Procedimiento para cancelar un bloque de flujo de datosHow to: Cancel a Dataflow Block

En este ejemplo se explica cómo habilitar la cancelación en la aplicación.This document demonstrates how to enable cancellation in your application. Este ejemplo usa Windows Forms para mostrar dónde están activos los elementos de trabajo en una canalización de flujo de datos y también los efectos de la canalización.This example uses Windows Forms to show where work items are active in a dataflow pipeline and also the effects of cancellation.

Nota

La biblioteca de flujos de datos TPL (el espacio de nombres System.Threading.Tasks.Dataflow) no se distribuye con .NET.The TPL Dataflow Library (the System.Threading.Tasks.Dataflow namespace) is not distributed with .NET. Para instalar el espacio de nombres System.Threading.Tasks.Dataflow en Visual Studio, abra el proyecto, seleccione Administrar paquetes NuGet en el menú Proyecto y busque en línea el paquete System.Threading.Tasks.Dataflow.To install the System.Threading.Tasks.Dataflow namespace in Visual Studio, open your project, choose Manage NuGet Packages from the Project menu, and search online for the System.Threading.Tasks.Dataflow package. Como alternativa, para realizar la instalación con la CLI de .Net Core, ejecute dotnet add package System.Threading.Tasks.Dataflow.Alternatively, to install it using the .Net Core CLI, run dotnet add package System.Threading.Tasks.Dataflow.

Para crear la aplicación de Windows FormsTo Create the Windows Forms Application

  1. Cree un proyecto de Aplicación de Windows Forms de Visual Basic o C#.Create a C# or Visual Basic Windows Forms Application project. En los pasos siguientes, el proyecto se denomina CancellationWinForms.In the following steps, the project is named CancellationWinForms.

  2. En el diseñador de formularios del formulario principal, Form1.cs (Form1.vb para Visual Basic), agregue un control ToolStrip.On the form designer for the main form, Form1.cs (Form1.vb for Visual Basic), add a ToolStrip control.

  3. Agregue un control ToolStripButton al control ToolStrip.Add a ToolStripButton control to the ToolStrip control. Establezca la propiedad DisplayStyle en Text y la propiedad Text en Agregar elementos de trabajo.Set the DisplayStyle property to Text and the Text property to Add Work Items.

  4. Agregue un segundo control ToolStripButton al control ToolStrip.Add a second ToolStripButton control to the ToolStrip control. Establezca la propiedad DisplayStyle en Text, la propiedad Text en Cancelar y la propiedad Enabled en False.Set the DisplayStyle property to Text, the Text property to Cancel, and the Enabled property to False.

  5. Agregue cuatro objetos ToolStripProgressBar al control ToolStrip.Add four ToolStripProgressBar objects to the ToolStrip control.

Creación de la canalización de flujo de datosCreating the Dataflow Pipeline

En esta sección se describe cómo crear la canalización de flujo de datos que procesa los elementos de trabajo y actualiza las barras de progreso.This section describes how to create the dataflow pipeline that processes work items and updates the progress bars.

Para crear la canalización de flujo de datosTo Create the Dataflow Pipeline

  1. En el proyecto, agregue una referencia a System.Threading.Tasks.Dataflow.dll.In your project, add a reference to System.Threading.Tasks.Dataflow.dll.

  2. Asegúrese de que Form1.cs (Form1.vb para Visual Basic) contenga las siguientes instrucciones using (Imports en Visual Basic).Ensure that Form1.cs (Form1.vb for Visual Basic) contains the following using statements (Imports in Visual Basic).

    using System;
    using System.Threading;
    using System.Threading.Tasks;
    using System.Threading.Tasks.Dataflow;
    using System.Windows.Forms;
    
    Imports System.Threading
    Imports System.Threading.Tasks
    Imports System.Threading.Tasks.Dataflow
    
    
  3. Agregue la clase WorkItem como un tipo interno de la clase Form1.Add the WorkItem class as an inner type of the Form1 class.

    // A placeholder type that performs work.
    class WorkItem
    {
       // Performs work for the provided number of milliseconds.
       public void DoWork(int milliseconds)
       {
          // For demonstration, suspend the current thread.
          Thread.Sleep(milliseconds);
       }
    }
    
    ' A placeholder type that performs work.
    Private Class WorkItem
        ' Performs work for the provided number of milliseconds.
        Public Sub DoWork(ByVal milliseconds As Integer)
            ' For demonstration, suspend the current thread.
            Thread.Sleep(milliseconds)
        End Sub
    End Class
    
  4. Agregue a la clase Form1 los miembros de datos siguientes:Add the following data members to the Form1 class.

    // Enables the user interface to signal cancellation.
    CancellationTokenSource cancellationSource;
    
    // The first node in the dataflow pipeline.
    TransformBlock<WorkItem, WorkItem> startWork;
    
    // The second, and final, node in the dataflow pipeline.
    ActionBlock<WorkItem> completeWork;
    
    // Increments the value of the provided progress bar.
    ActionBlock<ToolStripProgressBar> incrementProgress;
    
    // Decrements the value of the provided progress bar.
    ActionBlock<ToolStripProgressBar> decrementProgress;
    
    // Enables progress bar actions to run on the UI thread.
    TaskScheduler uiTaskScheduler;
    
    ' Enables the user interface to signal cancellation.
    Private cancellationSource As CancellationTokenSource
    
    ' The first node in the dataflow pipeline.
    Private startWork As TransformBlock(Of WorkItem, WorkItem)
    
    ' The second, and final, node in the dataflow pipeline.
    Private completeWork As ActionBlock(Of WorkItem)
    
    ' Increments the value of the provided progress bar.
    Private incrementProgress As ActionBlock(Of ToolStripProgressBar)
    
    ' Decrements the value of the provided progress bar.
    Private decrementProgress As ActionBlock(Of ToolStripProgressBar)
    
    ' Enables progress bar actions to run on the UI thread.
    Private uiTaskScheduler As TaskScheduler
    
  5. Agregue el método siguiente, CreatePipeline, a la clase Form1.Add the following method, CreatePipeline, to the Form1 class.

    // Creates the blocks that participate in the dataflow pipeline.
    private void CreatePipeline()
    {
       // Create the cancellation source.
       cancellationSource = new CancellationTokenSource();
    
       // Create the first node in the pipeline. 
       startWork = new TransformBlock<WorkItem, WorkItem>(workItem =>
       {
          // Perform some work.
          workItem.DoWork(250);
    
          // Decrement the progress bar that tracks the count of 
          // active work items in this stage of the pipeline.
          decrementProgress.Post(toolStripProgressBar1);
    
          // Increment the progress bar that tracks the count of 
          // active work items in the next stage of the pipeline.
          incrementProgress.Post(toolStripProgressBar2);
    
          // Send the work item to the next stage of the pipeline.
          return workItem;
       },
       new ExecutionDataflowBlockOptions
       {
          CancellationToken = cancellationSource.Token
       });
    
       // Create the second, and final, node in the pipeline. 
       completeWork = new ActionBlock<WorkItem>(workItem =>
       {
          // Perform some work.
          workItem.DoWork(1000);
    
          // Decrement the progress bar that tracks the count of 
          // active work items in this stage of the pipeline.
          decrementProgress.Post(toolStripProgressBar2);
    
          // Increment the progress bar that tracks the overall 
          // count of completed work items.
          incrementProgress.Post(toolStripProgressBar3);
       }, 
       new ExecutionDataflowBlockOptions
       {
          CancellationToken = cancellationSource.Token,
          MaxDegreeOfParallelism = 2
       });
    
       // Connect the two nodes of the pipeline. When the first node completes, 
       // set the second node also to the completed state.
       startWork.LinkTo(
          completeWork, new DataflowLinkOptions { PropagateCompletion = true });
                
       // Create the dataflow action blocks that increment and decrement
       // progress bars.
       // These blocks use the task scheduler that is associated with
       // the UI thread.
    
       incrementProgress = new ActionBlock<ToolStripProgressBar>(
          progressBar => progressBar.Value++,
          new ExecutionDataflowBlockOptions
          {
             CancellationToken = cancellationSource.Token,
             TaskScheduler = uiTaskScheduler
          });
    
       decrementProgress = new ActionBlock<ToolStripProgressBar>(
          progressBar => progressBar.Value--,
          new ExecutionDataflowBlockOptions
          {
             CancellationToken = cancellationSource.Token,
             TaskScheduler = uiTaskScheduler
          });
    }
    
    ' Creates the blocks that participate in the dataflow pipeline.
    Private Sub CreatePipeline()
        ' Create the cancellation source.
        cancellationSource = New CancellationTokenSource()
    
        ' Create the first node in the pipeline. 
        startWork = New TransformBlock(Of WorkItem, WorkItem)(Function(workItem)
            ' Perform some work.
            ' Decrement the progress bar that tracks the count of 
            ' active work items in this stage of the pipeline.
            ' Increment the progress bar that tracks the count of 
            ' active work items in the next stage of the pipeline.
            ' Send the work item to the next stage of the pipeline.
            workItem.DoWork(250)
            decrementProgress.Post(toolStripProgressBar1)
            incrementProgress.Post(toolStripProgressBar2)
            Return workItem
        End Function,
        New ExecutionDataflowBlockOptions With {.CancellationToken = cancellationSource.Token})
    
        ' Create the second, and final, node in the pipeline. 
        completeWork = New ActionBlock(Of WorkItem)(Sub(workItem)
           ' Perform some work.
           ' Decrement the progress bar that tracks the count of 
           ' active work items in this stage of the pipeline.
           ' Increment the progress bar that tracks the overall 
           ' count of completed work items.
           workItem.DoWork(1000)
           decrementProgress.Post(toolStripProgressBar2)
           incrementProgress.Post(toolStripProgressBar3)
        End Sub,
        New ExecutionDataflowBlockOptions With {.CancellationToken = cancellationSource.Token,
                                                .MaxDegreeOfParallelism = 2 })
    
        ' Connect the two nodes of the pipeline. When the first node completes,
        ' set the second node also to the completed state.
        startWork.LinkTo(
           completeWork, New DataflowLinkOptions With {.PropagateCompletion = true})
    
        ' Create the dataflow action blocks that increment and decrement
        ' progress bars.
        ' These blocks use the task scheduler that is associated with
        ' the UI thread.
    
        incrementProgress = New ActionBlock(Of ToolStripProgressBar)(
           Sub(progressBar) progressBar.Value += 1,
           New ExecutionDataflowBlockOptions With {.CancellationToken = cancellationSource.Token,
                                                   .TaskScheduler = uiTaskScheduler})
    
        decrementProgress = New ActionBlock(Of ToolStripProgressBar)(
           Sub(progressBar) progressBar.Value -= 1,
           New ExecutionDataflowBlockOptions With {.CancellationToken = cancellationSource.Token,
                                                   .TaskScheduler = uiTaskScheduler})
    
    End Sub
    

Dado que los bloques de flujo de datos incrementProgress y decrementProgress actúan sobre la interfaz de usuario, es importante que esta acción se produzca en el subproceso de interfaz de usuario.Because the incrementProgress and decrementProgress dataflow blocks act on the user interface, it is important that these actions occur on the user-interface thread. Para lograrlo, durante la construcción, cada uno de estos objetos proporciona un objeto ExecutionDataflowBlockOptions que tiene la propiedad TaskScheduler establecida como TaskScheduler.FromCurrentSynchronizationContext.To accomplish this, during construction these objects each provide a ExecutionDataflowBlockOptions object that has the TaskScheduler property set to TaskScheduler.FromCurrentSynchronizationContext. El método TaskScheduler.FromCurrentSynchronizationContext crea un objeto TaskScheduler que funciona en el contexto de sincronización actual.The TaskScheduler.FromCurrentSynchronizationContext method creates a TaskScheduler object that performs work on the current synchronization context. Dado que al constructor Form1 se le llama desde el subproceso de interfaz de usuario, las acciones de los bloques de flujo de datos incrementProgress y decrementProgress se ejecutan también en el subproceso de interfaz de usuario.Because the Form1 constructor is called from the user-interface thread, the actions for the incrementProgress and decrementProgress dataflow blocks also run on the user-interface thread.

Este ejemplo establece la propiedad CancellationToken cuando construye los miembros de la canalización.This example sets the CancellationToken property when it constructs the members of the pipeline. Dado que la propiedad CancellationToken cancela de forma permanente la ejecución del bloque de flujo de datos, se debe volver a crear la canalización completa después de que el usuario cancela la operación, en caso de que después desee agregar más elementos de trabajo a la canalización.Because the CancellationToken property permanently cancels dataflow block execution, the whole pipeline must be recreated after the user cancels the operation and then wants to add more work items to the pipeline. Para consultar un ejemplo en el que se muestre una forma alternativa de cancelar un bloque de flujo de datos, a fin de que se pueda realizar otro trabajo después de cancelar una operación, consulte Tutorial: Uso de flujos de datos en aplicaciones de Windows Forms.For an example that demonstrates an alternative way to cancel a dataflow block so that other work can be performed after an operation is canceled, see Walkthrough: Using Dataflow in a Windows Forms Application.

Conexión de la canalización de flujo de datos a la interfaz de usuarioConnecting the Dataflow Pipeline to the User Interface

En esta sección se describe cómo conectar la canalización de flujo de datos a la interfaz de usuario.This section describes how to connect the dataflow pipeline to the user interface. Tanto la creación de la canalización como la adición de elementos de trabajo a la canalización se controlan mediante el controlador de eventos para el botón Agregar elementos de trabajo.Both creating the pipeline and adding work items to the pipeline are controlled by the event handler for the Add Work Items button. La cancelación se inicia con el botón Cancelar.Cancellation is initiated by the Cancel button. Cuando el usuario hace clic en cualquiera de estos botones, se inicia la acción correspondiente de forma asincrónica.When the user clicks either of these buttons, the appropriate action is initiated in an asynchronous manner.

Para conectar la canalización de flujo de datos a la interfaz de usuarioTo Connect the Dataflow Pipeline to the User Interface

  1. En el diseñador de formularios del formulario principal, cree un controlador de eventos para el evento Click del botón Agregar elementos de trabajo.On the form designer for the main form, create an event handler for the Click event for the Add Work Items button.

  2. Implemente el evento Click para el botón Agregar elementos de trabajo.Implement the Click event for the Add Work Items button.

    // Event handler for the Add Work Items button.
    private void toolStripButton1_Click(object sender, EventArgs e)
    {
       // The Cancel button is disabled when the pipeline is not active.
       // Therefore, create the pipeline and enable the Cancel button
       // if the Cancel button is disabled.
       if (!toolStripButton2.Enabled)
       {
          CreatePipeline();
    
          // Enable the Cancel button.
          toolStripButton2.Enabled = true;
       }
    
       // Post several work items to the head of the pipeline.
       for (int i = 0; i < 5; i++)
       {
          toolStripProgressBar1.Value++;
          startWork.Post(new WorkItem());
       }
    }
    
    ' Event handler for the Add Work Items button.
    Private Sub toolStripButton1_Click(ByVal sender As Object, ByVal e As EventArgs) Handles toolStripButton1.Click
        ' The Cancel button is disabled when the pipeline is not active.
        ' Therefore, create the pipeline and enable the Cancel button
        ' if the Cancel button is disabled.
        If Not toolStripButton2.Enabled Then
            CreatePipeline()
    
            ' Enable the Cancel button.
            toolStripButton2.Enabled = True
        End If
    
        ' Post several work items to the head of the pipeline.
        For i As Integer = 0 To 4
            toolStripProgressBar1.Value += 1
            startWork.Post(New WorkItem())
        Next i
    End Sub
    
  3. En el diseñador de formularios del formulario principal, cree un controlador de eventos Click para el botón Cancelar.On the form designer for the main form, create an event handler for the Click event handler for the Cancel button.

  4. Implemente el controlador de eventos Click para el botón Cancelar.Implement the Click event handler for the Cancel button.

    // Event handler for the Cancel button.
    private async void toolStripButton2_Click(object sender, EventArgs e)
    {
       // Disable both buttons.
       toolStripButton1.Enabled = false;
       toolStripButton2.Enabled = false;
    
       // Trigger cancellation.
       cancellationSource.Cancel();
    
       try
       {
          // Asynchronously wait for the pipeline to complete processing and for
          // the progress bars to update.
          await Task.WhenAll(
             completeWork.Completion, 
             incrementProgress.Completion, 
             decrementProgress.Completion);
       }
       catch (OperationCanceledException)
       {
       }
    
       // Increment the progress bar that tracks the number of cancelled 
       // work items by the number of active work items.
       toolStripProgressBar4.Value += toolStripProgressBar1.Value;         
       toolStripProgressBar4.Value += toolStripProgressBar2.Value;
    
       // Reset the progress bars that track the number of active work items.
       toolStripProgressBar1.Value = 0;
       toolStripProgressBar2.Value = 0;
    
       // Enable the Add Work Items button.      
       toolStripButton1.Enabled = true;
    }
    
    ' Event handler for the Cancel button.
    Private Async Sub toolStripButton2_Click(ByVal sender As Object, ByVal e As EventArgs) Handles toolStripButton2.Click
        ' Disable both buttons.
        toolStripButton1.Enabled = False
        toolStripButton2.Enabled = False
    
        ' Trigger cancellation.
        cancellationSource.Cancel()
    
        Try
            ' Asynchronously wait for the pipeline to complete processing and for
            ' the progress bars to update.
            Await Task.WhenAll(completeWork.Completion, incrementProgress.Completion, decrementProgress.Completion)
        Catch e1 As OperationCanceledException
        End Try
    
        ' Increment the progress bar that tracks the number of cancelled 
        ' work items by the number of active work items.
        toolStripProgressBar4.Value += toolStripProgressBar1.Value
        toolStripProgressBar4.Value += toolStripProgressBar2.Value
    
        ' Reset the progress bars that track the number of active work items.
        toolStripProgressBar1.Value = 0
        toolStripProgressBar2.Value = 0
    
        ' Enable the Add Work Items button.      
        toolStripButton1.Enabled = True
    End Sub
    

EjemploExample

En el siguiente ejemplo se muestra el código completo de Form1.cs (Form1.vb para Visual Basic).The following example shows the complete code for Form1.cs (Form1.vb for Visual Basic).

using System;
using System.Threading;
using System.Threading.Tasks;
using System.Threading.Tasks.Dataflow;
using System.Windows.Forms;

namespace CancellationWinForms
{
   public partial class Form1 : Form
   {
      // A placeholder type that performs work.
      class WorkItem
      {
         // Performs work for the provided number of milliseconds.
         public void DoWork(int milliseconds)
         {
            // For demonstration, suspend the current thread.
            Thread.Sleep(milliseconds);
         }
      }

      // Enables the user interface to signal cancellation.
      CancellationTokenSource cancellationSource;

      // The first node in the dataflow pipeline.
      TransformBlock<WorkItem, WorkItem> startWork;

      // The second, and final, node in the dataflow pipeline.
      ActionBlock<WorkItem> completeWork;

      // Increments the value of the provided progress bar.
      ActionBlock<ToolStripProgressBar> incrementProgress;

      // Decrements the value of the provided progress bar.
      ActionBlock<ToolStripProgressBar> decrementProgress;

      // Enables progress bar actions to run on the UI thread.
      TaskScheduler uiTaskScheduler;

      public Form1()
      {
         InitializeComponent();

         // Create the UI task scheduler from the current sychronization
         // context.
         uiTaskScheduler = TaskScheduler.FromCurrentSynchronizationContext();
      }

      // Creates the blocks that participate in the dataflow pipeline.
      private void CreatePipeline()
      {
         // Create the cancellation source.
         cancellationSource = new CancellationTokenSource();

         // Create the first node in the pipeline. 
         startWork = new TransformBlock<WorkItem, WorkItem>(workItem =>
         {
            // Perform some work.
            workItem.DoWork(250);

            // Decrement the progress bar that tracks the count of 
            // active work items in this stage of the pipeline.
            decrementProgress.Post(toolStripProgressBar1);

            // Increment the progress bar that tracks the count of 
            // active work items in the next stage of the pipeline.
            incrementProgress.Post(toolStripProgressBar2);

            // Send the work item to the next stage of the pipeline.
            return workItem;
         },
         new ExecutionDataflowBlockOptions
         {
            CancellationToken = cancellationSource.Token
         });

         // Create the second, and final, node in the pipeline. 
         completeWork = new ActionBlock<WorkItem>(workItem =>
         {
            // Perform some work.
            workItem.DoWork(1000);

            // Decrement the progress bar that tracks the count of 
            // active work items in this stage of the pipeline.
            decrementProgress.Post(toolStripProgressBar2);

            // Increment the progress bar that tracks the overall 
            // count of completed work items.
            incrementProgress.Post(toolStripProgressBar3);
         }, 
         new ExecutionDataflowBlockOptions
         {
            CancellationToken = cancellationSource.Token,
            MaxDegreeOfParallelism = 2
         });

         // Connect the two nodes of the pipeline. When the first node completes, 
         // set the second node also to the completed state.
         startWork.LinkTo(
            completeWork, new DataflowLinkOptions { PropagateCompletion = true });
                  
         // Create the dataflow action blocks that increment and decrement
         // progress bars.
         // These blocks use the task scheduler that is associated with
         // the UI thread.

         incrementProgress = new ActionBlock<ToolStripProgressBar>(
            progressBar => progressBar.Value++,
            new ExecutionDataflowBlockOptions
            {
               CancellationToken = cancellationSource.Token,
               TaskScheduler = uiTaskScheduler
            });

         decrementProgress = new ActionBlock<ToolStripProgressBar>(
            progressBar => progressBar.Value--,
            new ExecutionDataflowBlockOptions
            {
               CancellationToken = cancellationSource.Token,
               TaskScheduler = uiTaskScheduler
            });
      }

      // Event handler for the Add Work Items button.
      private void toolStripButton1_Click(object sender, EventArgs e)
      {
         // The Cancel button is disabled when the pipeline is not active.
         // Therefore, create the pipeline and enable the Cancel button
         // if the Cancel button is disabled.
         if (!toolStripButton2.Enabled)
         {
            CreatePipeline();

            // Enable the Cancel button.
            toolStripButton2.Enabled = true;
         }

         // Post several work items to the head of the pipeline.
         for (int i = 0; i < 5; i++)
         {
            toolStripProgressBar1.Value++;
            startWork.Post(new WorkItem());
         }
      }

      // Event handler for the Cancel button.
      private async void toolStripButton2_Click(object sender, EventArgs e)
      {
         // Disable both buttons.
         toolStripButton1.Enabled = false;
         toolStripButton2.Enabled = false;

         // Trigger cancellation.
         cancellationSource.Cancel();

         try
         {
            // Asynchronously wait for the pipeline to complete processing and for
            // the progress bars to update.
            await Task.WhenAll(
               completeWork.Completion, 
               incrementProgress.Completion, 
               decrementProgress.Completion);
         }
         catch (OperationCanceledException)
         {
         }

         // Increment the progress bar that tracks the number of cancelled 
         // work items by the number of active work items.
         toolStripProgressBar4.Value += toolStripProgressBar1.Value;         
         toolStripProgressBar4.Value += toolStripProgressBar2.Value;

         // Reset the progress bars that track the number of active work items.
         toolStripProgressBar1.Value = 0;
         toolStripProgressBar2.Value = 0;

         // Enable the Add Work Items button.      
         toolStripButton1.Enabled = true;
      }

      ~Form1()
      {
         cancellationSource.Dispose();
      }
   }
}
Imports System.Threading
Imports System.Threading.Tasks
Imports System.Threading.Tasks.Dataflow


Namespace CancellationWinForms
    Partial Public Class Form1
        Inherits Form
        ' A placeholder type that performs work.
        Private Class WorkItem
            ' Performs work for the provided number of milliseconds.
            Public Sub DoWork(ByVal milliseconds As Integer)
                ' For demonstration, suspend the current thread.
                Thread.Sleep(milliseconds)
            End Sub
        End Class

        ' Enables the user interface to signal cancellation.
        Private cancellationSource As CancellationTokenSource

        ' The first node in the dataflow pipeline.
        Private startWork As TransformBlock(Of WorkItem, WorkItem)

        ' The second, and final, node in the dataflow pipeline.
        Private completeWork As ActionBlock(Of WorkItem)

        ' Increments the value of the provided progress bar.
        Private incrementProgress As ActionBlock(Of ToolStripProgressBar)

        ' Decrements the value of the provided progress bar.
        Private decrementProgress As ActionBlock(Of ToolStripProgressBar)

        ' Enables progress bar actions to run on the UI thread.
        Private uiTaskScheduler As TaskScheduler

        Public Sub New()
            InitializeComponent()

            ' Create the UI task scheduler from the current sychronization
            ' context.
            uiTaskScheduler = TaskScheduler.FromCurrentSynchronizationContext()
        End Sub

        ' Creates the blocks that participate in the dataflow pipeline.
        Private Sub CreatePipeline()
            ' Create the cancellation source.
            cancellationSource = New CancellationTokenSource()

            ' Create the first node in the pipeline. 
            startWork = New TransformBlock(Of WorkItem, WorkItem)(Function(workItem)
                ' Perform some work.
                ' Decrement the progress bar that tracks the count of 
                ' active work items in this stage of the pipeline.
                ' Increment the progress bar that tracks the count of 
                ' active work items in the next stage of the pipeline.
                ' Send the work item to the next stage of the pipeline.
                workItem.DoWork(250)
                decrementProgress.Post(toolStripProgressBar1)
                incrementProgress.Post(toolStripProgressBar2)
                Return workItem
            End Function,
            New ExecutionDataflowBlockOptions With {.CancellationToken = cancellationSource.Token})

            ' Create the second, and final, node in the pipeline. 
            completeWork = New ActionBlock(Of WorkItem)(Sub(workItem)
               ' Perform some work.
               ' Decrement the progress bar that tracks the count of 
               ' active work items in this stage of the pipeline.
               ' Increment the progress bar that tracks the overall 
               ' count of completed work items.
               workItem.DoWork(1000)
               decrementProgress.Post(toolStripProgressBar2)
               incrementProgress.Post(toolStripProgressBar3)
            End Sub,
            New ExecutionDataflowBlockOptions With {.CancellationToken = cancellationSource.Token,
                                                    .MaxDegreeOfParallelism = 2 })

            ' Connect the two nodes of the pipeline. When the first node completes,
            ' set the second node also to the completed state.
            startWork.LinkTo(
               completeWork, New DataflowLinkOptions With {.PropagateCompletion = true})

            ' Create the dataflow action blocks that increment and decrement
            ' progress bars.
            ' These blocks use the task scheduler that is associated with
            ' the UI thread.

            incrementProgress = New ActionBlock(Of ToolStripProgressBar)(
               Sub(progressBar) progressBar.Value += 1,
               New ExecutionDataflowBlockOptions With {.CancellationToken = cancellationSource.Token,
                                                       .TaskScheduler = uiTaskScheduler})

            decrementProgress = New ActionBlock(Of ToolStripProgressBar)(
               Sub(progressBar) progressBar.Value -= 1,
               New ExecutionDataflowBlockOptions With {.CancellationToken = cancellationSource.Token,
                                                       .TaskScheduler = uiTaskScheduler})

        End Sub

        ' Event handler for the Add Work Items button.
        Private Sub toolStripButton1_Click(ByVal sender As Object, ByVal e As EventArgs) Handles toolStripButton1.Click
            ' The Cancel button is disabled when the pipeline is not active.
            ' Therefore, create the pipeline and enable the Cancel button
            ' if the Cancel button is disabled.
            If Not toolStripButton2.Enabled Then
                CreatePipeline()

                ' Enable the Cancel button.
                toolStripButton2.Enabled = True
            End If

            ' Post several work items to the head of the pipeline.
            For i As Integer = 0 To 4
                toolStripProgressBar1.Value += 1
                startWork.Post(New WorkItem())
            Next i
        End Sub

        ' Event handler for the Cancel button.
        Private Async Sub toolStripButton2_Click(ByVal sender As Object, ByVal e As EventArgs) Handles toolStripButton2.Click
            ' Disable both buttons.
            toolStripButton1.Enabled = False
            toolStripButton2.Enabled = False

            ' Trigger cancellation.
            cancellationSource.Cancel()

            Try
                ' Asynchronously wait for the pipeline to complete processing and for
                ' the progress bars to update.
                Await Task.WhenAll(completeWork.Completion, incrementProgress.Completion, decrementProgress.Completion)
            Catch e1 As OperationCanceledException
            End Try

            ' Increment the progress bar that tracks the number of cancelled 
            ' work items by the number of active work items.
            toolStripProgressBar4.Value += toolStripProgressBar1.Value
            toolStripProgressBar4.Value += toolStripProgressBar2.Value

            ' Reset the progress bars that track the number of active work items.
            toolStripProgressBar1.Value = 0
            toolStripProgressBar2.Value = 0

            ' Enable the Add Work Items button.      
            toolStripButton1.Enabled = True
        End Sub
        
        Protected Overrides Sub Finalize()
           cancellationSource.Dispose()
           MyBase.Finalize()
        End Sub
    End Class
End Namespace

En la ilustración siguiente se muestra la aplicación en ejecución.The following illustration shows the running application.

Aplicación de Windows FormsThe Windows Forms Application

Vea tambiénSee also