共用方式為

取消作業：銜接 .NET Framework 和 Windows 執行階段 (C# 和 Visual Basic)

  • 發行項

您可以結合 .NET Framework 與 Windows 執行階段中功能讓您的資源。本主題中的範例示範如何使用 .NET FrameworkCancellationToken 執行個體將取消按鈕設定為使用 Windows 執行階段 方法下載從網路的網誌饋送的應用程式。

注意事項注意事項

若要執行這個範例，必須在電腦上安裝 Windows 8。此外，因此，如果您要從 Visual Studio 執行這個範例，您也必須在 Visual Studio 2012 已安裝或 Visual Studio Express 2012 for Windows 8 。

AsTask 提供橋接器

取消語彙基元要求 Task 執行個體，不過， Windows 執行階段 方法產生 IAsyncOperationWithProgress 執行個體。您可以使用 .NET Framework AsTask 擴充方法橋接這兩者。

在此範例中的 DownloadBlogsAsync 方法完成大部分的工作。

Async Function DownloadBlogsAsync(ct As CancellationToken) As Task
    Dim client As Windows.Web.Syndication.SyndicationClient = New SyndicationClient()

    Dim uriList = CreateUriList()

    ' Force the SyndicationClient to download the information.
    client.BypassCacheOnRetrieve = True


    ' The following code avoids the use of implicit typing (var) so that you 
    ' can identify the types clearly.

    For Each uri In uriList
        ' ***These three lines are combined in the single statement that follows them.
        'Dim feedOp As IAsyncOperationWithProgress(Of SyndicationFeed, RetrievalProgress) =
        '    client.RetrieveFeedAsync(uri)
        'Dim feedTask As Task(Of SyndicationFeed) = feedOp.AsTask(ct)
        'Dim feed As SyndicationFeed = Await feedTask

        ' ***You can combine the previous three steps in one expression.
        Dim feed As SyndicationFeed = Await client.RetrieveFeedAsync(uri).AsTask(ct)

        DisplayResults(feed, ct)
    Next
End Function

async Task DownloadBlogsAsync(CancellationToken ct)
{
    Windows.Web.Syndication.SyndicationClient client = new SyndicationClient();

    var uriList = CreateUriList();

    // Force the SyndicationClient to download the information.
    client.BypassCacheOnRetrieve = true;

    // The following code avoids the use of implicit typing (var) so that you 
    // can identify the types clearly.

    foreach (var uri in uriList)
    {
        // ***These three lines are combined in the single statement that follows them.
        //IAsyncOperationWithProgress<SyndicationFeed, RetrievalProgress> feedOp = 
        //    client.RetrieveFeedAsync(uri);
        //Task<SyndicationFeed> feedTask = feedOp.AsTask(ct);
        //SyndicationFeed feed = await feedTask;

        // ***You can combine the previous three steps in one expression.
        SyndicationFeed feed = await client.RetrieveFeedAsync(uri).AsTask(ct);

        DisplayResults(feed);
    }
}

在迴圈的註解的部分詳細顯示轉換步驟。

  • 要啟動的 SyndicationClient.RetrieveFeedAsync 呼叫下載從指定的 URI 建立部落格摘要的非同步作業。非同步作業是 IAsyncOperationWithProgress 的執行個體。

    Dim feedOp As IAsyncOperationWithProgress(Of SyndicationFeed, RetrievalProgress) = 
    client.RetrieveFeedAsync(uri)

    IAsyncOperationWithProgress<SyndicationFeed, RetrievalProgress> feedOp =  
    client.RetrieveFeedAsync(uri);

  • 由於您要使用的 .NET Framework 的取消功能要求工作，程式碼會將 AsTask 表示 IAsyncOperationWithProgress 執行個體為 Task<TResult>。特別是，應用程式碼接受 CancellationToken 引數的 AsTask 多載。

    Dim feedTask As Task(Of SyndicationFeed) = feedOp.AsTask(ct)

    Task<SyndicationFeed> feedTask = feedOp.AsTask(ct);

  • 最後， await 或 Await 運算子等候工作 SyndicationFeed 會擷取結果。

    Dim feed As SyndicationFeed = Await feedTask

    SyndicationFeed feed = await feedTask;

如需 AsTask，請參閱 WhenAny：銜接 .NET Framework 和 Windows 執行階段 (C# 和 Visual Basic)中的 擴充起始程式碼 。

問題值得注意

您可以檢閱整個範例會移動到本主題結尾，透過下載這個範例至本機電腦，或是建立範例。如需詳細資訊和指示，請參閱 設定範例。

因為您檢閱這個範例中，您將注意重點強調星號。我們建議您閱讀本節進一步了解這些點，，特別是如果您之前尚未使用 CancellationToken 。

若要實作取消按鈕，您的程式碼必須包含下列項目。

  • CancellationTokenSource 變數，則為 cts，在所有方法的範圍存取它。

    Public NotInheritable Class MainPage
    Inherits Page

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


    public sealed partial class MainPage : Page
{
    // ***Declare a System.Threading.CancellationTokenSource.
    CancellationTokenSource cts;

  • [移除] 按鈕的事件處理常式。事件處理常式會使用 CancellationTokenSource.Cancel 方法告知 cts ，當使用者要求取消。

    ' ***Add an event handler for the Cancel button.
Private Sub cancelButton_Click(sender As Object, e As RoutedEventArgs)
    If cts IsNot Nothing Then
        cts.Cancel()
        ResultsTextBox.Text &= vbCrLf & "Downloads canceled by the Cancel button."
    End If
End Sub

    // ***Add an event handler for the Cancel button.
private void CancelButton_Click(object sender, RoutedEventArgs e)
{
    if (cts != null)
    {
        cts.Cancel();
        ResultsTextBox.Text += "\r\nDownloads canceled by the Cancel button.";
    }
}

  • [啟動] 按鈕的 StartButton_Click，這個事件的事件處理常式，包括下列動作。

    • 事件處理常式執行個體化 CancellationTokenSource，否則為 cts。

      cts = New CancellationTokenSource()

      // ***Instantiate the CancellationTokenSource.
cts = new CancellationTokenSource();

    • 在對 DownloadBlogsAsync的呼叫，下載網誌饋送，程式碼將 ctsCancellationTokenSource.Token 屬性做為引數。如果要求取消， Token 屬性傳播訊息。

      Await DownloadBlogsAsync(cts.Token)

      await DownloadBlogsAsync(cts.Token);

    • 對 DownloadBlogsAsync 的呼叫中包含之 OperationCanceledException 的 catch 區塊結果的 try-catch 陳述式上，當您選取 [移除] 按鈕時。非同步方法的呼叫端定義採取什麼動作。這個範例會顯示訊息。

      下列程式碼會顯示完整的 try-catch 陳述式。

      Try
    ' ***Send a token to carry the message if cancellation is requested.
    Await DownloadBlogsAsync(cts.Token)

    ' ***Check for cancellations.
Catch op As OperationCanceledException
    ' In practice, this catch block often is empty. It is used to absorb
    ' the exception,
    ResultsTextBox.Text &= vbCrLf & "Cancellation exception bubbles up to the caller."

    ' Check for other exceptions.
Catch ex As Exception
    ResultsTextBox.Text =
        "Page could not be loaded." & vbCrLf & "Exception: " & ex.ToString()
End Try

      try
{
    // ***Send a token to carry the message if cancellation is requested.
    await DownloadBlogsAsync(cts.Token);
}
// ***Check for cancellations.
catch (OperationCanceledException)
{
    // In practice, this catch block often is empty. It is used to absorb
    // the exception,
    ResultsTextBox.Text += "\r\nCancellation exception bubbles up to the caller.";
}
// Check for other exceptions.
catch (Exception ex)
{
    ResultsTextBox.Text =
        "Page could not be loaded.\r\n" + "Exception: " + ex.ToString();
}

  • 如先前所述本主題中， DownloadBlogsAsync 方法會呼叫 Windows 執行階段 方法，則為 RetrieveFeedAsync，並將 .NET Framework 擴充方法，則為 AsTask，則傳回的 IAsyncOperation 執行個體。AsTask 表示執行個體做為 Task，因此，您可以將取消語彙基元到非同步作業。請選擇 [移除] 按鈕時，語彙基元會傳送訊息。

    請注意使用，則為 AsTask，程式碼可以將相同的 CancellationToken 執行個體加入至 Windows 執行階段 方法 (RetrieveFeedAsync) 和 .NET Framework 方法 (DownloadBlogsAsync)。

    下列程式碼行顯示程式碼的這個部分。

    Dim feed As SyndicationFeed = Await client.RetrieveFeedAsync(uri).AsTask(ct)

    SyndicationFeed feed = await client.RetrieveFeedAsync(uri).AsTask(ct);

  • 如果您無法移除應用程式，它會產生下列輸出。

    Developing for Windows
    New blog for Windows 8 app developers, 5/1/2012 2:33:02 PM -07:00
    Trigger-Start Services Recipe, 3/24/2011 2:23:01 PM -07:00
    Windows Restart and Recovery Recipe, 3/21/2011 2:13:24 PM -07:00

Extreme Windows Blog
    Samsung Series 9 27” PLS Display: Amazing Picture, 8/20/2012 2:41:48 PM -07:00
    NVIDIA GeForce GTX 660 Ti Graphics Card: Affordable Graphics Powerhouse, 8/16/2012 10:56:19 AM -07:00
    HP Z820 Workstation: Rising To the Challenge, 8/14/2012 1:57:01 PM -07:00

Blogging Windows
    Windows Upgrade Offer Registration Now Available, 8/20/2012 1:01:00 PM -07:00
    Windows 8 has reached the RTM milestone, 8/1/2012 9:00:00 AM -07:00
    Windows 8 will be available on…, 7/18/2012 1:09:00 PM -07:00

Windows for your Business
    What Windows 8 RTM Means for Businesses, 8/1/2012 9:01:00 AM -07:00
    Higher-Ed Learning with Windows 8, 7/26/2012 12:03:00 AM -07:00
    Second Public Beta of App-V 5.0 Now Available with Office Integration, 7/24/2012 10:07:26 AM -07:00

Windows Experience Blog
    Tech Tuesday Live Twitter Chat with Microsoft Hardware, 8/20/2012 2:20:57 AM -07:00
    New Colors and New Artist Series Mice from Microsoft Hardware, 8/15/2012 12:06:35 AM -07:00
    Tech Tuesday Live Twitter Chat with HP on Keeping Kids Safe as They Head Back to School #winchat, 8/13/2012 12:24:18 PM -07:00

Windows Security Blog
    Dealing with Fake Tech Support & Phone Scams, 6/16/2011 1:53:00 PM -07:00
    Combating social engineering tactics, like cookiejacking, to stay safer online, 5/28/2011 12:02:26 PM -07:00
    Windows 7 is now Common Criteria Certified!, 4/27/2011 9:35:01 AM -07:00

Windows Home Server Blog
    Connecting Windows 8 Consumer Preview with Windows Home Server, 3/25/2012 9:06:00 AM -07:00
    Viridian PC Systems announces two new server models are available to order, 10/3/2011 12:36:00 PM -07:00
    PC Specialist to release Windows Home Server 2011, 9/27/2011 10:27:37 AM -07:00

Springboard Series Blog
    Windows 8 Is Ready For Your Enterprise, 8/16/2012 9:59:00 AM -07:00
    What to Expect in User Experience Virtualization Beta 2, 6/25/2012 11:03:27 PM -07:00
    Introducing Microsoft BitLocker Administration 2.0 Beta, 6/12/2012 8:08:23 AM -07:00

    如果您選擇 [移除] 按鈕，在應用程式完成下載內容之前，結果會類似下列的輸出。

    Developing for Windows
    New blog for Windows 8 app developers, 5/1/2012 2:33:02 PM -07:00
    Trigger-Start Services Recipe, 3/24/2011 2:23:01 PM -07:00
    Windows Restart and Recovery Recipe, 3/21/2011 2:13:24 PM -07:00

Extreme Windows Blog
    Samsung Series 9 27” PLS Display: Amazing Picture, 8/20/2012 2:41:48 PM -07:00
    NVIDIA GeForce GTX 660 Ti Graphics Card: Affordable Graphics Powerhouse, 8/16/2012 10:56:19 AM -07:00
    HP Z820 Workstation: Rising To the Challenge, 8/14/2012 1:57:01 PM -07:00

Blogging Windows
    Windows Upgrade Offer Registration Now Available, 8/20/2012 1:01:00 PM -07:00
    Windows 8 has reached the RTM milestone, 8/1/2012 9:00:00 AM -07:00
    Windows 8 will be available on…, 7/18/2012 1:09:00 PM -07:00

Windows for your Business
    What Windows 8 RTM Means for Businesses, 8/1/2012 9:01:00 AM -07:00
    Higher-Ed Learning with Windows 8, 7/26/2012 12:03:00 AM -07:00
    Second Public Beta of App-V 5.0 Now Available with Office Integration, 7/24/2012 10:07:26 AM -07:00

Downloads canceled by the Cancel button.
Cancellation exception bubbles up to the caller.

設定範例

您可以下載應用程式，建置自己或檢閱程式碼在這個主題的結尾，而不需要實作它。在您的電腦上安裝 Visual Studio 2012 和 Windows 8 上執行此應用程式。

下載完成的應用程式

  1. 下載壓縮檔) Async 範例:在 .NET 和 Windows 執行階段之間的橋接 AsTask & (取消)。

  2. 解壓縮您下載的檔案，然後啟動 Visual Studio。

  3. 在功能表列上，選擇 [檔案]、[開啟]、[專案/方案]。

  4. 巡覽至保存解壓縮的範例程式碼的資料夾，然後開啟方案 (.sln) 檔。

  5. 選擇 F5 鍵建置及執行專案。

    執行程式碼幾次驗證可以移除在不同的點。

建置完成的應用程式

  1. 啟動 Visual Studio。

  2. 在功能表列上，選擇 [檔案]、[新增]、[專案]。

    [新增專案] 對話方塊隨即開啟。

  3. 在 [安裝] 中， [ [範本] 分類中，選取 [Visual Basic] 或 [Visual C#]]，然後選取 [Windows 市集]。

  4. 從專案類型清單中，選取 [空白的應用程式 (XAML)]。

  5. 將專案命名為 BlogFeedWithCancellation，然後選擇 [OK] 按鈕。

    新專案即會出現於 [方案總管] 中。

  6. 在 [方案總管] 中，開啟 MainPage.xaml 的捷徑功能表，然後選擇 [開啟]。

  7. 在 [XAML] 視窗中 MainPage.xaml，以下列程式碼取代程式碼。

    <Page
    x:Class="BlogFeedWithCancellation.MainPage"
    xmlns="https://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="https://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:BlogFeedWithCancellation"
    xmlns:d="https://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="https://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d">

    <Grid Background="{StaticResource ApplicationPageBackgroundThemeBrush}">
        <Button x:Name="StartButton" Content="Start" HorizontalAlignment="Left" Margin="325,77,0,0" VerticalAlignment="Top" Click="StartButton_Click" Height="145" Background="#FFA89B9B" FontSize="36" Width="355"  />
        <Button x:Name="CancelButton" Content="Cancel" HorizontalAlignment="Left" Margin="684,77,0,0" VerticalAlignment="Top" Height="145" Background="#FFA89B9B" Click="CancelButton_Click" FontSize="36" Width="355"  />
        <TextBox x:Name="ResultsTextBox" HorizontalAlignment="Left" Margin="325,222,0,0" TextWrapping="Wrap" VerticalAlignment="Top" Height="546" FontSize="10" ScrollViewer.VerticalScrollBarVisibility="Visible" Width="711" />
    </Grid>
</Page>

    包含文字方塊的簡單視窗，開始按鈕和取消按鈕隨即出現在 [設計] 視窗中 MainPage.xaml。

  8. 在 [方案總管] 中，開啟或 MainPage.xaml.vb MainPage.xaml.cs 的捷徑功能表，然後選擇 [檢視程式碼]。

  9. 使用下列程式碼取代或 MainPage.xaml.vb MainPage.xaml.cs 的程式碼。

    ' Add an Imports statement for SyndicationClient.
Imports Windows.Web.Syndication
' Add an Imports statement for Tasks.
Imports System.Threading.Tasks
' Add an Imports statement for CancellationToken.
Imports System.Threading


Public NotInheritable Class MainPage
    Inherits Page

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

    Private Async Sub StartButton_Click(sender As Object, e As RoutedEventArgs)
        ResultsTextBox.Text = ""
        ' Prevent unexpected reentrance.
        StartButton.IsEnabled = False

        ' ***Instantiate the CancellationTokenSource.
        cts = New CancellationTokenSource()

        Try
            ' ***Send a token to carry the message if cancellation is requested.
            Await DownloadBlogsAsync(cts.Token)

            ' ***Check for cancellations.
        Catch op As OperationCanceledException
            ' In practice, this catch block often is empty. It is used to absorb
            ' the exception,
            ResultsTextBox.Text &= vbCrLf & "Cancellation exception bubbles up to the caller."

            ' Check for other exceptions.
        Catch ex As Exception
            ResultsTextBox.Text =
                "Page could not be loaded." & vbCrLf & "Exception: " & ex.ToString()
        End Try

        ' ***Set the CancellationTokenSource to null when the work is complete.
        cts = Nothing

        ' In case you want to try again.
        StartButton.IsEnabled = True
    End Sub


    ' Provide a parameter for the CancellationToken.
    Async Function DownloadBlogsAsync(ct As CancellationToken) As Task
        Dim client As Windows.Web.Syndication.SyndicationClient = New SyndicationClient()

        Dim uriList = CreateUriList()

        ' Force the SyndicationClient to download the information.
        client.BypassCacheOnRetrieve = True


        ' The following code avoids the use of implicit typing (var) so that you 
        ' can identify the types clearly.

        For Each uri In uriList
            ' ***These three lines are combined in the single statement that follows them.
            'Dim feedOp As IAsyncOperationWithProgress(Of SyndicationFeed, RetrievalProgress) =
            '    client.RetrieveFeedAsync(uri)
            'Dim feedTask As Task(Of SyndicationFeed) = feedOp.AsTask(ct)
            'Dim feed As SyndicationFeed = Await feedTask

            ' ***You can combine the previous three steps in one expression.
            Dim feed As SyndicationFeed = Await client.RetrieveFeedAsync(uri).AsTask(ct)

            DisplayResults(feed, ct)
        Next
    End Function


    ' ***Add an event handler for the Cancel button.
    Private Sub cancelButton_Click(sender As Object, e As RoutedEventArgs)
        If cts IsNot Nothing Then
            cts.Cancel()
            ResultsTextBox.Text &= vbCrLf & "Downloads canceled by the Cancel button."
        End If
    End Sub


    Function CreateUriList() As List(Of Uri)
        ' Create a list of URIs.
        Dim uriList = New List(Of Uri) From
                {
                    New Uri("https://windowsteamblog.com/windows/b/developers/atom.aspx"),
                    New Uri("https://windowsteamblog.com/windows/b/extremewindows/atom.aspx"),
                    New Uri("https://windowsteamblog.com/windows/b/bloggingwindows/atom.aspx"),
                    New Uri("https://windowsteamblog.com/windows/b/business/atom.aspx"),
                    New Uri("https://windowsteamblog.com/windows/b/windowsexperience/atom.aspx"),
                    New Uri("https://windowsteamblog.com/windows/b/windowssecurity/atom.aspx"),
                    New Uri("https://windowsteamblog.com/windows/b/windowshomeserver/atom.aspx"),
                    New Uri("https://windowsteamblog.com/windows/b/springboard/atom.aspx")
                }
        Return uriList
    End Function


    ' You can pass the CancellationToken to this method if you think you might use a
    ' cancellable API here in the future.
    Sub DisplayResults(sf As SyndicationFeed, ct As CancellationToken)
        ' Title of the blog.
        ResultsTextBox.Text &= sf.Title.Text & vbCrLf

        ' Titles and dates for the first three blog posts.
        For i As Integer = 0 To If(sf.Items.Count >= 3, 2, sf.Items.Count)
            ResultsTextBox.Text &= vbTab & sf.Items.ElementAt(i).Title.Text & ", " &
                    sf.Items.ElementAt(i).PublishedDate.ToString() & vbCrLf
        Next

        ResultsTextBox.Text &= vbCrLf
    End Sub
End Class

    using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using Windows.Foundation;
using Windows.Foundation.Collections;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Controls.Primitives;
using Windows.UI.Xaml.Data;
using Windows.UI.Xaml.Input;
using Windows.UI.Xaml.Media;
using Windows.UI.Xaml.Navigation;

// Add a using directive for SyndicationClient.
using Windows.Web.Syndication;
// Add a using directive for Tasks.
using System.Threading.Tasks;
// Add a using directive for CancellationToken.
using System.Threading;


namespace BlogFeedWithCancellation
{
    public sealed partial class MainPage : Page
    {
        // ***Declare a System.Threading.CancellationTokenSource.
        CancellationTokenSource cts;

        public MainPage()
        {
            this.InitializeComponent();
        }


        private async void StartButton_Click(object sender, RoutedEventArgs e)
        {
            ResultsTextBox.Text = "";
            // Prevent unexpected reentrance.
            StartButton.IsEnabled = false;

            // ***Instantiate the CancellationTokenSource.
            cts = new CancellationTokenSource();

            try
            {
                // ***Send a token to carry the message if cancellation is requested.
                await DownloadBlogsAsync(cts.Token);
            }
            // ***Check for cancellations.
            catch (OperationCanceledException)
            {
                // In practice, this catch block often is empty. It is used to absorb
                // the exception,
                ResultsTextBox.Text += "\r\nCancellation exception bubbles up to the caller.";
            }
            // Check for other exceptions.
            catch (Exception ex)
            {
                ResultsTextBox.Text =
                    "Page could not be loaded.\r\n" + "Exception: " + ex.ToString();
            }

            // ***Set the CancellationTokenSource to null when the work is complete.
            cts = null;

            // In case you want to try again.
            StartButton.IsEnabled = true;
        }


        // ***Provide a parameter for the CancellationToken.
        async Task DownloadBlogsAsync(CancellationToken ct)
        {
            Windows.Web.Syndication.SyndicationClient client = new SyndicationClient();

            var uriList = CreateUriList();

            // Force the SyndicationClient to download the information.
            client.BypassCacheOnRetrieve = true;

            // The following code avoids the use of implicit typing (var) so that you 
            // can identify the types clearly.

            foreach (var uri in uriList)
            {
                // ***These three lines are combined in the single statement that follows them.
                //IAsyncOperationWithProgress<SyndicationFeed, RetrievalProgress> feedOp = 
                //    client.RetrieveFeedAsync(uri);
                //Task<SyndicationFeed> feedTask = feedOp.AsTask(ct);
                //SyndicationFeed feed = await feedTask;

                // ***You can combine the previous three steps in one expression.
                SyndicationFeed feed = await client.RetrieveFeedAsync(uri).AsTask(ct);

                DisplayResults(feed);
            }
        }



        // ***Add an event handler for the Cancel button.
        private void CancelButton_Click(object sender, RoutedEventArgs e)
        {
            if (cts != null)
            {
                cts.Cancel();
                ResultsTextBox.Text += "\r\nDownloads canceled by the Cancel button.";
            }
        }


        List<Uri> CreateUriList()
        {
            // Create a list of URIs.
            List<Uri> uriList = new List<Uri> 
            { 
                new Uri("https://windowsteamblog.com/windows/b/developers/atom.aspx"),
                new Uri("https://windowsteamblog.com/windows/b/extremewindows/atom.aspx"),
                new Uri("https://windowsteamblog.com/windows/b/bloggingwindows/atom.aspx"),
                new Uri("https://windowsteamblog.com/windows/b/business/atom.aspx"),
                new Uri("https://windowsteamblog.com/windows/b/windowsexperience/atom.aspx"),
                new Uri("https://windowsteamblog.com/windows/b/windowssecurity/atom.aspx"),
                new Uri("https://windowsteamblog.com/windows/b/windowshomeserver/atom.aspx"),
                new Uri("https://windowsteamblog.com/windows/b/springboard/atom.aspx")
            };
            return uriList;
        }


        // You can pass the CancellationToken to this method if you think you might use a
        // cancellable API here in the future.
        void DisplayResults(SyndicationFeed sf)
        {
            // Title of the blog.
            ResultsTextBox.Text += sf.Title.Text + "\r\n";

            // Titles and dates for the first three blog posts.
            for (int i = 0; i < (sf.Items.Count < 3 ? sf.Items.Count : 3); i++)    // Is Math.Min better?
            {
                ResultsTextBox.Text += "\t" + sf.Items.ElementAt(i).Title.Text + ", " +
                    sf.Items.ElementAt(i).PublishedDate.ToString() + "\r\n";
            }

            ResultsTextBox.Text += "\r\n";
        }
    }
}

  10. 選取 F5 鍵執行程式，然後選取 [開始] 按鈕。

請參閱

概念

WhenAny：銜接 .NET Framework 和 Windows 執行階段 (C# 和 Visual Basic)

取消一項或多項工作 (C# 和 Visual Basic)

在一段時間後取消工作 (C# 和 Visual Basic)

當其中一項工作完成時，取消剩餘的工作 (C# 和 Visual Basic)

取消語彙基元