取消一个任务或一组任务(C# 和 Visual Basic)
可以设置可以使用取消异步应用程序的按钮,如果您不希望等待其结束。按照本主题中的示例,可以添加取消按钮到下载一个网站目录或网站列表的应用程序。
示例使用 微调异步应用程序(C# 和 Visual Basic) 描述的 UI。
![]() |
---|
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。 |
取消任务
第一个示例关联 取消 按钮与单个下载任务。如果选择按钮,当应用程序下载内容时,下载被取消。
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 CancelATask 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。
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
下面的更改添加一个 取消 按钮到下载网站的应用程序。如果不希望下载或生成该示例,您可以查看在“完整示例”部分的最终产品本主题末尾的。星号指出代码中的更改。
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 StarterCode 作为 启动项目 而不是 CancelATask。
然后添加到该项 MainWindow.xaml.vb 或 MainWindow.xaml.cs 文件进行以下更改。
声明 CancellationTokenSource 变量,cts,在任何方法的范围访问该元素
Class MainWindow ' ***Declare a System.Threading.CancellationTokenSource. Dim cts As CancellationTokenSource
public partial class MainWindow : Window { // ***Declare a System.Threading.CancellationTokenSource. CancellationTokenSource cts;
添加 取消 按钮的下列事件处理程序。事件处理程序使用 CancellationTokenSource.Cancel 方法通知 cts,当用户请求取消。
' ***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() 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(); } }
可以在事件处理程序的以下更改 启动 按钮的,startButton_Click。
实例化 CancellationTokenSource,cts。
' ***Instantiate the CancellationTokenSource. cts = New CancellationTokenSource()
// ***Instantiate the CancellationTokenSource. cts = new CancellationTokenSource();
在对 AccessTheWebAsync的调用,下载一个指定的网站的内容,请发送 ctsCancellationTokenSource.Token 属性作为参数。如果取消请求,Token 属性传播消息。将显示消息的 catch 块,如果用户选择取消下载操作。下面的代码演示更改。
Try ' ***Send a token to carry the message if cancellation is requested. Dim contentLength As Integer = Await AccessTheWebAsync(cts.Token) resultsTextBox.Text &= String.Format(vbCrLf & "Length of the downloaded string: {0}." & vbCrLf, contentLength) ' *** If cancellation is requested, an OperationCanceledException results. Catch ex As OperationCanceledException resultsTextBox.Text &= vbCrLf & "Download canceled." & vbCrLf Catch ex As Exception resultsTextBox.Text &= vbCrLf & "Download failed." & vbCrLf End Try
try { // ***Send a token to carry the message if cancellation is requested. int contentLength = await AccessTheWebAsync(cts.Token); resultsTextBox.Text += String.Format("\r\nLength of the downloaded string: {0}.\r\n", contentLength); } // *** If cancellation is requested, an OperationCanceledException results. catch (OperationCanceledException) { resultsTextBox.Text += "\r\nDownload canceled.\r\n"; } catch (Exception) { resultsTextBox.Text += "\r\nDownload failed.\r\n"; }
在 AccessTheWebAsync,请使用 GetAsync 方法的 HttpClient.GetAsync(String, CancellationToken) 超加载到 HttpClient 类型下载网站的内容。通过 ct,AccessTheWebAsync的 CancellationToken 参数,作为第二个参数。如果用户选择 取消 按钮,标记传播消息。
下面的代码演示在 AccessTheWebAsync的更改。
' ***Provide a parameter for the CancellationToken. Async Function AccessTheWebAsync(ct As CancellationToken) As Task(Of Integer) Dim client As HttpClient = New HttpClient() resultsTextBox.Text &= String.Format(vbCrLf & "Ready to download." & vbCrLf) ' You might need to slow things down to have a chance to cancel. Await Task.Delay(250) ' GetAsync returns a Task(Of HttpResponseMessage). ' ***The ct argument carries the message if the Cancel button is chosen. Dim response As HttpResponseMessage = Await client.GetAsync("https://msdn.microsoft.com/en-us/library/dd470362.aspx", ct) ' Retrieve the website contents from the HttpResponseMessage. Dim urlContents As Byte() = Await response.Content.ReadAsByteArrayAsync() ' The result of the method is the length of the downloaded website. Return urlContents.Length End Function
// ***Provide a parameter for the CancellationToken. async Task<int> AccessTheWebAsync(CancellationToken ct) { HttpClient client = new HttpClient(); resultsTextBox.Text += String.Format("\r\nReady to download.\r\n"); // You might need to slow things down to have a chance to cancel. await Task.Delay(250); // GetAsync returns a Task<HttpResponseMessage>. // ***The ct argument carries the message if the Cancel button is chosen. HttpResponseMessage response = await client.GetAsync("https://msdn.microsoft.com/en-us/library/dd470362.aspx", ct); // Retrieve the website contents from the HttpResponseMessage. byte[] urlContents = await response.Content.ReadAsByteArrayAsync(); // The result of the method is the length of the downloaded website. return urlContents.Length; }
如果不移除程序,它产生下面的输出。
Ready to download. Length of the downloaded string: 158125.
如果选择 取消 按钮,在过程完成下载该内容之前,程序将生成以下输出。
Ready to download. Download canceled.
取消任务列表
可以扩展前面的示例通过将同一 CancellationTokenSource 实例中移除许多任务与每个任务。如果选择 取消 按钮,则取消完成的所有任务。
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 CancelAListOfTasks 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。
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
若要扩展此示例,分步,后面部分的“下载示例的”命令,但是,选择 CancelATask 作为 启动项目。添加对该项目进行以下更改。星号指出在程序中的更改。
添加一个方法以生成 web 地址的。
' ***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
// ***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; }
调用 AccessTheWebAsync的方法。
' ***Call SetUpURLList to make a list of web addresses. Dim urlList As List(Of String) = SetUpURLList()
// ***Call SetUpURLList to make a list of web addresses. List<string> urlList = SetUpURLList();
将 AccessTheWebAsync 的以下循环处理该列表中的每个 web 地址。
' ***Add a loop to process 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
// ***Add a loop to process the list of web addresses. 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); }
由于 AccessTheWebAsync 显示长度,方法不需要返回任何内容。移除返回语句,并更改方法的返回类型为 Task 而不是 Task<TResult>。
Async Function AccessTheWebAsync(ct As CancellationToken) As Task
async Task AccessTheWebAsync(CancellationToken ct)
调用从 startButton_Click 的方法使用语句而不是表达式。
Await AccessTheWebAsync(cts.Token)
await AccessTheWebAsync(cts.Token);
如果不移除程序,它产生下面的输出。
Length of the downloaded string: 35939. Length of the downloaded string: 237682. Length of the downloaded string: 128607. Length of the downloaded string: 158124. Length of the downloaded string: 204890. Length of the downloaded string: 175488. Length of the downloaded string: 145790. Downloads complete.
如果选择 取消 按钮,在下载之前完成后,输出包含长度的下载该移除前完成列表中。
Length of the downloaded string: 35939. Length of the downloaded string: 237682. Length of the downloaded string: 128607. Downloads canceled.
完成示例
以下各节包含每个的代码示例。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。
取消任务示例
下面的代码是移除单个任务的示例的完整 MainWindow.xaml.vb 或 MainWindow.xaml.cs 文件。
' 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
' ***Send a token to carry the message if cancellation is requested.
Dim contentLength As Integer = Await AccessTheWebAsync(cts.Token)
resultsTextBox.Text &=
String.Format(vbCrLf & "Length of the downloaded string: {0}." & vbCrLf, contentLength)
' *** If cancellation is requested, an OperationCanceledException results.
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
' ***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()
End If
End Sub
' ***Provide a parameter for the CancellationToken.
Async Function AccessTheWebAsync(ct As CancellationToken) As Task(Of Integer)
Dim client As HttpClient = New HttpClient()
resultsTextBox.Text &=
String.Format(vbCrLf & "Ready to download." & vbCrLf)
' You might need to slow things down to have a chance to cancel.
Await Task.Delay(250)
' GetAsync returns a Task(Of HttpResponseMessage).
' ***The ct argument carries the message if the Cancel button is chosen.
Dim response As HttpResponseMessage = Await client.GetAsync("https://msdn.microsoft.com/en-us/library/dd470362.aspx", ct)
' Retrieve the website contents from the HttpResponseMessage.
Dim urlContents As Byte() = Await response.Content.ReadAsByteArrayAsync()
' The result of the method is the length of the downloaded website.
Return urlContents.Length
End Function
End Class
' Output for a successful download:
' Ready to download.
' Length of the downloaded string: 158125.
' Or, if you cancel:
' Ready to download.
' Download 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 for System.Threading.
using System.Threading;
namespace CancelATask
{
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
{
// ***Send a token to carry the message if cancellation is requested.
int contentLength = await AccessTheWebAsync(cts.Token);
resultsTextBox.Text +=
String.Format("\r\nLength of the downloaded string: {0}.\r\n", contentLength);
}
// *** If cancellation is requested, an OperationCanceledException results.
catch (OperationCanceledException)
{
resultsTextBox.Text += "\r\nDownload canceled.\r\n";
}
catch (Exception)
{
resultsTextBox.Text += "\r\nDownload failed.\r\n";
}
// ***Set the CancellationTokenSource to null when the download is complete.
cts = null;
}
// ***Add an event handler for the Cancel button.
private void cancelButton_Click(object sender, RoutedEventArgs e)
{
if (cts != null)
{
cts.Cancel();
}
}
// ***Provide a parameter for the CancellationToken.
async Task<int> AccessTheWebAsync(CancellationToken ct)
{
HttpClient client = new HttpClient();
resultsTextBox.Text +=
String.Format("\r\nReady to download.\r\n");
// You might need to slow things down to have a chance to cancel.
await Task.Delay(250);
// GetAsync returns a Task<HttpResponseMessage>.
// ***The ct argument carries the message if the Cancel button is chosen.
HttpResponseMessage response = await client.GetAsync("https://msdn.microsoft.com/en-us/library/dd470362.aspx", ct);
// Retrieve the website contents from the HttpResponseMessage.
byte[] urlContents = await response.Content.ReadAsByteArrayAsync();
// The result of the method is the length of the downloaded website.
return urlContents.Length;
}
}
// Output for a successful download:
// Ready to download.
// Length of the downloaded string: 158125.
// Or, if you cancel:
// Ready to download.
// Download canceled.
}
取消任务的示例列出
下面的代码是取消任务列表示例的完整 MainWindow.xaml.vb 或 MainWindow.xaml.cs 文件。
' 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
' ***AccessTheWebAsync returns a Task, not a Task(Of Integer).
Await AccessTheWebAsync(cts.Token)
' ***Small change in the display lines.
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
' 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()
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()
' ***Add a loop to process 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
' Output if you do not choose to cancel:
' Length of the downloaded string: 35939.
' Length of the downloaded string: 237682.
' Length of the downloaded string: 128607.
' Length of the downloaded string: 158124.
' Length of the downloaded string: 204890.
' Length of the downloaded string: 175488.
' Length of the downloaded string: 145790.
' Downloads complete.
' Sample output if you choose to cancel:
' Length of the downloaded string: 35939.
' Length of the downloaded string: 237682.
' Length of the downloaded string: 128607.
' 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 for System.Threading.
using System.Threading;
namespace CancelAListOfTasks
{
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);
// ***Small change in the display lines.
resultsTextBox.Text += "\r\nDownloads complete.";
}
catch (OperationCanceledException)
{
resultsTextBox.Text += "\r\nDownloads canceled.";
}
catch (Exception)
{
resultsTextBox.Text += "\r\nDownloads failed.";
}
// Set the CancellationTokenSource to null when the download is complete.
cts = null;
}
// Add an event handler for the Cancel button.
private void cancelButton_Click(object sender, RoutedEventArgs e)
{
if (cts != null)
{
cts.Cancel();
}
}
// Provide a parameter for the CancellationToken.
// ***Change the return type to Task because the method has no return statement.
async Task AccessTheWebAsync(CancellationToken ct)
{
// Declare an HttpClient object.
HttpClient client = new HttpClient();
// ***Call SetUpURLList to make a list of web addresses.
List<string> urlList = SetUpURLList();
// ***Add a loop to process the list of web addresses.
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);
}
}
// ***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;
}
}
// Output if you do not choose to cancel:
//Length of the downloaded string: 35939.
//Length of the downloaded string: 237682.
//Length of the downloaded string: 128607.
//Length of the downloaded string: 158124.
//Length of the downloaded string: 204890.
//Length of the downloaded string: 175488.
//Length of the downloaded string: 145790.
//Downloads complete.
// Sample output if you choose to cancel:
//Length of the downloaded string: 35939.
//Length of the downloaded string: 237682.
//Length of the downloaded string: 128607.
//Downloads canceled.
}
请参见
参考
概念
使用 Async 和 Await 的异步编程(C# 和 Visual Basic)