CountdownEvent Class

.NET Framework 4.6 and 4.5

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

System.Object
  System.Threading.CountdownEvent

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

[ComVisibleAttribute(false)]
[DebuggerDisplayAttribute("Initial Count={InitialCount}, Current Count={CurrentCount}")]
[HostProtectionAttribute(SecurityAction.LinkDemand, Synchronization = true, 
	ExternalThreading = true)]
public class CountdownEvent : IDisposable

The CountdownEvent type exposes the following members.

  NameDescription
Public methodSupported by Portable Class LibraryCountdownEventInitializes a new instance of CountdownEvent class with the specified count.
Top

  NameDescription
Public propertySupported by Portable Class LibraryCurrentCountGets the number of remaining signals required to set the event.
Public propertySupported by Portable Class LibraryInitialCountGets the numbers of signals initially required to set the event.
Public propertySupported by Portable Class LibraryIsSetDetermines whether the event is set.
Public propertySupported by Portable Class LibraryWaitHandleGets a WaitHandle that is used to wait for the event to be set.
Top

  NameDescription
Public methodSupported by Portable Class LibraryAddCount()Increments the CountdownEvent's current count by one.
Public methodSupported by Portable Class LibraryAddCount(Int32)Increments the CountdownEvent's current count by a specified value.
Public methodSupported by Portable Class LibraryDispose()Releases all resources used by the current instance of the CountdownEvent class.
Protected methodSupported by Portable Class LibraryDispose(Boolean)Releases the unmanaged resources used by the CountdownEvent, and optionally releases the managed resources.
Public methodSupported by Portable Class LibraryEquals(Object)Determines whether the specified object is equal to the current object. (Inherited from Object.)
Protected methodSupported by Portable Class LibraryFinalizeAllows an object to try to free resources and perform other cleanup operations before it is reclaimed by garbage collection. (Inherited from Object.)
Public methodSupported by Portable Class LibraryGetHashCodeServes as the default hash function. (Inherited from Object.)
Public methodSupported by Portable Class LibraryGetTypeGets the Type of the current instance. (Inherited from Object.)
Protected methodSupported by Portable Class LibraryMemberwiseCloneCreates a shallow copy of the current Object. (Inherited from Object.)
Public methodSupported by Portable Class LibraryReset()Resets the CurrentCount to the value of InitialCount.
Public methodSupported by Portable Class LibraryReset(Int32)Resets the InitialCount property to a specified value.
Public methodSupported by Portable Class LibrarySignal()Registers a signal with the CountdownEvent, decrementing the value of CurrentCount.
Public methodSupported by Portable Class LibrarySignal(Int32)Registers multiple signals with the CountdownEvent, decrementing the value of CurrentCount by the specified amount.
Public methodSupported by Portable Class LibraryToStringReturns a string that represents the current object. (Inherited from Object.)
Public methodSupported by Portable Class LibraryTryAddCount()Attempts to increment CurrentCount by one.
Public methodSupported by Portable Class LibraryTryAddCount(Int32)Attempts to increment CurrentCount by a specified value.
Public methodSupported by Portable Class LibraryWait()Blocks the current thread until the CountdownEvent is set.
Public methodSupported by Portable Class LibraryWait(CancellationToken)Blocks the current thread until the CountdownEvent is set, while observing a CancellationToken.
Public methodSupported by Portable Class LibraryWait(Int32)Blocks the current thread until the CountdownEvent is set, using a 32-bit signed integer to measure the timeout.
Public methodSupported by Portable Class LibraryWait(TimeSpan)Blocks the current thread until the CountdownEvent is set, using a TimeSpan to measure the timeout.
Public methodSupported by Portable Class LibraryWait(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 methodSupported by Portable Class LibraryWait(TimeSpan, CancellationToken)Blocks the current thread until the CountdownEvent is set, using a TimeSpan to measure the timeout, while observing a CancellationToken.
Top

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: 4.6, 4.5, 4

.NET Framework Client Profile

Supported in: 4

.NET for Windows Phone apps

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

Portable Class Library

Supported in: Portable Class Library

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");
        }
        finally
        {
           cts.Dispose();
        }
        // It's good for to release a CountdownEvent when you're done with it.
        cde.Dispose();
    }
}
Was this page helpful?
(1500 characters remaining)
Thank you for your feedback
Show:
© 2015 Microsoft