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EventWaitHandle Constructor (Boolean, EventResetMode)

Initializes a new instance of the EventWaitHandle class, specifying whether the wait handle is initially signaled, and whether it resets automatically or manually.

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

public EventWaitHandle(
	bool initialState,
	EventResetMode mode
)

Parameters

initialState
Type: System.Boolean

true to set the initial state to signaled; false to set it to nonsignaled.

mode
Type: System.Threading.EventResetMode

One of the EventResetMode values that determines whether the event resets automatically or manually.

If the initial state of the event is nonsignaled, threads that wait on the event will block. If the initial state is signaled, and the ManualReset flag is specified for mode, threads that wait on the event will not block. If the initial state is signaled, and mode is AutoReset, the first thread that waits on the event will be released immediately, after which the event will reset, and subsequent threads will block.

The following code example uses the SignalAndWait(WaitHandle, WaitHandle) method overload to allow the main thread to signal a blocked thread and then wait until the thread finishes a task.

The example starts five threads and allows them to block on an EventWaitHandle created with the EventResetMode.AutoReset flag, then releases one thread each time the user presses ENTER key. The example then queues another five threads and releases them all using an EventWaitHandle created with the EventResetMode.ManualReset flag.

using System;
using System.Threading;

public class Example
{
    // The EventWaitHandle used to demonstrate the difference 
    // between AutoReset and ManualReset synchronization events. 
    // 
    private static EventWaitHandle ewh;

    // A counter to make sure all threads are started and 
    // blocked before any are released. A Long is used to show 
    // the use of the 64-bit Interlocked methods. 
    // 
    private static long threadCount = 0;

    // An AutoReset event that allows the main thread to block 
    // until an exiting thread has decremented the count. 
    // 
    private static EventWaitHandle clearCount = 
        new EventWaitHandle(false, EventResetMode.AutoReset);

    [MTAThread]
    public static void Main()
    {
        // Create an AutoReset EventWaitHandle. 
        //
        ewh = new EventWaitHandle(false, EventResetMode.AutoReset);

        // Create and start five numbered threads. Use the 
        // ParameterizedThreadStart delegate, so the thread 
        // number can be passed as an argument to the Start  
        // method. 
        for (int i = 0; i <= 4; i++)
        {
            Thread t = new Thread(
                new ParameterizedThreadStart(ThreadProc)
            );
            t.Start(i);
        }

        // Wait until all the threads have started and blocked. 
        // When multiple threads use a 64-bit value on a 32-bit 
        // system, you must access the value through the 
        // Interlocked class to guarantee thread safety. 
        // 
        while (Interlocked.Read(ref threadCount) < 5)
        {
            Thread.Sleep(500);
        }

        // Release one thread each time the user presses ENTER, 
        // until all threads have been released. 
        // 
        while (Interlocked.Read(ref threadCount) > 0)
        {
            Console.WriteLine("Press ENTER to release a waiting thread.");
            Console.ReadLine();

            // SignalAndWait signals the EventWaitHandle, which 
            // releases exactly one thread before resetting,  
            // because it was created with AutoReset mode.  
            // SignalAndWait then blocks on clearCount, to  
            // allow the signaled thread to decrement the count 
            // before looping again. 
            //
            WaitHandle.SignalAndWait(ewh, clearCount);
        }
        Console.WriteLine();

        // Create a ManualReset EventWaitHandle. 
        //
        ewh = new EventWaitHandle(false, EventResetMode.ManualReset);

        // Create and start five more numbered threads. 
        // 
        for(int i=0; i<=4; i++)
        {
            Thread t = new Thread(
                new ParameterizedThreadStart(ThreadProc)
            );
            t.Start(i);
        }

        // Wait until all the threads have started and blocked. 
        // 
        while (Interlocked.Read(ref threadCount) < 5)
        {
            Thread.Sleep(500);
        }

        // Because the EventWaitHandle was created with 
        // ManualReset mode, signaling it releases all the 
        // waiting threads. 
        //
        Console.WriteLine("Press ENTER to release the waiting threads.");
        Console.ReadLine();
        ewh.Set();

    }

    public static void ThreadProc(object data)
    {
        int index = (int) data;

        Console.WriteLine("Thread {0} blocks.", data);
        // Increment the count of blocked threads.
        Interlocked.Increment(ref threadCount);

        // Wait on the EventWaitHandle.
        ewh.WaitOne();

        Console.WriteLine("Thread {0} exits.", data);
        // Decrement the count of blocked threads.
        Interlocked.Decrement(ref threadCount);

        // After signaling ewh, the main thread blocks on 
        // clearCount until the signaled thread has  
        // decremented the count. Signal it now. 
        //
        clearCount.Set();
    }
}

.NET Framework

Supported in: 4.5.2, 4.5.1, 4.5, 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone 8, Silverlight 8.1

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

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

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