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)

Inheritance Hierarchy
SystemObject
  System.Threading.TasksTask
     System.Threading.TasksTask

Syntax
[HostProtectionAttribute(SecurityAction.LinkDemand, Synchronization = true, 
	ExternalThreading = true)]
public class Task : IAsyncResult, IDisposable
[HostProtectionAttribute(SecurityAction::LinkDemand, Synchronization = true, 
	ExternalThreading = true)]
public ref class Task : IAsyncResult, IDisposable
[<HostProtectionAttribute(SecurityAction.LinkDemand, Synchronization = true,
    ExternalThreading = true)>]
type Task = 
    class
        interface IAsyncResult
        interface IDisposable
    end
<HostProtectionAttribute(SecurityAction.LinkDemand, Synchronization := True,
	ExternalThreading := True)>
Public Class Task
	Implements IAsyncResult, IDisposable
Constructors
NameDescription
System_CAPS_pubmethod Task

Initializes a new Task with the specified action.

System_CAPS_pubmethod Task

Initializes a new Task with the specified action and CancellationToken.

System_CAPS_pubmethod Task

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

System_CAPS_pubmethod Task

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

System_CAPS_pubmethod Task

Initializes a new Task with the specified action and state.

System_CAPS_pubmethod Task

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

System_CAPS_pubmethod Task

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

System_CAPS_pubmethod Task

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

Properties
NameDescription
System_CAPS_pubproperty

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

System_CAPS_pubproperty System_CAPS_static

Gets a task that has already completed successfully.

System_CAPS_pubproperty

Gets the TaskCreationOptions used to create this task.

System_CAPS_pubproperty System_CAPS_static

Returns the ID of the currently executing Task.

System_CAPS_pubproperty

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_pubproperty System_CAPS_static

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

System_CAPS_pubproperty

Gets an ID for this Task instance.

System_CAPS_pubproperty

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

System_CAPS_pubproperty

Gets whether this Task has completed.

System_CAPS_pubproperty

Gets whether the Task completed due to an unhandled exception.

System_CAPS_pubproperty

Gets the TaskStatus of this task.

Methods
NameDescription
System_CAPS_pubmethod ConfigureAwait

Configures an awaiter used to await this Task.

System_CAPS_pubmethod ContinueWith

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

System_CAPS_pubmethod ContinueWith

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

System_CAPS_pubmethod ContinueWith

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_pubmethod ContinueWith

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_pubmethod ContinueWith

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_pubmethod ContinueWith

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

System_CAPS_pubmethod ContinueWith

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

System_CAPS_pubmethod ContinueWith

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_pubmethod ContinueWith

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

System_CAPS_pubmethod ContinueWith

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

System_CAPS_pubmethod ContinueWith

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

System_CAPS_pubmethod ContinueWith

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

System_CAPS_pubmethod ContinueWith

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_pubmethod ContinueWith

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

System_CAPS_pubmethod ContinueWith

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

System_CAPS_pubmethod ContinueWith

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

System_CAPS_pubmethod ContinueWith

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_pubmethod ContinueWith

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_pubmethod ContinueWith

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_pubmethod ContinueWith

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_pubmethod System_CAPS_static Delay

Creates a task that completes after a time delay.

System_CAPS_pubmethod System_CAPS_static Delay

Creates a cancellable task that completes after a time delay.

System_CAPS_pubmethod System_CAPS_static Delay

Creates a task that completes after a specified time interval.

System_CAPS_pubmethod System_CAPS_static Delay

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

System_CAPS_pubmethod Dispose

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

System_CAPS_protmethod Dispose

Disposes the Task, releasing all of its unmanaged resources.

System_CAPS_pubmethod Equals

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

System_CAPS_protmethod Finalize

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_pubmethod System_CAPS_static FromCanceled

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

System_CAPS_pubmethod System_CAPS_static FromCanceled

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

System_CAPS_pubmethod System_CAPS_static FromException

Creates a Task that has completed with a specified exception.

System_CAPS_pubmethod System_CAPS_static FromException

Creates a Task that's completed with a specified exception.

System_CAPS_pubmethod System_CAPS_static FromResult

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

System_CAPS_pubmethod GetAwaiter

Gets an awaiter used to await this Task.

System_CAPS_pubmethod GetHashCode

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

System_CAPS_pubmethod GetType

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

System_CAPS_protmethod MemberwiseClone

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

System_CAPS_pubmethod System_CAPS_static Run

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

System_CAPS_pubmethod System_CAPS_static Run

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_pubmethod System_CAPS_static Run

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

System_CAPS_pubmethod System_CAPS_static Run

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

System_CAPS_pubmethod System_CAPS_static Run

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

System_CAPS_pubmethod System_CAPS_static Run

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_pubmethod System_CAPS_static Run

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

System_CAPS_pubmethod System_CAPS_static Run

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

System_CAPS_pubmethod RunSynchronously

Runs the Task synchronously on the current TaskScheduler.

System_CAPS_pubmethod RunSynchronously

Runs the Task synchronously on the TaskScheduler provided.

System_CAPS_pubmethod Start

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

System_CAPS_pubmethod Start

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

System_CAPS_pubmethod ToString

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

System_CAPS_pubmethod Wait

Waits for the Task to complete execution.

System_CAPS_pubmethod Wait

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

System_CAPS_pubmethod Wait

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

System_CAPS_pubmethod Wait

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_pubmethod Wait

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

System_CAPS_pubmethod System_CAPS_static WaitAll

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

System_CAPS_pubmethod System_CAPS_static WaitAll

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

System_CAPS_pubmethod System_CAPS_static WaitAll

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

System_CAPS_pubmethod System_CAPS_static WaitAll

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_pubmethod System_CAPS_static WaitAll

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

System_CAPS_pubmethod System_CAPS_static WaitAny

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

System_CAPS_pubmethod System_CAPS_static WaitAny

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

System_CAPS_pubmethod System_CAPS_static WaitAny

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

System_CAPS_pubmethod System_CAPS_static WaitAny

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_pubmethod System_CAPS_static WaitAny

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

System_CAPS_pubmethod System_CAPS_static WhenAll

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

System_CAPS_pubmethod System_CAPS_static WhenAll

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

System_CAPS_pubmethod System_CAPS_static WhenAll

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

System_CAPS_pubmethod System_CAPS_static WhenAll

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

System_CAPS_pubmethod System_CAPS_static WhenAny

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

System_CAPS_pubmethod System_CAPS_static WhenAny

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

System_CAPS_pubmethod System_CAPS_static WhenAny

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

System_CAPS_pubmethod System_CAPS_static WhenAny

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

System_CAPS_pubmethod System_CAPS_static Yield

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

Explicit Interface Implementations
NameDescription
System_CAPS_pubinterface System_CAPS_privproperty

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

System_CAPS_pubinterface System_CAPS_privproperty

Gets an indication of whether the operation completed synchronously.

Extension Methods
NameDescription
System_CAPS_pubmethod DispatcherOperationWait

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

System_CAPS_pubmethod DispatcherOperationWait

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

System_CAPS_pubmethod IsDispatcherOperationTask

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

Remarks
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 property, as well as the , , and 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 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

Creating and executing a task

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 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
Imports System.Threading.Tasks

Module Example
   Public Sub Main()
      Dim t As Task = Task.Run(Sub()
                                  ' Just loop.
                                  Dim ctr As Integer = 0
                                  For ctr = 0 to 1000000
                                  Next
                                  Console.WriteLine("Finished {0} loop iterations",
                                                    ctr)
                               End Sub)
      t.Wait()
   End Sub
End Module
' 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 TaskFactoryStartNew method. The property returns a TaskFactory object. Overloads of the TaskFactoryStartNew method let you specify parameters to pass to the task creation options and a task scheduler. The following example uses the TaskFactoryStartNew 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
Imports System.Threading.Tasks

Module Example
   Public Sub Main()
      Dim t As Task = Task.Factory.StartNew(Sub()
                                  ' Just loop.
                                  Dim ctr As Integer = 0
                                  For ctr = 0 to 1000000
                                  Next
                                  Console.WriteLine("Finished {0} loop iterations",
                                                    ctr)
                               End Sub)
      t.Wait()
   End Sub
End Module
' The example displays the following output:
'       Finished 1000001 loop iterations

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

Separating task creation and execution

The Task class also provides constructors that initialize the task but that do not schedule it for execution. For performance reasons, the TaskRun or TaskFactoryStartNew 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 TaskStart method to schedule the task for execution at a later time.

Waiting for one or more tasks to complete

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 TaskWait 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 ThreadSleep 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
Imports System.Threading
Imports System.Threading.Tasks

Module Example
   Public Sub Main()
      ' Wait on a single task with no timeout specified.
      Dim taskA = Task.Run( Sub() Thread.Sleep(2000))
      Console.WriteLine("taskA Status: {0}", taskA.Status)
      Try
        taskA.Wait()
        Console.WriteLine("taskA Status: {0}", taskA.Status)
      Catch e As AggregateException
         Console.WriteLine("Exception in taskA.")
      End Try
   End Sub
End Module
' 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 and Wait 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.
Imports System.Threading
Imports System.Threading.Tasks

Module Example
   Public Sub Main()
      ' Wait on a single task with a timeout specified.
      Dim taskA As Task = Task.Run( Sub() Thread.Sleep(2000))
      Try
         taskA.Wait(1000)        ' Wait for 1 second.
         Dim completed As Boolean = taskA.IsCompleted
         Console.WriteLine("Task.Completed: {0}, Status: {1}",
                           completed, taskA.Status)
         If Not completed Then 
            Console.WriteLine("Timed out before task A completed.")
         End If                     
      Catch e As AggregateException
         Console.WriteLine("Exception in taskA.")
      End Try
   End Sub
End Module
' The example displays the following output:
'     Task A completed: False, Status: Running
'     Timed out before task A completed.

You can also supply a cancellation token by calling the Wait and Wait methods. If the token's 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 TaskWaitAll method. The following example creates three tasks, each of which sleeps for an interval determine by a random number generator. The WaitAny 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
Imports System.Threading
Imports System.Threading.Tasks

Module Example
   Public Sub Main()
      Dim tasks(2) As Task
      Dim rnd As New Random()
      For ctr As Integer = 0 To 2
         tasks(ctr) = Task.Run( Sub()  Thread.Sleep(rnd.Next(500, 3000)))
      Next

      Try 
         Dim index As Integer= Task.WaitAny(tasks)
         Console.WriteLine("Task #{0} completed first.", tasks(index).Id)
         Console.WriteLine()
         Console.WriteLine("Status of all tasks:")
         For Each t in tasks
            Console.WriteLine("   Task #{0}: {1}", t.Id, t.Status)
         Next   
      Catch e As AggregateException
         Console.WriteLine("An exception occurred.")
      End Try
   End Sub
End Module
' 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
Imports System.Threading
Imports System.Threading.Tasks

Module Example
   Public Sub Main()
      ' Wait for all tasks to complete.
      Dim tasks(9) As Task
      For i As Integer = 0 To 9
          tasks(i) = Task.Run( Sub() Thread.Sleep(2000) )
      Next
      Try 
         Task.WaitAll(tasks)
      Catch ae As AggregateException
         Console.WriteLine("One or more exceptions occurred: ")
         For Each ex In ae.Flatten().InnerExceptions
            Console.WriteLine("   {0}", ex.Message)
         Next
      End Try   

      Console.WriteLine("Status of completed tasks:")
      For Each t in tasks
         Console.WriteLine("   Task #{0}: {1}", t.Id, t.Status)
      Next   
   End Sub
End Module
' 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
Imports System.Threading
Imports System.Threading.Tasks

Module Example
   Public Sub Main()
      ' Create a cancellation token and cancel it.
      Dim source1 As New CancellationTokenSource()
      Dim token1 As CancellationToken = source1.Token
      source1.Cancel()
      ' Create a cancellation token for later cancellation.
      Dim source2 As New CancellationTokenSource()
      Dim token2 As CancellationToken = source2.Token

      ' Create a series of tasks that will complete, be cancelled, 
      ' timeout, or throw an exception.
      Dim tasks(11) As Task
      For i As Integer = 0 To 11
          Select Case i Mod 4 
             ' Task should run to completion.
             Case 0
                tasks(i) = Task.Run( Sub() Thread.Sleep(2000))
             ' Task should be set to canceled state.
             Case 1   
                tasks(i) = Task.Run( Sub() Thread.Sleep(2000), token1)
             Case 2
                ' Task should throw an exception.
                tasks(i) = Task.Run( Sub() 
                                        Throw New NotSupportedException() 
                                     End Sub)
             Case 3
                ' Task should examine cancellation token.
                tasks(i) = Task.Run( Sub() 
                                        Thread.Sleep(2000) 
                                        If token2.IsCancellationRequested
                                           token2.ThrowIfCancellationRequested()
                                        End If
                                        Thread.Sleep(500) 
                                     End Sub, token2)   
          End Select
      Next
      Thread.Sleep(250)
      source2.Cancel()

      Try 
         Task.WaitAll(tasks)
      Catch ae As AggregateException
         Console.WriteLine("One or more exceptions occurred:")
         For Each ex in ae.InnerExceptions
             Console.WriteLine("   {0}: {1}", ex.GetType().Name, ex.Message)
         Next
      End Try   
      Console.WriteLine()

      Console.WriteLine("Status of tasks:")
      For Each t in tasks
         Console.WriteLine("   Task #{0}: {1}", t.Id, t.Status)
         If t.Exception IsNot Nothing Then
            For Each ex in t.Exception.InnerExceptions
               Console.WriteLine("      {0}: {1}", ex.GetType().Name,
                                 ex.Message)
            Next
         End If
      Next
   End Sub
End Module
' 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).

Tasks and culture

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 debugger developers

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 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 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.

Examples

The following example creates and executes four tasks. Three tasks execute an Action 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 TaskFactoryStartNew method.

  • Task t3 is instantiated and started in a single method call by calling the Run 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
Imports System.Threading
Imports System.Threading.Tasks

Module Example
    Public Sub Main()
        Dim action As Action(Of Object) = 
              Sub(obj As Object)
                 Console.WriteLine("Task={0}, obj={1}, Thread={2}", 
                 Task.CurrentId, obj,
                 Thread.CurrentThread.ManagedThreadId)
              End Sub

        ' Construct an unstarted task
        Dim t1 As New Task(action, "alpha")

        ' Construct a started task
        Dim t2 As Task = 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.
        Dim taskData As String = "delta"
        Dim t3 As Task = Task.Run(Sub()
                                     Console.WriteLine("Task={0}, obj={1}, Thread={2}",
                                     Task.CurrentId, taskData,
                                     Thread.CurrentThread.ManagedThreadId)
                                  End Sub)
        ' Wait for the task to finish.
        t3.Wait()

        ' Construct an unstarted task
        Dim t4 As 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()
    End Sub
End Module
' 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=3
'       Task=3, obj=delta, Thread=3
'       Task=4, obj=gamma, Thread=1
Version Information
Universal Windows Platform
Available since 4.5
.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
Thread Safety

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

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