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

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藉由使用 Task.WhenAny 方法搭配 CancellationToken，您可以在一個工作完成之後，取消所有剩下的工作。 WhenAny 方法需要一個是工作集合的引數。 方法會啟動所有工作並傳回單一工作。 集合中的任何工作完成時，就完成單一工作。

這個範例展示如何使用消除標誌結合WhenAny 來保留第一個工作，讓其從工作集合中結束，並取消剩下的工作。 每個工作都會下載網站的內容。 這個範例顯示第一個下載內容完成的長度並移除其他下載。

注意事項注意事項

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。

  1. 解壓縮您下載的檔案，然後啟動 Visual Studio。

  2. 在功能表列上，選擇 [檔案]、[開啟]、[專案/方案]。

  3. 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。

  4. 在 [方案總管] 中，開啟 [CancelAfterOneTask] 專案的捷徑功能表，然後選取 [設定為啟始專案]。

  5. 選擇 F5 鍵以執行專案。

    選取 Ctrl+F5 鍵執行專案，但不偵錯。

  6. 執行程式幾次，驗證不同下載先完成。

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。

在 [CancelAListOfTasks] 專案的 MainWindow.xaml.vb 或 MainWindow.xaml.cs 檔案中，將每個網站的處理步驟從 AccessTheWebAsync 中的迴圈移至下列非同步方法，以開始轉換。

' ***Bundle the processing steps for a website into one async method.
Async Function ProcessURLAsync(url As String, client As HttpClient, ct As CancellationToken) As Task(Of Integer)

    ' GetAsync returns a Task(Of HttpResponseMessage).  
    Dim response As HttpResponseMessage = Await client.GetAsync(url, ct)

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

    Return urlContents.Length
End Function

// ***Bundle the processing steps for a website into one async method.
async Task<int> ProcessURLAsync(string url, HttpClient client, CancellationToken ct)
{
    // GetAsync returns a Task<HttpResponseMessage>. 
    HttpResponseMessage response = await client.GetAsync(url, ct);

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

    return urlContents.Length;
}

在 AccessTheWebAsync 中，本範例使用查詢、ToArray``1 方法或 WhenAny 方法來建立和啟動工作陣列。 WhenAny 應用到陣列上會傳回單一的工作，當等候時，可確保工作陣列中的第一個工作會完成。

在 AccessTheWebAsync 進行下列變更。 星號標示出程式碼檔案中的變更。

  1. 將迴圈標記為註解或刪除。

  2. 建立一個會在執行時產生泛型工作集合的查詢。 對 ProcessURLAsync 的每個呼叫都會傳回 Task，其中 TResult 為整數。

    ' ***Create a query that, when executed, returns a collection of tasks. 
Dim downloadTasksQuery As IEnumerable(Of Task(Of Integer)) =
    From url In urlList Select ProcessURLAsync(url, client, ct)

    // ***Create a query that, when executed, returns a collection of tasks.
IEnumerable<Task<int>> downloadTasksQuery =
    from url in urlList select ProcessURLAsync(url, client, ct);

  3. 呼叫 ToArray 來執行查詢並啟動工作。 在下一個步驟中應用 WhenAny 方法將會執行查詢並啟動工作，而不需要使用 ToArray，不過，其他方法可能不是如此。 最安全的做法是明確強制查詢的執行。

    ' ***Use ToArray to execute the query and start the download tasks.  
Dim downloadTasks As Task(Of Integer)() = downloadTasksQuery.ToArray()

    // ***Use ToArray to execute the query and start the download tasks. 
Task<int>[] downloadTasks = downloadTasksQuery.ToArray();

  4. 在工作的集合上呼叫 WhenAny。 WhenAny 傳回 Task(Of Task(Of Integer)) 或 Task<Task<int>>。也就是說，WhenAny 會在等候時傳回相當於單一 Task(Of Integer) 或 Task<int> 的工作。 該單一工作是集合中要第一個完成的工作。 先完成的工作被指派給 firstFinishedTask。 firstFinishedTask 的類型是 Task，其中 TResult是一個整數，因為那是 ProcessURLAsync 的回傳類型。

    ' ***Call WhenAny and then await the result. The task that finishes  
' first is assigned to firstFinishedTask. 
Dim firstFinishedTask As Task(Of Integer) = Await Task.WhenAny(downloadTasks)

    // ***Call WhenAny and then await the result. The task that finishes  
// first is assigned to firstFinishedTask.
Task<int> firstFinishedTask = await Task.WhenAny(downloadTasks);

  5. 在此範例中，您只需要專注在先完成的工作上。 因此，請使用 CancellationTokenSource.Cancel 來取消剩餘工作。

    ' ***Cancel the rest of the downloads. You just want the first one.
cts.Cancel()

    // ***Cancel the rest of the downloads. You just want the first one.
cts.Cancel();

  6. 最後，等候 firstFinishedTask 擷取下載內容的長度。

    Dim length = Await firstFinishedTask
resultsTextBox.Text &= String.Format(vbCrLf & "Length of the downloaded website:  {0}" & vbCrLf, length)

    var length = await firstFinishedTask;
resultsTextBox.Text += String.Format("\r\nLength of the downloaded website:  {0}\r\n", length);

執行程式幾次，驗證不同下載先完成。

完整範例

下列程式碼是範例的完整 MainWindow.xaml.vb 或 MainWindow.xaml.cs 檔案。 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
            Await AccessTheWebAsync(cts.Token)
            resultsTextBox.Text &= vbCrLf & "Download complete." 

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

        Catch ex As Exception
            resultsTextBox.Text &= vbCrLf & "Download 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()

        '' Comment out or delete the loop. 
        ''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 

        ' ***Create a query that, when executed, returns a collection of tasks. 
        Dim downloadTasksQuery As IEnumerable(Of Task(Of Integer)) =
            From url In urlList Select ProcessURLAsync(url, client, ct)

        ' ***Use ToArray to execute the query and start the download tasks.  
        Dim downloadTasks As Task(Of Integer)() = downloadTasksQuery.ToArray()

        ' ***Call WhenAny and then await the result. The task that finishes  
        ' first is assigned to firstFinishedTask. 
        Dim firstFinishedTask As Task(Of Integer) = Await Task.WhenAny(downloadTasks)

        ' ***Cancel the rest of the downloads. You just want the first one.
        cts.Cancel()

        ' ***Await the first completed task and display the results 
        ' Run the program several times to demonstrate that different 
        ' websites can finish first. 
        Dim length = Await firstFinishedTask
        resultsTextBox.Text &= String.Format(vbCrLf & "Length of the downloaded website:  {0}" & vbCrLf, length)
    End Function 


    ' ***Bundle the processing steps for a website into one async method.
    Async Function ProcessURLAsync(url As String, client As HttpClient, ct As CancellationToken) As Task(Of Integer)

        ' GetAsync returns a Task(Of HttpResponseMessage).  
        Dim response As HttpResponseMessage = Await client.GetAsync(url, ct)

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

        Return urlContents.Length
    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 website:  158856 

' Download complete.

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 CancelAfterOneTask
{
    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
            {
                await AccessTheWebAsync(cts.Token);
                resultsTextBox.Text += "\r\nDownload complete.";
            }
            catch (OperationCanceledException)
            {
                resultsTextBox.Text += "\r\nDownload canceled.";
            }
            catch (Exception)
            {
                resultsTextBox.Text += "\r\nDownload failed.";
            }

            // Set the CancellationTokenSource to null when the download is complete.
            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();
            }
        }


        // Provide a parameter for the CancellationToken.
        async Task AccessTheWebAsync(CancellationToken ct)
        {
            HttpClient client = new HttpClient();

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

            // ***Comment out or delete the loop. 
            //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 can cancel 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);
            //} 

            // ***Create a query that, when executed, returns a collection of tasks.
            IEnumerable<Task<int>> downloadTasksQuery =
                from url in urlList select ProcessURLAsync(url, client, ct);

            // ***Use ToArray to execute the query and start the download tasks. 
            Task<int>[] downloadTasks = downloadTasksQuery.ToArray();

            // ***Call WhenAny and then await the result. The task that finishes  
            // first is assigned to firstFinishedTask.
            Task<int> firstFinishedTask = await Task.WhenAny(downloadTasks);

            // ***Cancel the rest of the downloads. You just want the first one.
            cts.Cancel();

            // ***Await the first completed task and display the results.  
            // Run the program several times to demonstrate that different 
            // websites can finish first. 
            var length = await firstFinishedTask;
            resultsTextBox.Text += String.Format("\r\nLength of the downloaded website:  {0}\r\n", length);
        }


        // ***Bundle the processing steps for a website into one async method.
        async Task<int> ProcessURLAsync(string url, HttpClient client, CancellationToken ct)
        {
            // GetAsync returns a Task<HttpResponseMessage>. 
            HttpResponseMessage response = await client.GetAsync(url, ct);

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

            return urlContents.Length;
        }


        // Add a method that creates a list of web addresses. 
        private List<string> SetUpURLList()
        {
            List<string> urls = new List<string> 
            { 
                "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;
        }
    }
    // Sample output: 

    // Length of the downloaded website:  158856 

    // Download complete.
}

請參閱

參考

WhenAny

概念

微調非同步應用程式 (C# 和 Visual Basic)

使用 Async 和 Await 設計非同步程式 (C# 和 Visual Basic)

其他資源

非同步範例：微調應用程式