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Task Class

Represents an asynchronous operation.

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

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

The Task type exposes the following members.

  NameDescription
Public methodTask(Action)Initializes a new Task with the specified action.
Public methodTask(Action, CancellationToken)Initializes a new Task with the specified action and CancellationToken.
Public methodTask(Action, TaskCreationOptions)Initializes a new Task with the specified action and creation options.
Public methodTask(Action<Object>, Object)Initializes a new Task with the specified action and state.
Public methodTask(Action, CancellationToken, TaskCreationOptions)Initializes a new Task with the specified action and creation options.
Public methodTask(Action<Object>, Object, CancellationToken)Initializes a new Task with the specified action, state, and options.
Public methodTask(Action<Object>, Object, TaskCreationOptions)Initializes a new Task with the specified action, state, and options.
Public methodTask(Action<Object>, Object, CancellationToken, TaskCreationOptions)Initializes a new Task with the specified action, state, and options.
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  NameDescription
Public propertyAsyncStateGets the state object supplied when the Task was created, or null if none was supplied.
Public propertyCreationOptionsGets the TaskCreationOptions used to create this task.
Public propertyStatic memberCurrentIdReturns the unique ID of the currently executing Task.
Public propertyExceptionGets 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.
Public propertyStatic memberFactoryProvides access to factory methods for creating Task and Task<TResult> instances.
Public propertyIdGets a unique ID for this Task instance.
Public propertyIsCanceledGets whether this Task instance has completed execution due to being canceled.
Public propertyIsCompletedGets whether this Task has completed.
Public propertyIsFaultedGets whether the Task completed due to an unhandled exception.
Public propertyStatusGets the TaskStatus of this Task.
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  NameDescription
Public methodContinueWith(Action<Task>)Creates a continuation that executes asynchronously when the target Task completes.
Public methodContinueWith(Action<Task>, CancellationToken)Creates a continuation that executes asynchronously when the target Task completes.
Public methodContinueWith(Action<Task>, TaskContinuationOptions)Creates a continuation that executes according to the specified TaskContinuationOptions.
Public methodContinueWith(Action<Task>, TaskScheduler)Creates a continuation that executes asynchronously when the target Task completes.
Public methodContinueWith(Action<Task>, CancellationToken, TaskContinuationOptions, TaskScheduler)Creates a continuation that executes according to the specified TaskContinuationOptions.
Public methodContinueWith<TResult>(Func<Task, TResult>)Creates a continuation that executes asynchronously when the target Task completes.
Public methodContinueWith<TResult>(Func<Task, TResult>, CancellationToken)Creates a continuation that executes asynchronously when the target Task completes.
Public methodContinueWith<TResult>(Func<Task, TResult>, TaskContinuationOptions)Creates a continuation that executes according to the condition specified in continuationOptions.
Public methodContinueWith<TResult>(Func<Task, TResult>, TaskScheduler)Creates a continuation that executes asynchronously when the target Task completes.
Public methodContinueWith<TResult>(Func<Task, TResult>, CancellationToken, TaskContinuationOptions, TaskScheduler)Creates a continuation that executes according to the condition specified in continuationOptions.
Public methodDispose()Releases all resources used by the current instance of the Task class.
Protected methodDispose(Boolean)Disposes the Task, releasing all of its unmanaged resources.
Public methodEquals(Object)Determines whether the specified Object is equal to the current Object. (Inherited from Object.)
Protected methodFinalizeAllows an object to try to free resources and perform other cleanup operations before it is reclaimed by garbage collection. (Inherited from Object.)
Public methodGetHashCodeServes as a hash function for a particular type. (Inherited from Object.)
Public methodGetTypeGets the Type of the current instance. (Inherited from Object.)
Protected methodMemberwiseCloneCreates a shallow copy of the current Object. (Inherited from Object.)
Public methodRunSynchronously()Runs the Task synchronously on the current TaskScheduler.
Public methodRunSynchronously(TaskScheduler)Runs the Task synchronously on the TaskScheduler provided.
Public methodStart()Starts the Task, scheduling it for execution to the current TaskScheduler.
Public methodStart(TaskScheduler)Starts the Task, scheduling it for execution to the specified TaskScheduler.
Public methodToStringReturns a string that represents the current object. (Inherited from Object.)
Public methodWait()Waits for the Task to complete execution.
Public methodWait(CancellationToken)Waits for the Task to complete execution.
Public methodWait(Int32)Waits for the Task to complete execution.
Public methodWait(TimeSpan)Waits for the Task to complete execution.
Public methodWait(Int32, CancellationToken)Waits for the Task to complete execution.
Public methodStatic memberWaitAll(Task[])Waits for all of the provided Task objects to complete execution.
Public methodStatic memberWaitAll(Task[], Int32)Waits for all of the provided Task objects to complete execution.
Public methodStatic memberWaitAll(Task[], CancellationToken)Waits for all of the provided Task objects to complete execution.
Public methodStatic memberWaitAll(Task[], TimeSpan)Waits for all of the provided Task objects to complete execution.
Public methodStatic memberWaitAll(Task[], Int32, CancellationToken)Waits for all of the provided Task objects to complete execution.
Public methodStatic memberWaitAny(Task[])Waits for any of the provided Task objects to complete execution.
Public methodStatic memberWaitAny(Task[], Int32)Waits for any of the provided Task objects to complete execution.
Public methodStatic memberWaitAny(Task[], CancellationToken)Waits for any of the provided Task objects to complete execution.
Public methodStatic memberWaitAny(Task[], TimeSpan)Waits for any of the provided Task objects to complete execution.
Public methodStatic memberWaitAny(Task[], Int32, CancellationToken)Waits for any of the provided Task objects to complete execution.
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  NameDescription
Explicit interface implemetationPrivate propertyIAsyncResult.AsyncWaitHandleGets a WaitHandle that can be used to wait for the task to complete.
Explicit interface implemetationPrivate propertyIAsyncResult.CompletedSynchronouslyGets an indication of whether the operation completed synchronously.
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Task instances may be created in a variety of ways. The most common approach is by using the Task type's Factory property to retrieve a TaskFactory instance that can be used to create tasks for several purposes. For example, to create a Task that runs an action, the factory's StartNew method may be used:

// C#
var t = Task.Factory.StartNew(() => DoAction());

' Visual Basic
 Dim t = Task.Factory.StartNew(Function() DoAction())

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 factory's StartNew method should be the preferred mechanism for creating and scheduling computational tasks, but for scenarios where creation and scheduling must be separated, the constructors may be used, and the task's Start method may then be used to schedule the task for execution at a later time.

For operations that return values, the Task<TResult> class should be used.

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 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.

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 shows how to start a task by using the StartNew() method:


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

class StartNewDemo
{
    // Demonstrated features:
    //		Task ctor()
    // 		Task.Factory
    //		Task.Wait()
    //		Task.RunSynchronously()
    // Expected results:
    // 		Task t1 (alpha) is created unstarted.
    //		Task t2 (beta) is created started.
    //		Task t1's (alpha) start is held until after t2 (beta) is started.
    //		Both tasks t1 (alpha) and t2 (beta) are potentially executed on threads other than the main thread on multi-core machines.
    //		Task t3 (gamma) is executed synchronously on the main thread.
    // Documentation:
    //		http://msdn.microsoft.com/en-us/library/system.threading.tasks.task_members(VS.100).aspx
    static void Main()
    {
        Action<object> action = (object obj) =>
        {
            Console.WriteLine("Task={0}, obj={1}, Thread={2}", Task.CurrentId, obj.ToString(), Thread.CurrentThread.ManagedThreadId);
        };

        // Construct an unstarted task
        Task t1 = new Task(action, "alpha");

        // Cosntruct 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.
        // You may optionally provide a timeout interval or a cancellation token
        // to mitigate situations when the task takes too long to finish.
        t1.Wait();

        // Construct an unstarted task
        Task t3 = new Task(action, "gamma");

        // Run it synchronously
        t3.RunSynchronously();

        // Although the task was run synchrounously, it is a good practice to wait for it which observes for 
        // exceptions potentially thrown by that task.
        t3.Wait();
    }


}


.NET Framework

Supported in: 4

.NET Framework Client Profile

Supported in: 4

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

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

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