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


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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


Type: System.Threading.Tasks.Task()

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

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

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


The tasks array is null.


The continuationAction argument is null.


The tasks array contains a null value.


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()
                                                  Console.WriteLine("Task={0}, Thread={1}, x=5", Task.CurrentId, Thread.CurrentThread.ManagedThreadId)
                                                  Return 5
                                              End Function),
            Task(Of Integer).Factory.StartNew(Function()
                                                  Console.WriteLine("Task={0}, Thread={1}, x=3", Task.CurrentId, Thread.CurrentThread.ManagedThreadId)
                                                  Return 3
                                              End Function),
            Task(Of Integer).Factory.StartNew(Function()
                                                  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
                                                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
Available since 5.0
Windows Phone Silverlight
Available since 8.0
Windows Phone
Available since 8.1
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