WaitHandle.WaitAll Method (WaitHandle[])


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

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

public static bool WaitAll(
	WaitHandle[] waitHandles


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.

Return Value

Type: System.Boolean

true when every element in waitHandles has received a signal; otherwise the method never returns.

Exception Condition

The waitHandles parameter is null. -or-

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


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


In the .NET for Windows Store apps or the Portable Class Library, catch the base class exception, ArgumentException, instead.

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.


The wait terminated 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.

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 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 a DuplicateWaitObjectException.


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

Calling this method overload is equivalent to calling the WaitAll(WaitHandle[], Int32, Boolean) method overload and specifying -1 (or Timeout.Infinite) for millisecondsTimeoutand true for exitContext.

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.
        Console.WriteLine("Files written - 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}.", 

Universal Windows Platform
Available since 8
.NET Framework
Available since 1.1
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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