

using System;
using System.Threading.Tasks;

public class Example
{
   public static void Main()
   {
      Task t = Task.Factory.StartNew( () => {
                                  // Just loop.
                                  int ctr = 0;
                                  for (ctr = 0; ctr <= 1000000; ctr++)
                                  {}
                                  Console.WriteLine("Finished {0} loop iterations",
                                                    ctr);
                               } );
      t.Wait();
   }
}
// The example displays the following output:
//        Finished 1000001 loop iterations

For more complete examples, see Task Parallelism (Task Parallel Library).

The Task class also provides constructors that initialize the task but that do not schedule it for execution. For performance reasons, the Task.Run or TaskFactory.StartNew method is the preferred mechanism for creating and scheduling computational tasks, but for scenarios where creation and scheduling must be separated, you can use the constructors and then call the Task.Start method to schedule the task for execution at a later time.

Because tasks typically run asynchronously on a thread pool thread, the thread that creates and starts the task continues execution as soon as the task has been instantiated. In some cases, when the calling thread is the main application thread, the app may terminate before any the task actually begins execution. In others, your application's logic may require that the calling thread continue execution only when one or more tasks has completed execution. You can synchronize the execution of the calling thread and the asynchronous tasks it launches by calling a Wait method to wait for one or more tasks to complete.

To wait for a single task to complete, you can call its Task.Wait method. A call to the Wait method blocks the calling thread until the single class instance has completed execution.

The following example calls the parameterless Wait() method to wait unconditionally until a task completes. The task simulates work by calling the Thread.Sleep method to sleep for two seconds.

using System;   
using System.Threading;
using System.Threading.Tasks;

class Program
{
    static Random rand = new Random();

    static void Main()
    {
        // Wait on a single task with no timeout specified.
        Task taskA = Task.Run( () => Thread.Sleep(2000));
        Console.WriteLine("taskA Status: {0}", taskA.Status);
        try {
          taskA.Wait();
          Console.WriteLine("taskA Status: {0}", taskA.Status);
       } 
       catch (AggregateException) {
          Console.WriteLine("Exception in taskA.");
       }   
    }    
}
// The example displays output like the following:
//     taskA Status: WaitingToRun
//     taskA Status: RanToCompletion

You can also conditionally wait for a task to complete. The Wait(Int32) and Wait(TimeSpan) methods block the calling thread until the task finishes or a timeout interval elapses, whichever comes first. Since the following example launches a task that sleeps for two seconds but defines a one-second timeout value, the calling thread blocks until the timeout expires and before the task has completed execution.

using System;
using System.Threading;
using System.Threading.Tasks;

public class Example
{
   public static void Main()
   {
      // Wait on a single task with a timeout specified.
      Task taskA = Task.Run( () => Thread.Sleep(2000));
      try {
        taskA.Wait(1000);       // Wait for 1 second.
        bool completed = taskA.IsCompleted;
        Console.WriteLine("Task A completed: {0}, Status: {1}",
                         completed, taskA.Status);
        if (! completed)
           Console.WriteLine("Timed out before task A completed.");                 
       }
       catch (AggregateException) {
          Console.WriteLine("Exception in taskA.");
       }   
   }
}
// The example displays output like the following:
//     Task A completed: False, Status: Running
//     Timed out before task A completed.

You can also supply a cancellation token by calling the Wait(CancellationToken) and Wait(Int32, CancellationToken) methods. If the token's IsCancellationRequested property is true, the wait is cancelled; if it becomes true while the Wait method terminates.

In some cases, you may want to wait for the first of a series of executing tasks to complete, but don't care which task it is. For this purpose, you can call one of the overloads of the Task.WaitAll method. The following example creates three tasks, each of which sleeps for an interval determine by a random number generator. The WaitAny(Task[]) method waits for the first task to complete. The example then displays information about the status of all three tasks.

using System;
using System.Threading;
using System.Threading.Tasks;

public class Example
{
   public static void Main()
   {
      var tasks = new Task[3];
      var rnd = new Random();
      for (int ctr = 0; ctr <= 2; ctr++)
         tasks[ctr] = Task.Run( () => Thread.Sleep(rnd.Next(500, 3000)));

      try {
         int index = Task.WaitAny(tasks);
         Console.WriteLine("Task #{0} completed first.\n", tasks[index].Id);
         Console.WriteLine("Status of all tasks:");
         foreach (var t in tasks)
            Console.WriteLine("   Task #{0}: {1}", t.Id, t.Status);
      }
      catch (AggregateException) {
         Console.WriteLine("An exception occurred.");
      }
   }
}
// The example displays output like the following:
//     Task #1 completed first.
//     
//     Status of all tasks:
//        Task #3: Running
//        Task #1: RanToCompletion
//        Task #4: Running

You can also wait for all of a series of tasks to complete by calling the WaitAll method. The following example creates ten tasks, waits for all ten to complete, and then displays their status.

using System;
using System.Threading;
using System.Threading.Tasks;

public class Example
{
   public static void Main()
   {
      // Wait for all tasks to complete.
      Task[] tasks = new Task[10];
      for (int i = 0; i < 10; i++)
      {
          tasks[i] = Task.Run(() => Thread.Sleep(2000));
      }
      try {
         Task.WaitAll(tasks);
      }
      catch (AggregateException ae) {
         Console.WriteLine("One or more exceptions occurred: ");
         foreach (var ex in ae.Flatten().InnerExceptions)
            Console.WriteLine("   {0}", ex.Message);
      }   

      Console.WriteLine("Status of completed tasks:");
      foreach (var t in tasks)
         Console.WriteLine("   Task #{0}: {1}", t.Id, t.Status);
   }
}
// The example displays the following output:
//     Status of completed tasks:
//        Task #2: RanToCompletion
//        Task #1: RanToCompletion
//        Task #3: RanToCompletion
//        Task #4: RanToCompletion
//        Task #6: RanToCompletion
//        Task #5: RanToCompletion
//        Task #7: RanToCompletion
//        Task #8: RanToCompletion
//        Task #9: RanToCompletion
//        Task #10: RanToCompletion

Note that when you wait for one or more tasks to complete, any exceptions thrown in the running tasks are propagated on the thread that calls the Wait method, as the following example shows. It launches 12 tasks, three of which complete normally and three of which throw an exception. Of the remaining six tasks, three are cancelled before the start, and three are cancelled while they executing. Exceptions are thrown in the WaitAll method call and are handled by a try/catch block.

using System;
using System.Threading;
using System.Threading.Tasks;

public class Example
{
   public static void Main()
   {
      // Create a cancellation token and cancel it.
      var source1 = new CancellationTokenSource();
      var token1 = source1.Token;
      source1.Cancel();
      // Create a cancellation token for later cancellation.
      var source2 = new CancellationTokenSource();
      var token2 = source2.Token;

      // Create a series of tasks that will complete, be cancelled, 
      // timeout, or throw an exception.
      Task[] tasks = new Task[12];
      for (int i = 0; i < 12; i++)
      {
          switch (i % 4) 
          {
             // Task should run to completion.
             case 0:
                tasks[i] = Task.Run(() => Thread.Sleep(2000));
                break;
             // Task should be set to canceled state.
             case 1:   
                tasks[i] = Task.Run( () => Thread.Sleep(2000),
                         token1);
                break;         
             case 2:
                // Task should throw an exception.
                tasks[i] = Task.Run( () => { throw new NotSupportedException(); } );
                break;
             case 3:
                // Task should examine cancellation token.
                tasks[i] = Task.Run( () => { Thread.Sleep(2000); 
                                             if (token2.IsCancellationRequested)
                                                token2.ThrowIfCancellationRequested();
                                             Thread.Sleep(500); }, token2);   
                break;
          }
      }
      Thread.Sleep(250);
      source2.Cancel();

      try {
         Task.WaitAll(tasks);
      }
      catch (AggregateException ae) {
          Console.WriteLine("One or more exceptions occurred:");
          foreach (var ex in ae.InnerExceptions)
             Console.WriteLine("   {0}: {1}", ex.GetType().Name, ex.Message);
       }   

      Console.WriteLine("\nStatus of tasks:");
      foreach (var t in tasks) {
         Console.WriteLine("   Task #{0}: {1}", t.Id, t.Status);
         if (t.Exception != null) {
            foreach (var ex in t.Exception.InnerExceptions)
               Console.WriteLine("      {0}: {1}", ex.GetType().Name,
                                 ex.Message);
         }
      }
   }
}
// The example displays output like the following:
//   One or more exceptions occurred:
//      TaskCanceledException: A task was canceled.
//      NotSupportedException: Specified method is not supported.
//      TaskCanceledException: A task was canceled.
//      TaskCanceledException: A task was canceled.
//      NotSupportedException: Specified method is not supported.
//      TaskCanceledException: A task was canceled.
//      TaskCanceledException: A task was canceled.
//      NotSupportedException: Specified method is not supported.
//      TaskCanceledException: A task was canceled.
//   
//   Status of tasks:
//      Task #13: RanToCompletion
//      Task #1: Canceled
//      Task #3: Faulted
//         NotSupportedException: Specified method is not supported.
//      Task #8: Canceled
//      Task #14: RanToCompletion
//      Task #4: Canceled
//      Task #6: Faulted
//         NotSupportedException: Specified method is not supported.
//      Task #7: Canceled
//      Task #15: RanToCompletion
//      Task #9: Canceled
//      Task #11: Faulted
//         NotSupportedException: Specified method is not supported.
//      Task #12: Canceled

For more information on exception handling in task-based asynchronous operations, see Exception Handling (Task Parallel Library).

Starting with desktop apps that target the .NET Framework 4.6, the culture of the thread that creates and invokes a task becomes part of the thread's context. That is, regardless of the current culture of the thread on which the task executes, the current culture of the task is the culture of the calling thread. For apps that target versions of the .NET Framework prior to the .NET Framework 4.6, the culture of the task is the culture of the thread on which the task executes. For more information, see the "Culture and task-based asynchronous operations" section in the CultureInfo topic.

System_CAPS_noteNote

Store apps follow the Windows Runtime in setting and getting the default culture.

For developers implementing custom debuggers, several internal and private members of task may be useful (these may change from release to release). The m_taskId field serves as the backing store for the Id property, however accessing this field directly from a debugger may be more efficient than accessing the same value through the property's getter method (the s_taskIdCounter counter is used to retrieve the next available ID for a task). Similarly, the m_stateFlags field stores information about the current lifecycle stage of the task, information also accessible through the Status property. The m_action field stores a reference to the task's delegate, and the m_stateObject field stores the async state passed to the task by the developer. Finally, for debuggers that parse stack frames, the InternalWait method serves a potential marker for when a task is entering a wait operation.

The following example creates and executes four tasks. Three tasks execute an Action<T> delegate named action, which accepts an argument of type Object. A fourth task executes a lambda expression (an Action delegate) that is defined inline in the call to the task creation method. Each task is instantiated and run in a different way:

  • Task t1 is instantiated by calling a Task class constructor, but is started by calling its Start() method only after task t2 has started.

  • Task t2 is instantiated and started in a single method call by calling the TaskFactory.StartNew(Action<Object>, Object) method.

  • Task t3 is instantiated and started in a single method call by calling the Run(Action) method.

  • Task t4 is executed synchronously on the main thread by calling the RunSynchronously() method.

Because task t4 executes synchronously, it executes on the main application thread. The remaining tasks execute asynchronously typically on one or more thread pool threads.

using System;
using System.Threading;
using System.Threading.Tasks;

class Example
{
    static void Main()
    {
        Action<object> action = (object obj) =>
                                {
                                   Console.WriteLine("Task={0}, obj={1}, Thread={2}",
                                   Task.CurrentId, obj,
                                   Thread.CurrentThread.ManagedThreadId);
                                };

        // Create a task but do not start it.
        Task t1 = new Task(action, "alpha");

        // Construct a started task
        Task t2 = Task.Factory.StartNew(action, "beta");
        // Block the main thread to demonstate that t2 is executing
        t2.Wait();

        // Launch t1 
        t1.Start();
        Console.WriteLine("t1 has been launched. (Main Thread={0})",
                          Thread.CurrentThread.ManagedThreadId);
        // Wait for the task to finish.
        t1.Wait();

        // Construct a started task using Task.Run.
        String taskData = "delta";
        Task t3 = Task.Run( () => {Console.WriteLine("Task={0}, obj={1}, Thread={2}",
                                                     Task.CurrentId, taskData,
                                                      Thread.CurrentThread.ManagedThreadId);
                                   });
        // Wait for the task to finish.
        t3.Wait();

        // Construct an unstarted task
        Task t4 = new Task(action, "gamma");
        // Run it synchronously
        t4.RunSynchronously();
        // Although the task was run synchronously, it is a good practice
        // to wait for it in the event exceptions were thrown by the task.
        t4.Wait();
    }
}
// The example displays output like the following:
//       Task=1, obj=beta, Thread=3
//       t1 has been launched. (Main Thread=1)
//       Task=2, obj=alpha, Thread=4
//       Task=3, obj=delta, Thread=3
//       Task=4, obj=gamma, Thread=1

Universal Windows Platform
Available since 8
.NET Framework
Available since 4.0
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 5.0
Windows Phone Silverlight
Available since 8.0
Windows Phone
Available since 8.1

All members of Task, except for Dispose, are thread-safe and may be used from multiple threads concurrently.

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