CountdownEvent
System.Threading.CountdownEvent 是一個同步處理基本類型，在發出了特定次數的訊號給它之後，就會解除封鎖其等候中的執行緒。 CountdownEvent 是針對下列案例所設計：您必須使用 ManualResetEvent 或 ManualResetEventSlim，並在發出訊號給事件之前手動遞減變數。 例如，在分支/聯結案例中，您可以只建立訊號計數為 5 的 CountdownEvent，然後在執行緒集區上啟動五個工作項目，並讓每個工作項目在其完成時呼叫 Signal。 每次呼叫 Signal 就會將訊號計數遞減 1。 在主執行緒上，對 Wait 的呼叫將會封鎖，直到訊號計數為零為止。
注意
對於不需與舊版 .NET Framework 同步處理 API 互動的程式碼，請考慮使用 System.Threading.Tasks.Task 物件或 Invoke 方法，以甚至更簡單的方式表達分支-聯結的平行處理原則。
CountdownEvent 具有下列其他功能：
等候作業可以使用取消語彙基元來取消。
建立執行個體之後，即可遞增其訊號計數。
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
下列範例示範如何將 CountdownEvent 與 ThreadPool 工作項目搭配使用。
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()
含有取消的 CountdownEvent
下列範例示範如何使用取消語彙基元，來取消 CountdownEvent 上的等候作業。 基本模式遵循在 .NET Framework 4 中導入的統一取消模型。 如需詳細資訊，請參閱受控執行緒中的取消作業。
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 {0}", dataWithToken.Data.Num);
return;
}
for (int i = 0; i < 10000; i++)
{
if (dataWithToken.Token.IsCancellationRequested)
{
Console.WriteLine("Cancelling while executing {0}", dataWithToken.Data.Num);
return;
}
// Increase this value to slow down the program.
Thread.SpinWait(100000);
}
Console.WriteLine("Processed {0}", 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，等候作業不會取消正在發出訊號給它的執行緒。 一般而言，會將取消套用至邏輯作業，其中可以包括在事件上等候，以及正在同步處理等候的所有工作項目。 在此範例中，會針對每個工作項目傳遞相同取消語彙基元的複本，讓它可以回應取消要求。
