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WaitHandle Class

Encapsulates operating system–specific objects that wait for exclusive access to shared resources.

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

'Declaration
<ComVisibleAttribute(True)> _
Public MustInherit Class WaitHandle _
	Inherits MarshalByRefObject _
	Implements IDisposable
'Usage
Dim instance As WaitHandle

This class is typically used as a base class for synchronization objects. Classes derived from WaitHandle define a signaling mechanism to indicate taking or releasing access to a shared resource, but use the inherited WaitHandle methods to block while waiting for access to shared resources.

Use the static methods of this class to block a thread until one or more synchronization objects receive a signal.

WaitHandle implements the Dispose pattern. See Implementing Finalize and Dispose to Clean Up Unmanaged Resources. When you derive from WaitHandle, use the SafeWaitHandle property to store your native handle operating system handle. You do not need to override the protected Dispose method unless you use additional unmanaged resources.

The following code example shows how two threads can do background tasks while the Main thread waits for the tasks to complete using the static WaitAny and WaitAll methods of the WaitHandle class.

Imports System
Imports System.Threading

NotInheritable Public Class App
    ' Define an array with two AutoResetEvent WaitHandles. 
    Private Shared waitHandles() As WaitHandle = _
        {New AutoResetEvent(False), New AutoResetEvent(False)}

    ' Define a random number generator for testing. 
    Private Shared r As New Random()

    <MTAThreadAttribute> _
    Public Shared Sub Main() 
        ' Queue two tasks on two different threads;  
        ' wait until all tasks are completed. 
        Dim dt As DateTime = DateTime.Now
        Console.WriteLine("Main thread is waiting for BOTH tasks to complete.")
        ThreadPool.QueueUserWorkItem(AddressOf DoTask, waitHandles(0))
        ThreadPool.QueueUserWorkItem(AddressOf DoTask, waitHandles(1))
        WaitHandle.WaitAll(waitHandles)
        ' The time shown below should match the longest task.
        Console.WriteLine("Both tasks are completed (time waited={0})", _
            (DateTime.Now - dt).TotalMilliseconds)

        ' Queue up two tasks on two different threads;  
        ' wait until any tasks are completed.
        dt = DateTime.Now
        Console.WriteLine()
        Console.WriteLine("The main thread is waiting for either task to complete.")
        ThreadPool.QueueUserWorkItem(AddressOf DoTask, waitHandles(0))
        ThreadPool.QueueUserWorkItem(AddressOf DoTask, waitHandles(1))
        Dim index As Integer = WaitHandle.WaitAny(waitHandles)
        ' The time shown below should match the shortest task.
        Console.WriteLine("Task {0} finished first (time waited={1}).", _
            index + 1,(DateTime.Now - dt).TotalMilliseconds)

    End Sub 'Main

    Shared Sub DoTask(ByVal state As [Object]) 
        Dim are As AutoResetEvent = CType(state, AutoResetEvent)
        Dim time As Integer = 1000 * r.Next(2, 10)
        Console.WriteLine("Performing a task for {0} milliseconds.", time)
        Thread.Sleep(time)
        are.Set()

    End Sub 'DoTask
End Class 'App

' This code produces output similar to the following: 

'  Main thread is waiting for BOTH tasks to complete. 
'  Performing a task for 7000 milliseconds. 
'  Performing a task for 4000 milliseconds. 
'  Both tasks are completed (time waited=7064.8052) 
'  
'  The main thread is waiting for either task to complete. 
'  Performing a task for 2000 milliseconds. 
'  Performing a task for 2000 milliseconds. 
'  Task 1 finished first (time waited=2000.6528).

This type is thread safe.

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 3.5, 2.0, 1.0

XNA Framework

Supported in: 3.0, 2.0, 1.0
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