TaskCompletionSource(Of TResult) Class

Represents the producer side of a Task(Of TResult) unbound to a delegate, providing access to the consumer side through the Task property.

System.Object
  System.Threading.Tasks.TaskCompletionSource(Of TResult)

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

'Declaration
<HostProtectionAttribute(SecurityAction.LinkDemand, Synchronization := True,  _
	ExternalThreading := True)> _
Public Class TaskCompletionSource(Of TResult)

Type Parameters

TResult

The type of the result value assocatied with this TaskCompletionSource(Of TResult).

The TaskCompletionSource(Of TResult) type exposes the following members.

  NameDescription
Public methodSupported by Portable Class LibraryTaskCompletionSource(Of TResult)Creates a TaskCompletionSource(Of TResult).
Public methodSupported by Portable Class LibraryTaskCompletionSource(Of TResult)(Object)Creates a TaskCompletionSource(Of TResult) with the specified state.
Public methodSupported by Portable Class LibraryTaskCompletionSource(Of TResult)(TaskCreationOptions)Creates a TaskCompletionSource(Of TResult) with the specified options.
Public methodSupported by Portable Class LibraryTaskCompletionSource(Of TResult)(Object, TaskCreationOptions)Creates a TaskCompletionSource(Of TResult) with the specified state and options.
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  NameDescription
Public propertySupported by Portable Class LibraryTaskGets the Task(Of TResult) created by this TaskCompletionSource(Of TResult).
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  NameDescription
Public methodSupported by Portable Class LibraryEquals(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.)
Public methodSupported by Portable Class LibraryGetHashCodeServes as the default hash function. (Inherited from Object.)
Public methodSupported by Portable Class LibraryGetTypeGets the Type of the current instance. (Inherited from Object.)
Protected methodSupported by Portable Class LibraryMemberwiseCloneCreates a shallow copy of the current Object. (Inherited from Object.)
Public methodSupported by Portable Class LibrarySetCanceledTransitions the underlying Task(Of TResult) into the Canceled state.
Public methodSupported by Portable Class LibrarySetException(IEnumerable(Of Exception))Transitions the underlying Task(Of TResult) into the Faulted state.
Public methodSupported by Portable Class LibrarySetException(Exception)Transitions the underlying Task(Of TResult) into the Faulted state.
Public methodSupported by Portable Class LibrarySetResultTransitions the underlying Task(Of TResult) into the RanToCompletion state.
Public methodSupported by Portable Class LibraryToStringReturns a string that represents the current object. (Inherited from Object.)
Public methodSupported by Portable Class LibraryTrySetCanceledAttempts to transition the underlying Task(Of TResult) into the Canceled state.
Public methodTrySetCanceled(CancellationToken)Attempts to transition the underlying Task(Of TResult) into the TaskStatus.Canceled state and enables a cancellation token to be stored in the canceled task.
Public methodSupported by Portable Class LibraryTrySetException(IEnumerable(Of Exception))Attempts to transition the underlying Task(Of TResult) into the Faulted state.
Public methodSupported by Portable Class LibraryTrySetException(Exception)Attempts to transition the underlying Task(Of TResult) into the Faulted state.
Public methodSupported by Portable Class LibraryTrySetResultAttempts to transition the underlying Task(Of TResult) into the RanToCompletion state.
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In many scenarios, it is useful to enable a Task(Of TResult) to represent an external asynchronous operation. TaskCompletionSource(Of TResult) is provided for this purpose. It enables the creation of a task that can be handed out to consumers, and those consumers can use the members of the task as they would any other. However, unlike most tasks, the state of a task created by a TaskCompletionSource is controlled explicitly by the methods on TaskCompletionSource. This enables the completion of the external asynchronous operation to be propagated to the underlying Task. The separation also ensures that consumers are not able to transition the state without access to the corresponding TaskCompletionSource. For more information, see the entry The Nature of TaskCompletionSource<TResult> in the Parallel Programming with .NET blog.

The Parallel Extensions samples also contain examples of how to use TaskCompletionSource(Of TResult).

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 use a TaskCompletionSource(Of TResult):

Imports System.Diagnostics
Imports System.Threading
Imports System.Threading.Tasks

Module TCSDemo
    ' Demonstrated features: 
    '   TaskCompletionSource ctor() 
    '   TaskCompletionSource.SetResult() 
    '   TaskCompletionSource.SetException() 
    '   Task.Result 
    ' Expected results: 
    '   The attempt to get t1.Result blocks for ~1000ms until tcs1 gets signaled. 15 is printed out. 
    '   The attempt to get t2.Result blocks for ~1000ms until tcs2 gets signaled. An exception is printed out. 
    ' Documentation: 
    '   http://msdn.microsoft.com/en-us/library/dd449199(VS.100).aspx 

    Private Sub Main()
        Dim tcs1 As New TaskCompletionSource(Of Integer)()
        Dim t1 As Task(Of Integer) = tcs1.Task

        ' Start a background task that will complete tcs1.Task
        Task.Factory.StartNew(Sub()
                                  Thread.Sleep(1000)
                                  tcs1.SetResult(15)
                              End Sub)

        ' The attempt to get the result of t1 blocks the current thread until the completion source gets signaled. 
        ' It should be a wait of ~1000 ms. 
        Dim sw As Stopwatch = Stopwatch.StartNew()
        Dim result As Integer = t1.Result
        sw.Stop()

        Console.WriteLine("(ElapsedTime={0}): t1.Result={1} (expected 15) ", sw.ElapsedMilliseconds, result)

        ' ------------------------------------------------------------------ 

        ' Alternatively, an exception can be manually set on a TaskCompletionSource.Task 
        Dim tcs2 As New TaskCompletionSource(Of Integer)()
        Dim t2 As Task(Of Integer) = tcs2.Task

        ' Start a background Task that will complete tcs2.Task with an exception
        Task.Factory.StartNew(Sub()
                                  Thread.Sleep(1000)
                                  tcs2.SetException(New InvalidOperationException("SIMULATED EXCEPTION"))
                              End Sub)

        ' The attempt to get the result of t2 blocks the current thread until the completion source gets signaled with either a result or an exception. 
        ' In either case it should be a wait of ~1000 ms.
        sw = Stopwatch.StartNew()
        Try
            result = t2.Result

            Console.WriteLine("t2.Result succeeded. THIS WAS NOT EXPECTED.")
        Catch e As AggregateException
            Console.Write("(ElapsedTime={0}): ", sw.ElapsedMilliseconds)
            Console.WriteLine("The following exceptions have been thrown by t2.Result: (THIS WAS EXPECTED)")
            For j As Integer = 0 To e.InnerExceptions.Count - 1
                Console.WriteLine(vbLf & "-------------------------------------------------" & vbLf & "{0}", e.InnerExceptions(j).ToString())
            Next 
        End Try 
    End Sub 

End Module

.NET Framework

Supported in: 4.6, 4.5, 4

.NET Framework Client Profile

Supported in: 4

Portable Class Library

Supported in: Portable Class Library

Supported in: Windows Phone 8.1

Supported in: Windows Phone Silverlight 8.1

Supported in: Windows Phone Silverlight 8

All members of TaskCompletionSource(Of TResult) are thread-safe and may be used from multiple threads concurrently.

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