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ManualResetEvent Class

Updated: February 2010

Notifies one or more waiting threads that an event has occurred. This class cannot be inherited.

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

[HostProtectionAttribute(SecurityAction.LinkDemand, Synchronization = true, 
	ExternalThreading = true)]
public sealed class ManualResetEvent : EventWaitHandle


The HostProtectionAttribute attribute applied to this type or member has the following Resources property value: Synchronization | ExternalThreading. The HostProtectionAttribute does not affect desktop applications (which are typically started by double-clicking an icon, typing a command, or entering a URL in a browser). For more information, see the HostProtectionAttribute class or SQL Server Programming and Host Protection Attributes.

In the .NET Framework version 2.0, ManualResetEvent derives from the new EventWaitHandle class. A ManualResetEvent is functionally equivalent to an EventWaitHandle created with EventResetMode.ManualReset.


Unlike the ManualResetEvent class, the EventWaitHandle class provides access to named system synchronization events.

ManualResetEvent allows threads to communicate with each other by signaling. Typically, this communication concerns a task which one thread must complete before other threads can proceed.

When a thread begins an activity that must complete before other threads proceed, it calls Reset to put ManualResetEvent in the non-signaled state. This thread can be thought of as controlling the ManualResetEvent. Threads that call WaitOne on the ManualResetEvent will block, awaiting the signal. When the controlling thread completes the activity, it calls Set to signal that the waiting threads can proceed. All waiting threads are released.

Once it has been signaled, ManualResetEvent remains signaled until it is manually reset. That is, calls to WaitOne return immediately.

You can control the initial state of a ManualResetEvent by passing a Boolean value to the constructor, true if the initial state is signaled and false otherwise.

ManualResetEvent can also be used with the static WaitAll and WaitAny methods.

For more information about thread synchronization mechanisms, see ManualResetEvent in the conceptual documentation.

The following example demonstrates how ManualResetEvent works. The example starts with a ManualResetEvent in the unsignaled state (that is, false is passed to the constructor). The example creates three threads, each of which blocks on the ManualResetEvent by calling its WaitOne method. When you press the Enter key, the example calls the Set method, which releases all three threads. Contrast this with the behavior of the AutoResetEvent class, which releases threads one at a time, resetting automatically after each release.

Pressing the Enter key again demonstrates that the ManualResetEvent remains in the signaled state until its Reset method is called: The example starts two more threads. These threads do not block when they call the WaitOne method, but instead run to completion.

Pressing the Enter key again causes the example to call the Reset method and to start one more thread, which blocks when it calls WaitOne. Pressing the Enter key one final time calls Set to release the last thread, and the program ends.

using System;
using System.Threading;

public class Example
    // mre is used to block and release threads manually. It is 
    // created in the unsignaled state. 
    private static ManualResetEvent mre = new ManualResetEvent(false);

    static void Main()
        Console.WriteLine("\nStart 3 named threads that block on a ManualResetEvent:\n");

        for(int i = 0; i <= 2; i++)
            Thread t = new Thread(ThreadProc);
            t.Name = "Thread_" + i;

        Console.WriteLine("\nWhen all three threads have started, press Enter to call Set()" +
                          "\nto release all the threads.\n");


        Console.WriteLine("\nWhen a ManualResetEvent is signaled, threads that call WaitOne()" +
                          "\ndo not block. Press Enter to show this.\n");

        for(int i = 3; i <= 4; i++)
            Thread t = new Thread(ThreadProc);
            t.Name = "Thread_" + i;

        Console.WriteLine("\nPress Enter to call Reset(), so that threads once again block" +
                          "\nwhen they call WaitOne().\n");


        // Start a thread that waits on the ManualResetEvent.
        Thread t5 = new Thread(ThreadProc);
        t5.Name = "Thread_5";

        Console.WriteLine("\nPress Enter to call Set() and conclude the demo.");


        // If you run this example in Visual Studio, uncomment the following line: 

    private static void ThreadProc()
        string name = Thread.CurrentThread.Name;

        Console.WriteLine(name + " starts and calls mre.WaitOne()");


        Console.WriteLine(name + " ends.");

/* This example produces output similar to the following:

Start 3 named threads that block on a ManualResetEvent:

Thread_0 starts and calls mre.WaitOne()
Thread_1 starts and calls mre.WaitOne()
Thread_2 starts and calls mre.WaitOne()

When all three threads have started, press Enter to call Set()
to release all the threads.

Thread_2 ends.
Thread_0 ends.
Thread_1 ends.

When a ManualResetEvent is signaled, threads that call WaitOne()
do not block. Press Enter to show this.

Thread_3 starts and calls mre.WaitOne()
Thread_3 ends.
Thread_4 starts and calls mre.WaitOne()
Thread_4 ends.

Press Enter to call Reset(), so that threads once again block
when they call WaitOne().

Thread_5 starts and calls mre.WaitOne()

Press Enter to call Set() and conclude the demo.

Thread_5 ends.

This class 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




February 2010

Improved the example.

Customer feedback.