WaitHandle.WaitAny Method (WaitHandle[], TimeSpan, Boolean)


The .NET API Reference documentation has a new home. Visit the .NET API Browser on docs.microsoft.com to see the new experience.

Waits for any of the elements in the specified array to receive a signal, using a TimeSpan 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 int WaitAny(
	WaitHandle[] waitHandles,
	TimeSpan timeout,
	bool exitContext


Type: System.Threading.WaitHandle[]

A WaitHandle array containing the objects for which the current instance will wait.

Type: System.TimeSpan

A TimeSpan that represents the number of milliseconds to wait, or a TimeSpan that represents -1 milliseconds 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.Int32

The array index of the object that satisfied the wait, or WaitTimeout if no object satisfied the wait and a time interval equivalent to timeout has passed.

Exception Condition

The waitHandles parameter is null.


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


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


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


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


timeout is greater than Int32.MaxValue.


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


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


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

If timeout 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 WaitAny method returns true if the wait completes because 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 WaitAny method throws an AbandonedMutexException only when the wait completes because of an abandoned mutex. If waitHandles contains a released mutex with a lower index number than the abandoned mutex, the WaitAny method completes normally and the exception is not thrown.


In versions of the .NET Framework earlier than version 2.0, if a thread exits or aborts without explicitly releasing a Mutex, and that Mutex is at index 0 (zero) in a WaitAny array on another thread, the index returned by WaitAny is 128 instead of 0.

This method returns when the wait terminates, either when any of the handles are signaled or when a time-out occurs. If more than one object becomes signaled during the call, the return value is the array index of the signaled object with the smallest index value of all the signaled objects. On some implementations, if more that 64 handles are passed, a NotSupportedException is thrown.

The maximum value for timeout is Int32.MaxValue.

The exitContext parameter has no effect unless the WaitAny 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 does not derive 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 WaitAny method. The thread returns to the original nondefault context after the call to the WaitAny method completes.

This can be useful when the context-bound class has SynchronizationAttribute. 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 WaitAny 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 WaitAny method returns, the thread that made the call must wait to reenter the synchronization domain.

The following code example demonstrates how to use the thread pool to simultaneously search for a file on multiple disks. For space considerations, only the root directory of each disk is searched.

using System;
using System.IO;
using System.Threading;

class Test
    static void Main()
        Search search = new Search();

class Search
    // Maintain state information to pass to FindCallback.
    class State
        public AutoResetEvent autoEvent;
        public string         fileName;

        public State(AutoResetEvent autoEvent, string fileName)
            this.autoEvent    = autoEvent;
            this.fileName     = fileName;

    AutoResetEvent[] autoEvents;
    String[] diskLetters;

    public Search()
        // Retrieve an array of disk letters.
        diskLetters = Environment.GetLogicalDrives();

        autoEvents = new AutoResetEvent[diskLetters.Length];
        for(int i = 0; i < diskLetters.Length; i++)
            autoEvents[i] = new AutoResetEvent(false);

    // Search for fileName in the root directory of all disks.
    public void FindFile(string fileName)
        for(int i = 0; i < diskLetters.Length; i++)
            Console.WriteLine("Searching for {0} on {1}.",
                fileName, diskLetters[i]);
                new WaitCallback(FindCallback), 
                new State(autoEvents[i], diskLetters[i] + fileName));

        // Wait for the first instance of the file to be found.
        int index = WaitHandle.WaitAny(
            autoEvents, new TimeSpan(0, 0, 3), false);
        if(index == WaitHandle.WaitTimeout)
            Console.WriteLine("\n{0} not found.", fileName);
            Console.WriteLine("\n{0} found on {1}.", fileName,

    // Search for stateInfo.fileName.
    void FindCallback(object state)
        State stateInfo = (State)state;

        // Signal if the file is found.

.NET Framework
Available since 1.1
Return to top