在一段時間後取消非同步工作 (C# 和 Visual Basic)

發行項
06/10/2015

如果不要等待作業完成，您可以取消非同步作業在一段時間之後，使用 CancellationTokenSource.CancelAfter 方法。這個方法會排程取消任何在 CancelAfter 運算式所指定的時間內沒有完成的所有關聯工作。

這個範例會加入至取消一項非同步工作或工作清單 (C# 和 Visual Basic) 中開發的程式碼，下載網站清單和顯示每一個清單內容長度。

注意事項
若要執行範例，您必須將 Visual Studio 2012、Visual Studio 2013、Visual Studio Express 2012 for Windows Desktop、Visual Studio Express 2013 for Windows 或 .NET Framework 4.5 或 4.5.1 安裝在您的電腦上。

下載範例

您可以從非同步範例：微調應用程式下載完整的 Windows Presentation Foundation (WPF) 專案，然後依照下列步驟執行。

解壓縮您下載的檔案，然後啟動 Visual Studio。
在功能表列上，選擇 [檔案]、[開啟]、[專案/方案]。
在 [開啟專案] 對話方塊中，開啟含有已解壓縮之範例程式碼的資料夾，然後開啟 AsyncFineTuningCS 或 AsyncFineTuningVB 的方案 (.sln) 檔。
在 [方案總管] 中，開啟 [CancelAfterTime] 專案的捷徑功能表，然後選取 [設定為啟始專案]。
選擇 F5 鍵以執行專案。

選取 Ctrl+F5 鍵執行專案，但不偵錯。
執行程式數次，確認輸出可能會顯示所有網站、沒有任何網站或是某些網站的輸出。

如果您不要下載專案，您可以檢視本主題結尾的 MainWindow.xaml.vb 和 MainWindow.xaml.cs 檔案。

建置範例

本主題中的範例會加入至取消一項非同步工作或工作清單 (C# 和 Visual Basic) 中開發的專案來消除工作清單。這個範例使用相同的 UI，雖然 [取消 ] 按鈕不會明確使用。

若要建置範例，請依照＜下載範例＞中的指示逐步執行，不過選取 CancelAListOfTasks 為 [啟始專案]。將本主題中的變更加入至該專案。

若要指定將工作標示為已取消之前的最大時間，請在 startButton_Click 加入 CancelAfter 呼叫，如下列範例所示。此加入以星號標記。

Private Async Sub startButton_Click(sender As Object, e As RoutedEventArgs)

    ' Instantiate the CancellationTokenSource.
    cts = New CancellationTokenSource()

    resultsTextBox.Clear()

    Try 
        ' ***Set up the CancellationTokenSource to cancel after 2.5 seconds. (You  
        ' can adjust the time.)
        cts.CancelAfter(2500)

        Await AccessTheWebAsync(cts.Token)
        resultsTextBox.Text &= vbCrLf & "Downloads complete." 

    Catch ex As OperationCanceledException
        resultsTextBox.Text &= vbCrLf & "Downloads canceled." & vbCrLf

    Catch ex As Exception
        resultsTextBox.Text &= vbCrLf & "Downloads failed." & vbCrLf
    End Try 

    ' Set the CancellationTokenSource to Nothing when the download is complete.
    cts = Nothing 
End Sub

private async void startButton_Click(object sender, RoutedEventArgs e)
{
    // Instantiate the CancellationTokenSource.
    cts = new CancellationTokenSource();

    resultsTextBox.Clear();

    try
    {
        // ***Set up the CancellationTokenSource to cancel after 2.5 seconds. (You 
        // can adjust the time.)
        cts.CancelAfter(2500);

        await AccessTheWebAsync(cts.Token);
        resultsTextBox.Text += "\r\nDownloads succeeded.\r\n";
    }
    catch (OperationCanceledException)
    {
        resultsTextBox.Text += "\r\nDownloads canceled.\r\n";
    }
    catch (Exception)
    {
        resultsTextBox.Text += "\r\nDownloads failed.\r\n";
    }

    cts = null; 
}

執行程式數次，確認輸出可能會顯示所有網站、沒有任何網站或是某些網站的輸出。以下是輸出範例。

Length of the downloaded string: 35990.

Length of the downloaded string: 407399.

Length of the downloaded string: 226091.

Downloads canceled.

完整範例

下列程式碼是範例的完整 MainWindow.xaml.vb 或 MainWindow.xaml.cs 檔案文字。星號標示出已為這個範例加入的項目。

請注意，您必須加入 System.Net.Http 的參考。

您可以從 Async 範例:微調應用程式下載專案。

' Add an Imports directive and a reference for System.Net.Http. 
Imports System.Net.Http

' Add the following Imports directive for System.Threading. 
Imports System.Threading

Class MainWindow

    ' Declare a System.Threading.CancellationTokenSource. 
    Dim cts As CancellationTokenSource


    Private Async Sub startButton_Click(sender As Object, e As RoutedEventArgs)

        ' Instantiate the CancellationTokenSource.
        cts = New CancellationTokenSource()

        resultsTextBox.Clear()

        Try 
            ' ***Set up the CancellationTokenSource to cancel after 2.5 seconds. (You  
            ' can adjust the time.)
            cts.CancelAfter(2500)

            Await AccessTheWebAsync(cts.Token)
            resultsTextBox.Text &= vbCrLf & "Downloads complete." 

        Catch ex As OperationCanceledException
            resultsTextBox.Text &= vbCrLf & "Downloads canceled." & vbCrLf

        Catch ex As Exception
            resultsTextBox.Text &= vbCrLf & "Downloads failed." & vbCrLf
        End Try 

        ' Set the CancellationTokenSource to Nothing when the download is complete.
        cts = Nothing 
    End Sub 


    ' You can still include a Cancel button if you want to. 
    Private Sub cancelButton_Click(sender As Object, e As RoutedEventArgs)

        If cts IsNot Nothing Then
            cts.Cancel()
        End If 
    End Sub 


    ' Provide a parameter for the CancellationToken. 
    ' Change the return type to Task because the method has no return statement.
    Async Function AccessTheWebAsync(ct As CancellationToken) As Task

        Dim client As HttpClient = New HttpClient()

        ' Call SetUpURLList to make a list of web addresses. 
        Dim urlList As List(Of String) = SetUpURLList()

        ' Process each element in the list of web addresses. 
        For Each url In urlList
            ' GetAsync returns a Task(Of HttpResponseMessage).  
            ' Argument ct carries the message if the Cancel button is chosen.  
            ' Note that the Cancel button can cancel all remaining downloads. 
            Dim response As HttpResponseMessage = Await client.GetAsync(url, ct)

            ' Retrieve the website contents from the HttpResponseMessage. 
            Dim urlContents As Byte() = Await response.Content.ReadAsByteArrayAsync()

            resultsTextBox.Text &=
                String.Format(vbCrLf & "Length of the downloaded string: {0}." & vbCrLf, urlContents.Length)
        Next 
    End Function 


    ' Add a method that creates a list of web addresses. 
    Private Function SetUpURLList() As List(Of String)

        Dim urls = New List(Of String) From
            {
                "https://msdn.microsoft.com",
                "https://msdn.microsoft.com/en-us/library/hh290138.aspx",
                "https://msdn.microsoft.com/en-us/library/hh290140.aspx",
                "https://msdn.microsoft.com/en-us/library/dd470362.aspx",
                "https://msdn.microsoft.com/en-us/library/aa578028.aspx",
                "https://msdn.microsoft.com/en-us/library/ms404677.aspx",
                "https://msdn.microsoft.com/en-us/library/ff730837.aspx"
            }
        Return urls
    End Function 

End Class 


' Sample output: 

' Length of the downloaded string: 35990. 

' Length of the downloaded string: 407399. 

' Length of the downloaded string: 226091. 

' Downloads canceled.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

// Add a using directive and a reference for System.Net.Http. 
using System.Net.Http;

// Add the following using directive. 
using System.Threading;

namespace CancelAfterTime
{
    public partial class MainWindow : Window
    {
        // Declare a System.Threading.CancellationTokenSource.
        CancellationTokenSource cts;

        public MainWindow()
        {
            InitializeComponent();
        }


        private async void startButton_Click(object sender, RoutedEventArgs e)
        {
            // Instantiate the CancellationTokenSource.
            cts = new CancellationTokenSource();

            resultsTextBox.Clear();

            try
            {
                // ***Set up the CancellationTokenSource to cancel after 2.5 seconds. (You 
                // can adjust the time.)
                cts.CancelAfter(2500);

                await AccessTheWebAsync(cts.Token);
                resultsTextBox.Text += "\r\nDownloads succeeded.\r\n";
            }
            catch (OperationCanceledException)
            {
                resultsTextBox.Text += "\r\nDownloads canceled.\r\n";
            }
            catch (Exception)
            {
                resultsTextBox.Text += "\r\nDownloads failed.\r\n";
            }

            cts = null; 
        }


        // You can still include a Cancel button if you want to. 
        private void cancelButton_Click(object sender, RoutedEventArgs e)
        {
            if (cts != null)
            {
                cts.Cancel();
            }
        }


        async Task AccessTheWebAsync(CancellationToken ct)
        {
            // Declare an HttpClient object.
            HttpClient client = new HttpClient();

            // Make a list of web addresses.
            List<string> urlList = SetUpURLList();

            foreach (var url in urlList)
            {
                // GetAsync returns a Task<HttpResponseMessage>.  
                // Argument ct carries the message if the Cancel button is chosen.  
                // Note that the Cancel button cancels all remaining downloads.
                HttpResponseMessage response = await client.GetAsync(url, ct);

                // Retrieve the website contents from the HttpResponseMessage. 
                byte[] urlContents = await response.Content.ReadAsByteArrayAsync();

                resultsTextBox.Text +=
                    String.Format("\r\nLength of the downloaded string: {0}.\r\n", urlContents.Length);
            }
        }


        private List<string> SetUpURLList()
        {
            List<string> urls = new List<string> 
            { 
                "https://msdn.microsoft.com",
                "https://msdn.microsoft.com/library/windows/apps/br211380.aspx",
                "https://msdn.microsoft.com/en-us/library/hh290136.aspx",
                "https://msdn.microsoft.com/en-us/library/ee256749.aspx",
                "https://msdn.microsoft.com/en-us/library/ms404677.aspx",
                "https://msdn.microsoft.com/en-us/library/ff730837.aspx"
            };
            return urls;
        }
    }

    // Sample Output: 

    // Length of the downloaded string: 35990. 

    // Length of the downloaded string: 407399. 

    // Length of the downloaded string: 226091. 

    // Downloads canceled.
}

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在一段時間後取消非同步工作 (C# 和 Visual Basic)

下載範例

建置範例

完整範例

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