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CountdownEvent Class

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

System.Object
  System.Threading.CountdownEvent

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

[ComVisibleAttribute(false)]
[HostProtectionAttribute(SecurityAction.LinkDemand, Synchronization = true, 
	ExternalThreading = true)]
public class CountdownEvent : IDisposable

The CountdownEvent type exposes the following members.

  NameDescription
Public methodCountdownEventInitializes a new instance of CountdownEvent class with the specified count.
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  NameDescription
Public propertyCurrentCountGets the number of remaining signals required to set the event.
Public propertyInitialCountGets the numbers of signals initially required to set the event.
Public propertyIsSetDetermines whether the event is set.
Public propertyWaitHandleGets a WaitHandle that is used to wait for the event to be set.
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  NameDescription
Public methodAddCount()Increments the CountdownEvent's current count by one.
Public methodAddCount(Int32)Increments the CountdownEvent's current count by a specified value.
Public methodDispose()Releases all resources used by the current instance of the CountdownEvent class.
Protected methodDispose(Boolean)When overridden in a derived class, releases the unmanaged resources used by the CountdownEvent, and optionally releases the managed resources.
Public methodEquals(Object)Determines whether the specified Object is equal to the current Object. (Inherited from Object.)
Protected methodFinalizeAllows an object to try to free resources and perform other cleanup operations before it is reclaimed by garbage collection. (Inherited from Object.)
Public methodGetHashCodeServes as a hash function for a particular type. (Inherited from Object.)
Public methodGetTypeGets the Type of the current instance. (Inherited from Object.)
Protected methodMemberwiseCloneCreates a shallow copy of the current Object. (Inherited from Object.)
Public methodReset()Resets the CurrentCount to the value of InitialCount.
Public methodReset(Int32)Resets the InitialCount property to a specified value.
Public methodSignal()Registers a signal with the CountdownEvent, decrementing the value of CurrentCount.
Public methodSignal(Int32)Registers multiple signals with the CountdownEvent, decrementing the value of CurrentCount by the specified amount.
Public methodToStringReturns a string that represents the current object. (Inherited from Object.)
Public methodTryAddCount()Attempts to increment CurrentCount by one.
Public methodTryAddCount(Int32)Attempts to increment CurrentCount by a specified value.
Public methodWait()Blocks the current thread until the CountdownEvent is set.
Public methodWait(CancellationToken)Blocks the current thread until the CountdownEvent is set, while observing a CancellationToken.
Public methodWait(Int32)Blocks the current thread until the CountdownEvent is set, using a 32-bit signed integer to measure the timeout.
Public methodWait(TimeSpan)Blocks the current thread until the CountdownEvent is set, using a TimeSpan to measure the timeout.
Public methodWait(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.
Public methodWait(TimeSpan, CancellationToken)Blocks the current thread until the CountdownEvent is set, using a TimeSpan to measure the timeout, while observing a CancellationToken.
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NoteNote

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.

.NET Framework

Supported in: 4

.NET Framework Client Profile

Supported in: 4

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

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

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:


using System;
using System.Collections.Concurrent;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
class CDESample
{
    // Demonstrates:
    //      CountdownEvent construction
    //      CountdownEvent.AddCount()
    //      CountdownEvent.Signal()
    //      CountdownEvent.Wait()
    //      CountdownEvent.Wait() w/ cancellation
    //      CountdownEvent.Reset()
    //      CountdownEvent.IsSet
    //      CountdownEvent.InitialCount
    //      CountdownEvent.CurrentCount
    static void Main()
    {
        // Initialize a queue and a CountdownEvent
        ConcurrentQueue<int> queue = new ConcurrentQueue<int>(Enumerable.Range(0, 10000));
        CountdownEvent cde = new CountdownEvent(10000); // initial count = 10000

        // This is the logic for all queue consumers
        Action consumer = () =>
        {
            int local;
            // decrement CDE count once for each element consumed from queue
            while (queue.TryDequeue(out local)) cde.Signal();
        };

        // Now empty the queue with a couple of asynchronous tasks
        Task t1 = Task.Factory.StartNew(consumer);
        Task t2 = 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 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
        CancellationTokenSource cts = new CancellationTokenSource();
        cts.Cancel(); // cancels the CancellationTokenSource
        try
        {
            cde.Wait(cts.Token);
        }
        catch (OperationCanceledException)
        {
            Console.WriteLine("cde.Wait(preCanceledToken) threw OCE, as expected");
        }

        // It's good for to release a CountdownEvent when you're done with it.
        cde.Dispose();

    }
}


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