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

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 static bool WaitAll(
	WaitHandle[] waitHandles,
	int millisecondsTimeout,
	bool exitContext


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.


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.

Notes on Exiting the Context

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.

using System;
using System.IO;
using System.Security.Permissions;
using System.Threading;

class Test
    static void Main()
        const int numberOfFiles = 5;
        string dirName = @"C:\TestTest";
        string fileName;

        byte[] byteArray;
        Random randomGenerator = new Random();

        ManualResetEvent[] manualEvents = 
            new ManualResetEvent[numberOfFiles];
        State stateInfo;


        // Queue the work items that create and write to the files.
        for(int i = 0; i < numberOfFiles; i++)
            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(new WaitCallback(
                Writer.WriteToFile), stateInfo);

        // Since ThreadPool threads are background threads, 
        // wait for the work items to signal before exiting.
        if(WaitHandle.WaitAll(manualEvents, 5000, false))
            Console.WriteLine("Files written - main exiting.");
            // The wait operation times out.
            Console.WriteLine("Error writing files - main exiting.");

// Maintain state to pass to WriteToFile.
class State
    public string fileName;
    public byte[] byteArray;
    public ManualResetEvent manualEvent;

    public State(string fileName, byte[] byteArray, 
        ManualResetEvent manualEvent)
        this.fileName = fileName;
        this.byteArray = byteArray;
        this.manualEvent = manualEvent;

class Writer
    static int workItemCount = 0;
    Writer() {}

    public static void WriteToFile(object state)
        int workItemNumber = workItemCount;
        Interlocked.Increment(ref workItemCount);
        Console.WriteLine("Starting work item {0}.",
        State stateInfo = (State)state;
        FileStream fileWriter = null;

        // Create and write to the file.
            fileWriter = new FileStream(
                stateInfo.fileName, FileMode.Create);
                0, stateInfo.byteArray.Length);
            if(fileWriter != null)

            // Signal Main that the work item has finished.
            Console.WriteLine("Ending work item {0}.", 

.NET Framework

Supported in: 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

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