TaskFactory.ContinueWhenAny Method (Task(), Action(Of Task))

 
System_CAPS_noteNote

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Creates a continuation Task that will be started upon the completion of any Task in the provided set.

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

Public Function ContinueWhenAny (
	tasks As Task(),
	continuationAction As Action(Of Task)
) As Task

Parameters

tasks
Type: System.Threading.Tasks.Task()

The array of tasks from which to continue when one task completes.

continuationAction
Type: System.Action(Of Task)

The action delegate to execute when one task in the tasks array completes.

Return Value

Type: System.Threading.Tasks.Task

The new continuation Task.

Exception Condition
ObjectDisposedException

One of the elements in the tasks array has been disposed.

ArgumentNullException

The tasks array is null.

-or-

The continuationAction argument is null.

ArgumentException

The tasks array contains a null value.

-or-

The tasks array is empty.

The following example shows how to use ContinueWhenAny and ContinueWhenAll:

Imports System.Threading
Imports System.Threading.Tasks

Module ContinuationWhenMulti
    ' Demonstrated features:
    '   Task.Factory
    '   TaskFactory.ContinueWhenAll()
    '   TaskFactory.ContinueWhenAny()
    '   Task.Wait()
    ' Expected results:
    '   Three tasks are created in parallel. 
    '   Each task for a different period of time prints a number and returns it.
    '   A ContinueWhenAny() task indicates the first of the three tasks to complete.
    '   A ContinueWhenAll() task sums up the results of the three tasks and prints out the total.
    ' Documentation:
    '   http://msdn.microsoft.com/en-us/library/system.threading.tasks.taskfactory_members(VS.100).aspx
    Sub Main()
        ' Schedule a list of tasks that return integer
        Dim tasks As Task(Of Integer)() = New Task(Of Integer)() {
            Task(Of Integer).Factory.StartNew(Function()
                                                  Thread.Sleep(500)
                                                  Console.WriteLine("Task={0}, Thread={1}, x=5", Task.CurrentId, Thread.CurrentThread.ManagedThreadId)
                                                  Return 5
                                              End Function),
            Task(Of Integer).Factory.StartNew(Function()
                                                  Thread.Sleep(10)
                                                  Console.WriteLine("Task={0}, Thread={1}, x=3", Task.CurrentId, Thread.CurrentThread.ManagedThreadId)
                                                  Return 3
                                              End Function),
            Task(Of Integer).Factory.StartNew(Function()
                                                  Thread.Sleep(200)
                                                  Console.WriteLine("Task={0}, Thread={1}, x=2", Task.CurrentId, Thread.CurrentThread.ManagedThreadId)
                                                  Return 2
                                              End Function)}


        ' Schedule a continuation to indicate the result of the first task to complete
        Task.Factory.ContinueWhenAny(tasks, Sub(winner)
                                                ' You would expect winning result = 3 on multi-core systems, because you expect
                                                ' tasks[1] to finish first.
                                                Console.WriteLine("Task={0}, Thread={1} (ContinueWhenAny): Winning result = {2}", Task.CurrentId, Thread.CurrentThread.ManagedThreadId, winner.Result)
                                            End Sub)


        ' Schedule a continuation that sums up the results of all tasks, then wait on it.
        ' The list of antecendent tasks is passed as an argument by the runtime.
        Task.Factory.ContinueWhenAll(tasks, Sub(antecendents)
                                                Dim sum As Integer = 0
                                                For Each task__1 As Task(Of Integer) In antecendents
                                                    sum += task__1.Result
                                                Next
                                                Console.WriteLine("Task={0}, Thread={1}, (ContinueWhenAll): Total={2} (expected 10)", Task.CurrentId, Thread.CurrentThread.ManagedThreadId, sum)
                                            End Sub).Wait()
    End Sub
End Module

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
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