如何：取消链接数据流块
本文档介绍如何取消目标数据流块与其源的链接。
注意
TPL 数据流库（System.Threading.Tasks.Dataflow 命名空间）不随 .NET 一起分发。 若要在 Visual Studio 中安装 System.Threading.Tasks.Dataflow 命名空间，请打开项目，选择“项目”菜单中的“管理 NuGet 包”，再在线搜索 System.Threading.Tasks.Dataflow 包。 或者，若要使用 .NET Core CLI 进行安装，请运行 dotnet add package System.Threading.Tasks.Dataflow。
示例
下面的示例创建了三个 TransformBlock<TInput,TOutput> 对象，每个对象调用 TrySolution 方法来计算值。 此示例只需使用第一次调用 TrySolution 的结果即可完成。
using System;
using System.Threading;
using System.Threading.Tasks.Dataflow;
// Demonstrates how to unlink dataflow blocks.
class DataflowReceiveAny
{
// Receives the value from the first provided source that has
// a message.
public static T ReceiveFromAny<T>(params ISourceBlock<T>[] sources)
{
// Create a WriteOnceBlock<T> object and link it to each source block.
var writeOnceBlock = new WriteOnceBlock<T>(e => e);
foreach (var source in sources)
{
// Setting MaxMessages to one instructs
// the source block to unlink from the WriteOnceBlock<T> object
// after offering the WriteOnceBlock<T> object one message.
source.LinkTo(writeOnceBlock, new DataflowLinkOptions { MaxMessages = 1 });
}
// Return the first value that is offered to the WriteOnceBlock object.
return writeOnceBlock.Receive();
}
// Demonstrates a function that takes several seconds to produce a result.
static int TrySolution(int n, CancellationToken ct)
{
// Simulate a lengthy operation that completes within three seconds
// or when the provided CancellationToken object is cancelled.
SpinWait.SpinUntil(() => ct.IsCancellationRequested,
new Random().Next(3000));
// Return a value.
return n + 42;
}
static void Main(string[] args)
{
// Create a shared CancellationTokenSource object to enable the
// TrySolution method to be cancelled.
var cts = new CancellationTokenSource();
// Create three TransformBlock<int, int> objects.
// Each TransformBlock<int, int> object calls the TrySolution method.
Func<int, int> action = n => TrySolution(n, cts.Token);
var trySolution1 = new TransformBlock<int, int>(action);
var trySolution2 = new TransformBlock<int, int>(action);
var trySolution3 = new TransformBlock<int, int>(action);
// Post data to each TransformBlock<int, int> object.
trySolution1.Post(11);
trySolution2.Post(21);
trySolution3.Post(31);
// Call the ReceiveFromAny<T> method to receive the result from the
// first TransformBlock<int, int> object to finish.
int result = ReceiveFromAny(trySolution1, trySolution2, trySolution3);
// Cancel all calls to TrySolution that are still active.
cts.Cancel();
// Print the result to the console.
Console.WriteLine("The solution is {0}.", result);
cts.Dispose();
}
}
/* Sample output:
The solution is 53.
*/
Imports System.Threading
Imports System.Threading.Tasks.Dataflow
' Demonstrates how to unlink dataflow blocks.
Friend Class DataflowReceiveAny
' Receives the value from the first provided source that has
' a message.
Public Shared Function ReceiveFromAny(Of T)(ParamArray ByVal sources() As ISourceBlock(Of T)) As T
' Create a WriteOnceBlock<T> object and link it to each source block.
Dim writeOnceBlock = New WriteOnceBlock(Of T)(Function(e) e)
For Each source In sources
' Setting MaxMessages to one instructs
' the source block to unlink from the WriteOnceBlock<T> object
' after offering the WriteOnceBlock<T> object one message.
source.LinkTo(writeOnceBlock, New DataflowLinkOptions With {.MaxMessages = 1})
Next source
' Return the first value that is offered to the WriteOnceBlock object.
Return writeOnceBlock.Receive()
End Function
' Demonstrates a function that takes several seconds to produce a result.
Private Shared Function TrySolution(ByVal n As Integer, ByVal ct As CancellationToken) As Integer
' Simulate a lengthy operation that completes within three seconds
' or when the provided CancellationToken object is cancelled.
SpinWait.SpinUntil(Function() ct.IsCancellationRequested, New Random().Next(3000))
' Return a value.
Return n + 42
End Function
Shared Sub Main(ByVal args() As String)
' Create a shared CancellationTokenSource object to enable the
' TrySolution method to be cancelled.
Dim cts = New CancellationTokenSource()
' Create three TransformBlock<int, int> objects.
' Each TransformBlock<int, int> object calls the TrySolution method.
Dim action As Func(Of Integer, Integer) = Function(n) TrySolution(n, cts.Token)
Dim trySolution1 = New TransformBlock(Of Integer, Integer)(action)
Dim trySolution2 = New TransformBlock(Of Integer, Integer)(action)
Dim trySolution3 = New TransformBlock(Of Integer, Integer)(action)
' Post data to each TransformBlock<int, int> object.
trySolution1.Post(11)
trySolution2.Post(21)
trySolution3.Post(31)
' Call the ReceiveFromAny<T> method to receive the result from the
' first TransformBlock<int, int> object to finish.
Dim result As Integer = ReceiveFromAny(trySolution1, trySolution2, trySolution3)
' Cancel all calls to TrySolution that are still active.
cts.Cancel()
' Print the result to the console.
Console.WriteLine("The solution is {0}.", result)
cts.Dispose()
End Sub
End Class
' Sample output:
'The solution is 53.
'
为了接收完成的第一个 TransformBlock<TInput,TOutput> 对象的值，该示例定义了 ReceiveFromAny(T) 方法。 ReceiveFromAny(T) 方法接受一个 ISourceBlock<TOutput> 对象的数组，并将其中每个对象链接到 WriteOnceBlock<T> 对象。 当使用 LinkTo 方法将源数据流块链接到目标块时，源会在数据可用时将消息传播到目标。 因为 WriteOnceBlock<T> 类只接受为其提供的第一条消息，ReceiveFromAny(T) 方法通过调用 Receive 方法来生成其结果。 这将生成提供给 WriteOnceBlock<T> 对象的第一条消息。 LinkTo 方法有一个重载版本，其含有一个具有 MaxMessages 属性的 DataflowLinkOptions 对象，当该属性设置为 1 时，则指示源块在目标收到来自源的一条消息后取消与目标的链接。 取消 WriteOnceBlock<T> 对象与其源的链接非常重要，因为当 WriteOnceBlock<T> 对象收到一条消息后，便不再需要源数组与 WriteOnceBlock<T> 对象之间的关系。
为了使 TrySolution 的剩余调用能够在其中一个调用计算了一个值后结束，TrySolution 方法采用一个 CancellationToken 对象，该对象在对 ReceiveFromAny(T) 的调用返回后将被取消。 当此 SpinUntil 对象取消时，CancellationToken 方法将返回。
