WaitAll Method (WaitHandle[])
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WaitHandle.WaitAll Method (WaitHandle())

[ This article is for Windows Phone 8 developers. If you’re developing for Windows 10, see the latest documentation. ]

Waits for all the elements in the specified array to receive a signal.

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

Public Shared Function WaitAll ( _
	waitHandles As WaitHandle() _
) As Boolean


Type: System.Threading.WaitHandle ()
An array that contains the objects for which the current instance will wait. This array cannot contain multiple references to the same object.

Return Value

Type: System.Boolean
true when every element in waitHandles has received a signal; otherwise, the method never returns.


The waitHandles parameter is Nothing.


One or more of the objects in the waitHandles array are Nothing.


The waitHandles array contains elements that are duplicates.


waitHandles is an array with no elements.


The number of objects in waitHandles is greater than the system permits.

The WaitAll method returns when all the handles are signaled. On some implementations, if more than 64 handles are passed, a NotSupportedException is thrown. If the array contains duplicates, the call fails with an ArgumentException.

Calling this method overload is equivalent to calling the WaitAll(WaitHandle(), Int32) method overload and specifying -1 (or Timeout.Infinite) for millisecondsTimeout.

If you call this method from a single-threaded apartment, and waitHandles contains more than one wait handle, the method deadlocks.

Version Notes

Windows Phone

 WaitAll is present but not supported in Windows Phone.

The following example shows how to divide work among three thread pool threads, and how to use the static WaitAll and WaitAny methods to wait until the subtasks are finished.

The example creates a BackgroundWorker that reports completion to the user interface. By using a BackgroundWorker, the example insulates the user interface thread from the effects of the WaitAny and WaitAll methods, and thus allows the user interface to remain responsive.

The BackgroundWorker runs a DoWork method that creates three tasks by using the ThreadPool.QueueUserWorkItem method, and assigns each task a random amount of work. The example defines a Subtask class to hold the data and thread procedure for each task. Each task has a ManualResetEvent, which it signals when its work is complete.

After starting the tasks, the DoWork method uses the WaitAny(WaitHandle()) method overload to wait for the shortest subtask to finish. The BackgroundWorker then uses the WaitAll(WaitHandle()) method overload to wait until the rest of the tasks are complete. The DoWork method then produces a report using the results from all three tasks.


The shortest task is not necessarily the first to complete. The thread pool threads may not all start immediately and may not be treated equally by the scheduler.

After you start the example, it changes the mouse button event to show user clicks, demonstrating that the user interface remains responsive during the execution of the background tasks.

Imports System.Threading

' The following imports simplify the supporting code; they are not required for 
' WaitHandle:
Imports System.Windows.Controls
Imports System.Windows.Input
Imports System.ComponentModel

Public Class Example

   Private Shared outputBlock As TextBlock

   Public Shared Sub Demo(ByVal outputBlock As TextBlock)

      Example.outputBlock = outputBlock
      Example.outputBlock.Text = "Click to start the demo."

      AddHandler outputBlock.MouseLeftButtonUp, AddressOf MouseUpStart

   End Sub

   Private Shared Sub MouseUpStart(ByVal sender As Object, ByVal e As MouseEventArgs)

      ' Replace the startup mouse button handler with a handler that 
      ' displays a message.
      RemoveHandler outputBlock.MouseLeftButtonUp, AddressOf MouseUpStart
      AddHandler outputBlock.MouseLeftButtonUp, AddressOf MouseUp

      outputBlock.Text = _
         "Demo is running. The BackgroundWorker waits for the first subtask to complete," & vbLf _
         & "then waits for all subtasks to complete and produces a report." & vbLf _
         & "Click here at any time to show that the user interface is responsive." & vbLf

      Dim worker As New System.ComponentModel.BackgroundWorker
      AddHandler worker.DoWork, AddressOf DoWork
      AddHandler worker.RunWorkerCompleted, AddressOf Completed

   End Sub

   ' The only purpose of this mouse button handler is to show that the user
   ' interface is responsive while the background tasks are running.
   Private Shared Sub MouseUp(ByVal sender As Object, ByVal e As MouseEventArgs)
      outputBlock.Text &= "Mouse clicked." & vbLf
   End Sub

   Private Shared Sub DoWork(ByVal sender As Object, ByVal e As DoWorkEventArgs)

      Dim worker As BackgroundWorker = CType(sender, BackgroundWorker)

      ' Divide the "work" into three parts, and queue three tasks to run on
      ' threadpool threads. Provide random data for each task.

      Dim r As New Random()
      ' Keep a list of subtasks and a list of their ManualResetEvent objects.
      Dim subtasks As New System.Collections.Generic.List(Of Subtask)
      Dim finished As New System.Collections.Generic.List(Of WaitHandle)

      For i As Integer = 1 To 3
         Dim task As New Subtask(i, 3000 + r.Next(4000))
         ThreadPool.QueueUserWorkItem(AddressOf task.DoSubtask)

      ' Wait for ANY subtask to complete.

      ' Create an array of ManualResetEvent wait handles. Each subtask will
      ' signal its ManualResetEvent when it is finished.
      Dim waitHandles() As WaitHandle = finished.ToArray()
      Dim index As Integer = WaitHandle.WaitTimeout

      index = WaitHandle.WaitAny(waitHandles)

      ' In an actual application, the result of the first subtask could be 
      ' processed now. 

      ' Wait for ALL subtasks to complete.

      ' Generate a report and return it as the result.
      Dim first As Subtask = subtasks(index)
      Dim total As Double = 0.0

      For Each task As Subtask In subtasks
         total += task.Result.TotalMilliseconds
      Next task

      e.Result = String.Format( _
         "Task {0} was the first to complete, with a duration of {1} seconds." _
            & vbLf & "The total duration of all tasks was {2} seconds." & vbLf, _
         first.SubtaskNumber, _
         first.Result.TotalMilliseconds / 1000, _
         total / 1000)

   End Sub 

   Private Shared Sub Completed(ByVal sender As Object, ByVal e As RunWorkerCompletedEventArgs)

      Dim worker As BackgroundWorker = CType(sender, BackgroundWorker)
      RemoveHandler worker.DoWork, AddressOf DoWork
      RemoveHandler worker.RunWorkerCompleted, AddressOf Completed

      outputBlock.Text &= e.Result & vbLf & "To repeat the demo, refresh the page."
   End Sub

End Class 

Class Subtask

   ' Signal this ManualResetEvent when the task is finished.
   Friend Finished As New ManualResetEvent(False)
   Friend SubtaskNumber As Integer
   Friend Result As TimeSpan
   Private data As Integer

   Friend Sub New(ByVal number As Integer, data As Integer)
      SubtaskNumber = number
      Me.data = data
   End Sub

   Friend Sub DoSubTask(ByVal state As Object)

      Dim start As DateTime = DateTime.Now
      ' Return a TimeSpan that represents the duration of the task.
      Result = DateTime.Now - start

   End Sub 

End Class

' This code produces output similar to the following:
'Demo is running. The BackgroundWorker waits for the first subtask to complete,
'then waits for all subtasks to complete and produces a report.
'Click here at any time to show that the user interface is responsive.
'Mouse clicked.
'Mouse clicked.
'Task 2 was the first to complete, with a duration of 3.431934 seconds.
'The total duration of all tasks was 11.1537855 seconds.
'To repeat the demo, refresh the page.

Windows Phone OS

Supported in: 8.1, 8.0, 7.1, 7.0

Windows Phone

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