AutoResetEvent Class

Definition

Namespace:

System.Threading

Assembly:

System.Threading.dll

Source:

AutoResetEvent.cs

Represents a thread synchronization event that, when signaled, releases one single waiting thread and then resets automatically. This class cannot be inherited.

public ref class AutoResetEvent sealed : System::Threading::EventWaitHandle

public sealed class AutoResetEvent : System.Threading.EventWaitHandle

type AutoResetEvent = class
    inherit EventWaitHandle

Public NotInheritable Class AutoResetEvent
Inherits EventWaitHandle

Inheritance

Object

MarshalByRefObject

WaitHandle

EventWaitHandle

AutoResetEvent

Examples

The following example shows how to use AutoResetEvent to release one thread at a time, by calling the Set method (on the base class) each time the user presses the Enter key. The example starts three threads, which wait on an AutoResetEvent that was created in the signaled state. The first thread is released immediately, because the AutoResetEvent is already in the signaled state. This resets the AutoResetEvent to the non-signaled state, so that subsequent threads block. The blocked threads are not released until the user releases them one at a time by pressing the Enter key.

After the threads are released from the first AutoResetEvent, they wait on another AutoResetEvent that was created in the non-signaled state. All three threads block, so the Set method must be called three times to release them all.

using namespace System;
using namespace System::Threading;

ref class Example
{
private:
    static AutoResetEvent^ event_1 = gcnew AutoResetEvent(true);
    static AutoResetEvent^ event_2 = gcnew AutoResetEvent(false);

    static void ThreadProc()
    {
        String^ name = Thread::CurrentThread->Name;

        Console::WriteLine("{0} waits on AutoResetEvent #1.", name);
        event_1->WaitOne();
        Console::WriteLine("{0} is released from AutoResetEvent #1.", name);

        Console::WriteLine("{0} waits on AutoResetEvent #2.", name);
        event_2->WaitOne();
        Console::WriteLine("{0} is released from AutoResetEvent #2.", name);

        Console::WriteLine("{0} ends.", name);
    }

public:
    static void Demo()
    {
        Console::WriteLine("Press Enter to create three threads and start them.\r\n" +
                           "The threads wait on AutoResetEvent #1, which was created\r\n" +
                           "in the signaled state, so the first thread is released.\r\n" +
                           "This puts AutoResetEvent #1 into the unsignaled state.");
        Console::ReadLine();
            
        for (int i = 1; i < 4; i++)
        {
            Thread^ t = gcnew Thread(gcnew ThreadStart(&ThreadProc));
            t->Name = "Thread_" + i;
            t->Start();
        }
        Thread::Sleep(250);

        for (int i = 0; i < 2; i++)
        {
            Console::WriteLine("Press Enter to release another thread.");
            Console::ReadLine();
            event_1->Set();
            Thread::Sleep(250);
        }

        Console::WriteLine("\r\nAll threads are now waiting on AutoResetEvent #2.");
        for (int i = 0; i < 3; i++)
        {
            Console::WriteLine("Press Enter to release a thread.");
            Console::ReadLine();
            event_2->Set();
            Thread::Sleep(250);
        }

        // Visual Studio: Uncomment the following line.
        //Console::Readline();
    }
};

void main()
{
    Example::Demo();
}

/* This example produces output similar to the following:

Press Enter to create three threads and start them.
The threads wait on AutoResetEvent #1, which was created
in the signaled state, so the first thread is released.
This puts AutoResetEvent #1 into the unsignaled state.

Thread_1 waits on AutoResetEvent #1.
Thread_1 is released from AutoResetEvent #1.
Thread_1 waits on AutoResetEvent #2.
Thread_3 waits on AutoResetEvent #1.
Thread_2 waits on AutoResetEvent #1.
Press Enter to release another thread.

Thread_3 is released from AutoResetEvent #1.
Thread_3 waits on AutoResetEvent #2.
Press Enter to release another thread.

Thread_2 is released from AutoResetEvent #1.
Thread_2 waits on AutoResetEvent #2.

All threads are now waiting on AutoResetEvent #2.
Press Enter to release a thread.

Thread_2 is released from AutoResetEvent #2.
Thread_2 ends.
Press Enter to release a thread.

Thread_1 is released from AutoResetEvent #2.
Thread_1 ends.
Press Enter to release a thread.

Thread_3 is released from AutoResetEvent #2.
Thread_3 ends.
 */

using System;
using System.Threading;

// Visual Studio: Replace the default class in a Console project with 
//                the following class.
class Example
{
    private static AutoResetEvent event_1 = new AutoResetEvent(true);
    private static AutoResetEvent event_2 = new AutoResetEvent(false);

    static void Main()
    {
        Console.WriteLine("Press Enter to create three threads and start them.\r\n" +
                          "The threads wait on AutoResetEvent #1, which was created\r\n" +
                          "in the signaled state, so the first thread is released.\r\n" +
                          "This puts AutoResetEvent #1 into the unsignaled state.");
        Console.ReadLine();
            
        for (int i = 1; i < 4; i++)
        {
            Thread t = new Thread(ThreadProc);
            t.Name = "Thread_" + i;
            t.Start();
        }
        Thread.Sleep(250);

        for (int i = 0; i < 2; i++)
        {
            Console.WriteLine("Press Enter to release another thread.");
            Console.ReadLine();
            event_1.Set();
            Thread.Sleep(250);
        }

        Console.WriteLine("\r\nAll threads are now waiting on AutoResetEvent #2.");
        for (int i = 0; i < 3; i++)
        {
            Console.WriteLine("Press Enter to release a thread.");
            Console.ReadLine();
            event_2.Set();
            Thread.Sleep(250);
        }

        // Visual Studio: Uncomment the following line.
        //Console.Readline();
    }

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

        Console.WriteLine("{0} waits on AutoResetEvent #1.", name);
        event_1.WaitOne();
        Console.WriteLine("{0} is released from AutoResetEvent #1.", name);

        Console.WriteLine("{0} waits on AutoResetEvent #2.", name);
        event_2.WaitOne();
        Console.WriteLine("{0} is released from AutoResetEvent #2.", name);

        Console.WriteLine("{0} ends.", name);
    }
}

/* This example produces output similar to the following:

Press Enter to create three threads and start them.
The threads wait on AutoResetEvent #1, which was created
in the signaled state, so the first thread is released.
This puts AutoResetEvent #1 into the unsignaled state.

Thread_1 waits on AutoResetEvent #1.
Thread_1 is released from AutoResetEvent #1.
Thread_1 waits on AutoResetEvent #2.
Thread_3 waits on AutoResetEvent #1.
Thread_2 waits on AutoResetEvent #1.
Press Enter to release another thread.

Thread_3 is released from AutoResetEvent #1.
Thread_3 waits on AutoResetEvent #2.
Press Enter to release another thread.

Thread_2 is released from AutoResetEvent #1.
Thread_2 waits on AutoResetEvent #2.

All threads are now waiting on AutoResetEvent #2.
Press Enter to release a thread.

Thread_2 is released from AutoResetEvent #2.
Thread_2 ends.
Press Enter to release a thread.

Thread_1 is released from AutoResetEvent #2.
Thread_1 ends.
Press Enter to release a thread.

Thread_3 is released from AutoResetEvent #2.
Thread_3 ends.
 */

Imports System.Threading

' Visual Studio: Replace the default class in a Console project with 
'                the following class.
Class Example

    Private Shared event_1 As New AutoResetEvent(True)
    Private Shared event_2 As New AutoResetEvent(False)

    <MTAThread()> _
    Shared Sub Main()
    
        Console.WriteLine("Press Enter to create three threads and start them." & vbCrLf & _
                          "The threads wait on AutoResetEvent #1, which was created" & vbCrLf & _
                          "in the signaled state, so the first thread is released." & vbCrLf & _
                          "This puts AutoResetEvent #1 into the unsignaled state.")
        Console.ReadLine()
            
        For i As Integer = 1 To 3
            Dim t As New Thread(AddressOf ThreadProc)
            t.Name = "Thread_" & i
            t.Start()
        Next
        Thread.Sleep(250)

        For i As Integer = 1 To 2
            Console.WriteLine("Press Enter to release another thread.")
            Console.ReadLine()

            event_1.Set()
            Thread.Sleep(250)
        Next

        Console.WriteLine(vbCrLf & "All threads are now waiting on AutoResetEvent #2.")
        For i As Integer = 1 To 3
            Console.WriteLine("Press Enter to release a thread.")
            Console.ReadLine()

            event_2.Set()
            Thread.Sleep(250)
        Next

        ' Visual Studio: Uncomment the following line.
        'Console.Readline()
    End Sub

    Shared Sub ThreadProc()
    
        Dim name As String = Thread.CurrentThread.Name

        Console.WriteLine("{0} waits on AutoResetEvent #1.", name)
        event_1.WaitOne()
        Console.WriteLine("{0} is released from AutoResetEvent #1.", name)

        Console.WriteLine("{0} waits on AutoResetEvent #2.", name)
        event_2.WaitOne()
        Console.WriteLine("{0} is released from AutoResetEvent #2.", name)

        Console.WriteLine("{0} ends.", name)
    End Sub
End Class

' This example produces output similar to the following:
'
'Press Enter to create three threads and start them.
'The threads wait on AutoResetEvent #1, which was created
'in the signaled state, so the first thread is released.
'This puts AutoResetEvent #1 into the unsignaled state.
'
'Thread_1 waits on AutoResetEvent #1.
'Thread_1 is released from AutoResetEvent #1.
'Thread_1 waits on AutoResetEvent #2.
'Thread_3 waits on AutoResetEvent #1.
'Thread_2 waits on AutoResetEvent #1.
'Press Enter to release another thread.
'
'Thread_3 is released from AutoResetEvent #1.
'Thread_3 waits on AutoResetEvent #2.
'Press Enter to release another thread.
'
'Thread_2 is released from AutoResetEvent #1.
'Thread_2 waits on AutoResetEvent #2.
'
'All threads are now waiting on AutoResetEvent #2.
'Press Enter to release a thread.
'
'Thread_2 is released from AutoResetEvent #2.
'Thread_2 ends.
'Press Enter to release a thread.
'
'Thread_1 is released from AutoResetEvent #2.
'Thread_1 ends.
'Press Enter to release a thread.
'
'Thread_3 is released from AutoResetEvent #2.
'Thread_3 ends.

Remarks

You use AutoResetEvent, ManualResetEvent, and EventWaitHandle for thread interaction (or thread signaling). For more information, see Thread interaction.

A thread waits for a signal by calling AutoResetEvent.WaitOne. If the AutoResetEvent is in the non-signaled state, the thread blocks until AutoResetEvent.Set is called. Calling Set signals AutoResetEvent to release a waiting thread. AutoResetEvent remains signaled until Reset is called or a single waiting thread is released, at which time it automatically returns to the non-signaled state.

If no threads are waiting when the AutoResetEvent goes into a signaled state, the state remains signaled until a thread observes the signal (by calling WaitOne). That thread does not block: the AutoResetEvent releases the thread immediately and returns to the non-signaled state.

Important

There's no guarantee that every call to the Set method will release a thread. 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's as if the second call did not happen. Also, if Set is called when there are no threads waiting and the AutoResetEvent is already signaled, the call has no effect.

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

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

AutoResetEvent derives from the EventWaitHandle class. An AutoResetEvent is functionally equivalent to an EventWaitHandle created with EventResetMode.AutoReset.

Note

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

Important

This type implements the IDisposable interface. When you have finished using the type, you should dispose of it either directly or indirectly. To dispose of the type directly, call its Dispose method in a try/catch block. To dispose of it indirectly, use a language construct such as using (in C#) or Using (in Visual Basic). For more information, see the "Using an Object that Implements IDisposable" section on the IDisposable interface page.

Constructors

AutoResetEvent(Boolean)

Initializes a new instance of the AutoResetEvent class with a Boolean value indicating whether to set the initial state to signaled.

Fields

WaitTimeout

Indicates that a WaitAny(WaitHandle[], Int32, Boolean) operation timed out before any of the wait handles were signaled. This field is constant. (Inherited from WaitHandle)

Properties

Handle	Obsolete. Gets or sets the native operating system handle. (Inherited from WaitHandle)
SafeWaitHandle	Gets or sets the native operating system handle. (Inherited from WaitHandle)

Methods

Close()	Releases all resources held by the current WaitHandle. (Inherited from WaitHandle)
CreateObjRef(Type)	Creates an object that contains all the relevant information required to generate a proxy used to communicate with a remote object. (Inherited from MarshalByRefObject)
Dispose()	Releases all resources used by the current instance of the WaitHandle class. (Inherited from WaitHandle)
Dispose(Boolean)	When overridden in a derived class, releases the unmanaged resources used by the WaitHandle, and optionally releases the managed resources. (Inherited from WaitHandle)
Equals(Object)	Determines whether the specified object is equal to the current object. (Inherited from Object)
GetAccessControl()	Gets an EventWaitHandleSecurity object that represents the access control security for the named system event represented by the current EventWaitHandle object. (Inherited from EventWaitHandle)
GetHashCode()	Serves as the default hash function. (Inherited from Object)
GetLifetimeService()	Obsolete. Retrieves the current lifetime service object that controls the lifetime policy for this instance. (Inherited from MarshalByRefObject)
GetType()	Gets the Type of the current instance. (Inherited from Object)
InitializeLifetimeService()	Obsolete. Obtains a lifetime service object to control the lifetime policy for this instance. (Inherited from MarshalByRefObject)
MemberwiseClone()	Creates a shallow copy of the current Object. (Inherited from Object)
MemberwiseClone(Boolean)	Creates a shallow copy of the current MarshalByRefObject object. (Inherited from MarshalByRefObject)
Reset()	Sets the state of the event to nonsignaled, causing threads to block. (Inherited from EventWaitHandle)
Set()	Sets the state of the event to signaled, allowing one or more waiting threads to proceed. (Inherited from EventWaitHandle)
SetAccessControl(EventWaitHandleSecurity)	Sets the access control security for a named system event. (Inherited from EventWaitHandle)
ToString()	Returns a string that represents the current object. (Inherited from Object)
WaitOne()	Blocks the current thread until the current WaitHandle receives a signal. (Inherited from WaitHandle)
WaitOne(Int32, Boolean)	Blocks the current thread until the current WaitHandle receives a signal, using a 32-bit signed integer to specify the time interval and specifying whether to exit the synchronization domain before the wait. (Inherited from WaitHandle)
WaitOne(Int32)	Blocks the current thread until the current WaitHandle receives a signal, using a 32-bit signed integer to specify the time interval in milliseconds. (Inherited from WaitHandle)
WaitOne(TimeSpan, Boolean)	Blocks the current thread until the current instance receives a signal, using a TimeSpan to specify the time interval and specifying whether to exit the synchronization domain before the wait. (Inherited from WaitHandle)
WaitOne(TimeSpan)	Blocks the current thread until the current instance receives a signal, using a TimeSpan to specify the time interval. (Inherited from WaitHandle)

Extension Methods

GetAccessControl(EventWaitHandle)	Returns the security descriptors for the specified handle.
SetAccessControl(EventWaitHandle, EventWaitHandleSecurity)	Sets the security descriptors for the specified event wait handle.
GetSafeWaitHandle(WaitHandle)	Gets the safe handle for a native operating system wait handle.
SetSafeWaitHandle(WaitHandle, SafeWaitHandle)	Sets a safe handle for a native operating system wait handle.

Thread Safety

This class is thread safe.

See also

• WaitHandle
• Managed Threading
• Overview of synchronization primitives