System.Threading.CountdownEvent 是同步處理基本類型，會在收到特定次數的訊號后解除封鎖其等候的線程。 CountdownEvent 是針對在發出事件訊號之前，您必須使用 ManualResetEvent 或 ManualResetEventSlim 並手動遞減變數的案例所設計。 例如，在分叉/合併情境中，您可以建立具有 5 個信號計數的 CountdownEvent，然後在執行緒集區上啟動五個工作項目，並在完成時讓每個工作項目呼叫 Signal。 每次呼叫 Signal 時，訊號計數會減少 1。 在主線程上，對的呼叫 Wait 將會封鎖，直到訊號計數為零為止。
備註
對於不需要與舊版 .NET Framework 同步處理 API 互動的程式代碼，請考慮使用 System.Threading.Tasks.Task 物件或 Invoke 方法，以更輕鬆地表示分支聯結平行處理原則。
CountdownEvent 具有下列其他功能：
您可以使用取消令牌來取消等候作業。
建立實例之後，其訊號計數可以遞增。
執行個體會公開 WaitHandle ，以便與其他 .NET 同步處理 API 整合，例如 WaitAll。
基本使用方式
下列範例示範如何使用 CountdownEvent 搭配 ThreadPool工作專案。
IEnumerable<Data> source = GetData();
using (CountdownEvent e = new CountdownEvent(1))
{
// fork work:
foreach (Data element in source)
{
// Dynamically increment signal count.
e.AddCount();
ThreadPool.QueueUserWorkItem(delegate(object state)
{
try
{
ProcessData(state);
}
finally
{
e.Signal();
}
},
element);
}
e.Signal();
// The first element could be run on this thread.
// Join with work.
e.Wait();
}
// .,.
Dim source As IEnumerable(Of Data) = GetData()
Dim e = New CountdownEvent(1)
' Fork work:
For Each element As Data In source
' Dynamically increment signal count.
e.AddCount()
ThreadPool.QueueUserWorkItem(Sub(state)
Try
ProcessData(state)
Finally
e.Signal()
End Try
End Sub,
element)
Next
' Decrement the signal count by the one we added
' in the constructor.
e.Signal()
' The first element could also be run on this thread.
' ProcessData(New Data(0))
' Join with work:
e.Wait()
倒數事件具取消功能
下列範例示範如何使用取消令牌取消等候作業 CountdownEvent 。 基本模式遵循 .NET Framework 4 中引進的統一取消模型。 如需詳細資訊，請參閱 管理線程中的取消操作。
class CancelableCountdownEvent
{
class Data
{
public int Num { get; set; }
public Data(int i) { Num = i; }
public Data() { }
}
class DataWithToken
{
public CancellationToken Token { get; set; }
public Data Data { get; private set; }
public DataWithToken(Data data, CancellationToken ct)
{
this.Data = data;
this.Token = ct;
}
}
static IEnumerable<Data> GetData()
{
return new List<Data>() { new Data(1), new Data(2), new Data(3), new Data(4), new Data(5) };
}
static void ProcessData(object obj)
{
DataWithToken dataWithToken = (DataWithToken)obj;
if (dataWithToken.Token.IsCancellationRequested)
{
Console.WriteLine($"Canceled before starting {dataWithToken.Data.Num}");
return;
}
for (int i = 0; i < 10000; i++)
{
if (dataWithToken.Token.IsCancellationRequested)
{
Console.WriteLine($"Cancelling while executing {dataWithToken.Data.Num}");
return;
}
// Increase this value to slow down the program.
Thread.SpinWait(100000);
}
Console.WriteLine($"Processed {dataWithToken.Data.Num}");
}
static void Main(string[] args)
{
EventWithCancel();
Console.WriteLine("Press enter to exit.");
Console.ReadLine();
}
static void EventWithCancel()
{
IEnumerable<Data> source = GetData();
CancellationTokenSource cts = new CancellationTokenSource();
//Enable cancellation request from a simple UI thread.
Task.Factory.StartNew(() =>
{
if (Console.ReadKey().KeyChar == 'c')
cts.Cancel();
});
// Event must have a count of at least 1
CountdownEvent e = new CountdownEvent(1);
// fork work:
foreach (Data element in source)
{
DataWithToken item = new DataWithToken(element, cts.Token);
// Dynamically increment signal count.
e.AddCount();
ThreadPool.QueueUserWorkItem(delegate(object state)
{
ProcessData(state);
if (!cts.Token.IsCancellationRequested)
e.Signal();
},
item);
}
// Decrement the signal count by the one we added
// in the constructor.
e.Signal();
// The first element could be run on this thread.
// Join with work or catch cancellation.
try
{
e.Wait(cts.Token);
}
catch (OperationCanceledException oce)
{
if (oce.CancellationToken == cts.Token)
{
Console.WriteLine("User canceled.");
}
else
{
throw; //We don't know who canceled us!
}
}
finally {
e.Dispose();
cts.Dispose();
}
//...
} //end method
} //end class
Option Strict On
Option Explicit On
Imports System.Collections
Imports System.Collections.Generic
Imports System.Linq
Imports System.Threading
Imports System.Threading.Tasks
Module CancelEventWait
Class Data
Public Num As Integer
Public Sub New(ByVal i As Integer)
Num = i
End Sub
Public Sub New()
End Sub
End Class
Class DataWithToken
Public Token As CancellationToken
Public _data As Data
Public Sub New(ByVal d As Data, ByVal ct As CancellationToken)
Me._data = d
Me.Token = ct
End Sub
End Class
Class Program
Shared Function GetData() As IEnumerable(Of Data)
Dim nums = New List(Of Data)
For i As Integer = 1 To 5
nums.Add(New Data(i))
Next
Return nums
End Function
Shared Sub ProcessData(ByVal obj As Object)
Dim dataItem As DataWithToken = CType(obj, DataWithToken)
If dataItem.Token.IsCancellationRequested = True Then
Console.WriteLine("Canceled before starting {0}", dataItem._data.Num)
Exit Sub
End If
' Increase this value to slow down the program.
For i As Integer = 0 To 10000
If dataItem.Token.IsCancellationRequested = True Then
Console.WriteLine("Cancelling while executing {0}", dataItem._data.Num)
Exit Sub
End If
Thread.SpinWait(100000)
Next
Console.WriteLine("Processed {0}", dataItem._data.Num)
End Sub
Shared Sub Main()
DoEventWithCancel()
Console.WriteLine("Press the enter key to exit.")
Console.ReadLine()
End Sub
Shared Sub DoEventWithCancel()
Dim source As IEnumerable(Of Data) = GetData()
Dim cts As CancellationTokenSource = New CancellationTokenSource()
' Enable cancellation request from a simple UI thread.
Task.Factory.StartNew(Sub()
If Console.ReadKey().KeyChar = "c"c Then
cts.Cancel()
End If
End Sub)
' Must have a count of at least 1 or else it is signaled.
Dim e As CountdownEvent = New CountdownEvent(1)
For Each element As Data In source
Dim item As DataWithToken = New DataWithToken(element, cts.Token)
' Dynamically increment signal count.
e.AddCount()
ThreadPool.QueueUserWorkItem(Sub(state)
ProcessData(state)
If cts.Token.IsCancellationRequested = False Then
e.Signal()
End If
End Sub,
item)
Next
' Decrement the signal count by the one we added
' in the constructor.
e.Signal()
' The first element could be run on this thread.
' ProcessData(source(0))
' Join with work or catch cancellation exception
Try
e.Wait(cts.Token)
Catch ex As OperationCanceledException
If ex.CancellationToken = cts.Token Then
Console.WriteLine("User canceled.")
Else : Throw ' we don't know who canceled us.
End If
Finally
e.Dispose()
cts.Dispose()
End Try
End Sub
End Class
End Module
請注意，等候作業不會取消發出訊號的線程。 一般而言，取消會套用至邏輯運算，且可以包括等候事件以及等候時同步處理的所有工作項目。 在此範例中，每個工作專案都會傳遞相同取消令牌的複本，以便回應取消要求。
