CountdownEvent Class

.NET Framework (current version)
 

Represents a synchronization primitive that is signaled when its count reaches zero.

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

System.Object
  System.Threading.CountdownEvent

<ComVisibleAttribute(False)>
<HostProtectionAttribute(SecurityAction.LinkDemand, Synchronization := True,
	ExternalThreading := True)>
Public Class CountdownEvent
	Implements IDisposable

NameDescription
System_CAPS_pubmethodCountdownEvent(Int32)

Initializes a new instance of CountdownEvent class with the specified count.

NameDescription
System_CAPS_pubpropertyCurrentCount

Gets the number of remaining signals required to set the event.

System_CAPS_pubpropertyInitialCount

Gets the numbers of signals initially required to set the event.

System_CAPS_pubpropertyIsSet

Indicates whether the CountdownEvent object's current count has reached zero..

System_CAPS_pubpropertyWaitHandle

Gets a WaitHandle that is used to wait for the event to be set.

NameDescription
System_CAPS_pubmethodAddCount()

Increments the CountdownEvent's current count by one.

System_CAPS_pubmethodAddCount(Int32)

Increments the CountdownEvent's current count by a specified value.

System_CAPS_pubmethodDispose()

Releases all resources used by the current instance of the CountdownEvent class.

System_CAPS_protmethodDispose(Boolean)

Releases the unmanaged resources used by the CountdownEvent, and optionally releases the managed resources.

System_CAPS_pubmethodEquals(Object)

Determines whether the specified object is equal to the current object.(Inherited from Object.)

System_CAPS_protmethodFinalize()

Allows an object to try to free resources and perform other cleanup operations before it is reclaimed by garbage collection.(Inherited from Object.)

System_CAPS_pubmethodGetHashCode()

Serves as the default hash function. (Inherited from Object.)

System_CAPS_pubmethodGetType()

Gets the Type of the current instance.(Inherited from Object.)

System_CAPS_protmethodMemberwiseClone()

Creates a shallow copy of the current Object.(Inherited from Object.)

System_CAPS_pubmethodReset()

Resets the CurrentCount to the value of InitialCount.

System_CAPS_pubmethodReset(Int32)

Resets the InitialCount property to a specified value.

System_CAPS_pubmethodSignal()

Registers a signal with the CountdownEvent, decrementing the value of CurrentCount.

System_CAPS_pubmethodSignal(Int32)

Registers multiple signals with the CountdownEvent, decrementing the value of CurrentCount by the specified amount.

System_CAPS_pubmethodToString()

Returns a string that represents the current object.(Inherited from Object.)

System_CAPS_pubmethodTryAddCount()

Attempts to increment CurrentCount by one.

System_CAPS_pubmethodTryAddCount(Int32)

Attempts to increment CurrentCount by a specified value.

System_CAPS_pubmethodWait()

Blocks the current thread until the CountdownEvent is set.

System_CAPS_pubmethodWait(CancellationToken)

Blocks the current thread until the CountdownEvent is set, while observing a CancellationToken.

System_CAPS_pubmethodWait(Int32)

Blocks the current thread until the CountdownEvent is set, using a 32-bit signed integer to measure the timeout.

System_CAPS_pubmethodWait(Int32, CancellationToken)

Blocks the current thread until the CountdownEvent is set, using a 32-bit signed integer to measure the timeout, while observing a CancellationToken.

System_CAPS_pubmethodWait(TimeSpan)

Blocks the current thread until the CountdownEvent is set, using a TimeSpan to measure the timeout.

System_CAPS_pubmethodWait(TimeSpan, CancellationToken)

Blocks the current thread until the CountdownEvent is set, using a TimeSpan to measure the timeout, while observing a CancellationToken.

Universal Windows Platform
Available since 8
.NET Framework
Available since 4.0
Portable Class Library
Supported in: portable .NET platforms
Windows Phone Silverlight
Available since 8.0
Windows Phone
Available since 8.1

All public and protected members of CountdownEvent are thread-safe and may be used concurrently from multiple threads, with the exception of Dispose, which must only be used when all other operations on the CountdownEvent have completed, and Reset, which should only be used when no other threads are accessing the event.

Example

The following example shows how to use a CountdownEvent:

Imports System.Collections.Concurrent
Imports System.Linq
Imports System.Threading
Imports System.Threading.Tasks

Module Example
    Sub Main()
        ' Initialize a queue and a CountdownEvent
        Dim queue As New ConcurrentQueue(Of Integer)(Enumerable.Range(0, 10000))
        Dim cde As New CountdownEvent(10000)
        ' initial count = 10000
        ' This is the logic for all queue consumers
        Dim consumer As Action =
            Sub()
                Dim local As Integer
                ' decrement CDE count once for each element consumed from queue
                While queue.TryDequeue(local)
                    cde.Signal()
                End While
            End Sub

        ' Now empty the queue with a couple of asynchronous tasks
        Dim t1 As Task = Task.Factory.StartNew(consumer)
        Dim t2 As Task = Task.Factory.StartNew(consumer)

        ' And wait for queue to empty by waiting on cde
        cde.Wait()
        ' will return when cde count reaches 0
        Console.WriteLine("Done emptying queue. InitialCount={0}, CurrentCount={1}, IsSet={2}", cde.InitialCount, cde.CurrentCount, cde.IsSet)

        ' Proper form is to wait for the tasks to complete, even if you know that their work
        ' is done already.
        Task.WaitAll(t1, t2)

        ' Resetting will cause the CountdownEvent to un-set, and resets InitialCount/CurrentCount
        ' to the specified value
        cde.Reset(10)

        ' AddCount will affect the CurrentCount, but not the InitialCount
        cde.AddCount(2)

        Console.WriteLine("After Reset(10), AddCount(2): InitialCount={0}, CurrentCount={1}, IsSet={2}", cde.InitialCount, cde.CurrentCount, cde.IsSet)

        ' Now try waiting with cancellation
        Dim cts As New CancellationTokenSource()
        cts.Cancel()
        ' cancels the CancellationTokenSource
        Try
            cde.Wait(cts.Token)
        Catch generatedExceptionName As OperationCanceledException
            Console.WriteLine("cde.Wait(preCanceledToken) threw OCE, as expected")
        Finally
           cts.Dispose()
        End Try

        ' It's good for to release a CountdownEvent when you're done with it.
        cde.Dispose()
    End Sub
End Module
' The example displays the following output:
'    Done emptying queue.  InitialCount=10000, CurrentCount=0, IsSet=True
'    After Reset(10), AddCount(2): InitialCount=10, CurrentCount=12, IsSet=False
'    cde.Wait(preCanceledToken) threw OCE, as expected
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