WaitHandle.WaitAll Method (WaitHandle(), Int32, Boolean)


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Waits for all the elements in the specified array to receive a signal, using an Int32 value to specify the time interval and specifying whether to exit the synchronization domain before the wait.

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

Public Shared Function WaitAll (
	waitHandles As WaitHandle(),
	millisecondsTimeout As Integer,
	exitContext As Boolean
) As Boolean


Type: System.Threading.WaitHandle()

A WaitHandle array containing the objects for which the current instance will wait. This array cannot contain multiple references to the same object (duplicates).

Type: System.Int32

The number of milliseconds to wait, or Timeout.Infinite (-1) to wait indefinitely.

Type: System.Boolean

true to exit the synchronization domain for the context before the wait (if in a synchronized context), and reacquire it afterward; otherwise, false.

Return Value

Type: System.Boolean

true when every element in waitHandles has received a signal; otherwise, false.

Exception Condition

The waitHandles parameter is null.


One or more of the objects in the waitHandles array is null.


waitHandles is an array with no elements and the .NET Framework version is 2.0 or later.


The waitHandles array contains elements that are duplicates.


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


The STAThreadAttribute attribute is applied to the thread procedure for the current thread, and waitHandles contains more than one element.


waitHandles is an array with no elements and the .NET Framework version is 1.0 or 1.1.


millisecondsTimeout is a negative number other than -1, which represents an infinite time-out.


The wait completed because a thread exited without releasing a mutex. This exception is not thrown on Windows 98 or Windows Millennium Edition.


The waitHandles array contains a transparent proxy for a WaitHandle in another application domain.

If millisecondsTimeout is zero, the method does not block. It tests the state of the wait handles and returns immediately.

AbandonedMutexException is new in the .NET Framework version 2.0. In previous versions, the WaitAll method returns true when a mutex is abandoned. An abandoned mutex often indicates a serious coding error. In the case of a system-wide mutex, it might indicate that an application has been terminated abruptly (for example, by using Windows Task Manager). The exception contains information useful for debugging.

The WaitAll method returns when the wait terminates, which means either when all the handles are signaled or when time-out occurs. On some implementations, if more than 64 handles are passed, a NotSupportedException is thrown. If there are duplicates in the array, the call fails with a DuplicateWaitObjectException.


The WaitAll method is not supported on threads that have STAThreadAttribute.

The exitContext parameter has no effect unless the WaitAll method is called from inside a nondefault managed context. This can happen if your thread is inside a call to an instance of a class derived from ContextBoundObject. Even if you are currently executing a method on a class that is not derived from ContextBoundObject, like String, you can be in a nondefault context if a ContextBoundObject is on your stack in the current application domain.

When your code is executing in a nondefault context, specifying true for exitContext causes the thread to exit the nondefault managed context (that is, to transition to the default context) before executing the WaitAll method. The thread returns to the original nondefault context after the call to the WaitAll method completes.

This can be useful when the context-bound class has the SynchronizationAttribute attribute. In that case, all calls to members of the class are automatically synchronized, and the synchronization domain is the entire body of code for the class. If code in the call stack of a member calls the WaitAll method and specifies true for exitContext, the thread exits the synchronization domain, allowing a thread that is blocked on a call to any member of the object to proceed. When the WaitAll method returns, the thread that made the call must wait to reenter the synchronization domain.

The following code example shows how to use the thread pool to asynchronously create and write to a group of files. Each write operation is queued as a work item and signals when it is finished. The main thread waits for all the items to signal and then exits.

Imports System
Imports System.IO
Imports System.Security.Permissions
Imports System.Threading

Public Class Test

    ' WaitHandle.WaitAll requires a multithreaded apartment 
    ' when using multiple wait handles.
    <MTAThreadAttribute> _
    Shared Sub Main()
        Const numberOfFiles As Integer = 5
        Dim dirName As String = "C:\TestTest"
        Dim fileName As String 

        Dim byteArray() As Byte 
        Dim randomGenerator As New Random()

        Dim manualEvents(numberOfFiles - 1) As ManualResetEvent
        Dim stateInfo As State 

        If Directory.Exists(dirName) <> True Then
        End If

        ' Queue the work items that create and write to the files.
        For i As Integer = 0 To numberOfFiles - 1
            fileName = String.Concat( _
                dirName, "\Test", i.ToString(), ".dat")

            ' Create random data to write to the file.
            byteArray = New Byte(1000000){}

            manualEvents(i) = New ManualResetEvent(false)

            stateInfo = _ 
                New State(fileName, byteArray, manualEvents(i))

            ThreadPool.QueueUserWorkItem(AddressOf _
                Writer.WriteToFile, stateInfo)
        Next i

        ' Since ThreadPool threads are background threads, 
        ' wait for the work items to signal before exiting.
        If WaitHandle.WaitAll(manualEvents, 5000, false) = True  Then

            Console.WriteLine("Files written - main exiting.")

            ' The wait operation times out.
            Console.WriteLine("Error writing files - main exiting.")
        End If
    End Sub

End Class

' Maintain state to pass to WriteToFile.
Public Class State

    Public fileName As String
    Public byteArray As Byte()
    Public manualEvent As ManualResetEvent

    Sub New(fileName As String, byteArray() As Byte, _
        manualEvent As ManualResetEvent)

        Me.fileName = fileName
        Me.byteArray = byteArray
        Me.manualEvent = manualEvent
    End Sub

End Class

Public Class Writer

    Private Sub New()
    End Sub

    Shared workItemCount As Integer = 0

    Shared Sub WriteToFile(state As Object)
        Dim workItemNumber As Integer = workItemCount
        Console.WriteLine("Starting work item {0}.", _
        Dim stateInfo As State = CType(state, State)
        Dim fileWriter As FileStream = Nothing

        ' Create and write to the file.
            fileWriter = New FileStream( _
                stateInfo.fileName, FileMode.Create)
            fileWriter.Write(stateInfo.byteArray, _
                0, stateInfo.byteArray.Length)
            If Not fileWriter Is Nothing Then
            End If

            ' Signal Main that the work item has finished.
            Console.WriteLine("Ending work item {0}.", _
        End Try
    End Sub

End Class

.NET Framework
Available since 1.1
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