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EventWaitHandle.Set Method

Sets the state of the event to signaled, allowing one or more waiting threads to proceed.

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

public bool Set()

Return Value

Type: System.Boolean
true if the operation succeeds; otherwise, false.

ExceptionCondition
ObjectDisposedException

The Close method was previously called on this System.Threading.EventWaitHandle.

For an EventWaitHandle with EventResetMode.AutoReset (including AutoResetEvent), the Set method releases a single thread. If there are no waiting threads, the wait handle remains signaled until a thread attempts to wait on it, or until its Reset method is called.

Important noteImportant

There is no guarantee that every call to the Set method will release a thread from an EventWaitHandle whose reset mode is EventResetMode.AutoReset. If two calls are too close together, so that the second call occurs before a thread has been released, only one thread is released. It is as if the second call did not happen. Also, if Set is called when there are no threads waiting and the EventWaitHandle is already signaled, the call has no effect.

For an EventWaitHandle with EventResetMode.ManualReset (including ManualResetEvent), calling the Set method leaves the wait handle in a signaled state until its Reset method is called.

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 the 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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