CountdownEvent Class

Definition

public ref class CountdownEvent : IDisposable

public class CountdownEvent : IDisposable

[System.Runtime.InteropServices.ComVisible(false)]
public class CountdownEvent : IDisposable

type CountdownEvent = class
 interface IDisposable

[<System.Runtime.InteropServices.ComVisible(false)>]
type CountdownEvent = class
 interface IDisposable

Public Class CountdownEvent
Implements IDisposable

Inheritance

Object

CountdownEvent

Attributes

ComVisibleAttribute

Implements

IDisposable

Examples

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 Example
{
 static async Task 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 know that their work
 // is done already.
 await Task.WhenAll(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 to release a CountdownEvent when you're done with it.
 cde.Dispose();
 }
}
// 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

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 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

Thread Safety

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.

See also

• CountdownEvent