在一段时间后取消任务(C# 和 Visual Basic)
如果您不希望等待该操作完成,可以取消异步操作一段时间后使用 CancellationTokenSource.CancelAfter 方法。此方法计划在时间不是完全中由 CancelAfter 表达式指定的所有关联的任务取消。
此示例添加到 取消一个任务或一组任务(C# 和 Visual Basic) 中开发下载网站列表和显示每个的内容长度的代码。
说明 |
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若要运行此示例,需要 Visual Studio 2012 中,Visual Studio express 2012 中的 windows 桌面或在您的计算机上安装 .NET framework 4.5。 |
下载示例
可以下载完整的 windows 演示基础 (WPF) 项从 Async 示例:优化应用程序 然后按照这些步骤。
对要下载的文件,然后启动 Visual Studio 2012。
在菜单栏上,依次选择**“文件”、“打开”和“项目/解决方案”**。
在 打开项目 对话框中,打开包含示例代码。解压缩,然后打开 AsyncFineTuningCS 或 AsyncFineTuningVB 的解决方案的文件夹 (.sln) 文件。
在 解决方案资源管理器,打开 CancelAfterTime 项目的快捷菜单,然后选择 设为启动项目。
选择 F5 键运行项目。
选择密钥 Ctrl+F5 运行项目,但不对其进行调试。
运行程序多次验证输出可能显示所有网站、没有网站,或者有些网站的输出。
如果不希望下载该项目,可以查看 MainWindow.xaml.vb 或 MainWindow.xaml.cs 文件本主题末尾的。
生成示例
本主题中的示例添加到 取消一个任务或一组任务(C# 和 Visual Basic) 中开发取消任务列表中选择项。该示例使用相同的 UI,不过,没有显式使用 取消 按钮。
若要生成此示例,分步,后面部分的“下载示例的”命令,但是,选择 CancelAListOfTasks 作为 启动项目。添加本主题中的更改到该项。
若要指定最长时间,在任务被标记为已取消之前,请添加对的调用 CancelAfter 到 startButton_Click,如下面的示例所示。添加标记为星号。
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;
}
运行程序多次验证输出可能显示所有网站、没有网站,或者有些网站的输出。下面的输出是示例。
Length of the downloaded string: 35990.
Length of the downloaded string: 407399.
Length of the downloaded string: 226091.
Downloads canceled.
完整的示例
下面的代码是 MainWindow.xaml.vb 或 MainWindow.xaml.cs 文件的全文该示例的。星号指出在本示例中添加了元素。
通知您必须添加 System.Net.Http的引用。
可以下载项目 Async 示例:优化应用程序。
' 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.
}
请参见
任务
演练:使用 Async 和 Await 访问 Web(C# 和 Visual Basic)
概念
使用 Async 和 Await 的异步编程(C# 和 Visual Basic)
取消一个任务或一组任务(C# 和 Visual Basic)