取消任务列表
如果不想等待异步控制台应用程序完成，可以取消该应用程序。 通过遵循本主题中的示例，可将取消添加到下载网站内容的应用程序。 可通过将 CancellationTokenSource 实例与每个任务进行关联来取消多个任务。 如果选择 Enter 键，则将取消所有尚未完成的任务。
本教程涉及：
- 创建 .NET 控制台应用程序
- 编写支持取消的异步应用程序
- 演示发出取消信号
必备条件
本教程需要的内容如下：
- .NET 5 或更高版本的 SDK
- 集成开发环境 (IDE)
创建示例应用程序
创建新的 .NET Core 控制台应用程序。 可通过使用 dotnet new console 命令或从 Visual Studio 进行创建。 在你最喜欢的编辑器中打开 Program.cs 文件。
替换 using 语句
将现有 using 语句替换为以下声明：
using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Net.Http;
using System.Threading;
using System.Threading.Tasks;
添加字段
在 Program 类定义中，添加以下三个字段：
static readonly CancellationTokenSource s_cts = new CancellationTokenSource();
static readonly HttpClient s_client = new HttpClient
{
MaxResponseContentBufferSize = 1_000_000
};
static readonly IEnumerable<string> s_urlList = new string[]
{
"https://learn.microsoft.com",
"https://learn.microsoft.com/aspnet/core",
"https://learn.microsoft.com/azure",
"https://learn.microsoft.com/azure/devops",
"https://learn.microsoft.com/dotnet",
"https://learn.microsoft.com/dynamics365",
"https://learn.microsoft.com/education",
"https://learn.microsoft.com/enterprise-mobility-security",
"https://learn.microsoft.com/gaming",
"https://learn.microsoft.com/graph",
"https://learn.microsoft.com/microsoft-365",
"https://learn.microsoft.com/office",
"https://learn.microsoft.com/powershell",
"https://learn.microsoft.com/sql",
"https://learn.microsoft.com/surface",
"https://learn.microsoft.com/system-center",
"https://learn.microsoft.com/visualstudio",
"https://learn.microsoft.com/windows",
"https://learn.microsoft.com/maui"
};
CancellationTokenSource 用于向 CancellationToken 发出请求取消的信号。 HttpClient 公开发送 HTTP 请求和接收 HTTP 响应的能力。 s_urlList 包括应用程序计划处理的所有 URL。
更新应用程序入口点
控制台应用程序的主入口点是 Main 方法。 将现有方法替换为以下内容：
static async Task Main()
{
Console.WriteLine("Application started.");
Console.WriteLine("Press the ENTER key to cancel...\n");
Task cancelTask = Task.Run(() =>
{
while (Console.ReadKey().Key != ConsoleKey.Enter)
{
Console.WriteLine("Press the ENTER key to cancel...");
}
Console.WriteLine("\nENTER key pressed: cancelling downloads.\n");
s_cts.Cancel();
});
Task sumPageSizesTask = SumPageSizesAsync();
Task finishedTask = await Task.WhenAny(new[] { cancelTask, sumPageSizesTask });
if (finishedTask == cancelTask)
{
// wait for the cancellation to take place:
try
{
await sumPageSizesTask;
Console.WriteLine("Download task completed before cancel request was processed.");
}
catch (TaskCanceledException)
{
Console.WriteLine("Download task has been cancelled.");
}
}
Console.WriteLine("Application ending.");
}
目前将已更新的 Main 方法视为异步 main 方法，这允许将异步入口点引入可执行文件中。 将几条说明性消息写入控制台，然后声明名为 cancelTask 的 Task 实例，这将读取控制台密钥笔画。 如果按 Enter，则会调用 CancellationTokenSource.Cancel()。 这将发出取消信号。 下一步，从 SumPageSizesAsync 方法分配 sumPageSizesTask 变量。 然后，将这两个任务传递到 Task.WhenAny(Task[])，这会在完成两个任务中的任意一个时继续。
下一个代码块可确保在取消得到处理之前不会退出应用程序。 如果要完成的第一个任务是 cancelTask，则等待 sumPageSizeTask。 如果已取消，则等待时会引发 System.Threading.Tasks.TaskCanceledException。 块捕获该异常，并输出消息。
创建异步总和页面大小方法
在 Main 方法下，添加 SumPageSizesAsync 方法：
static async Task SumPageSizesAsync()
{
var stopwatch = Stopwatch.StartNew();
int total = 0;
foreach (string url in s_urlList)
{
int contentLength = await ProcessUrlAsync(url, s_client, s_cts.Token);
total += contentLength;
}
stopwatch.Stop();
Console.WriteLine($"\nTotal bytes returned: {total:#,#}");
Console.WriteLine($"Elapsed time: {stopwatch.Elapsed}\n");
}
该方法从实例化和启动 Stopwatch 开始。 然后会在 s_urlList 的每个 URL 中进行循环，并调用 ProcessUrlAsync。 对于每次迭代，s_cts.Token 都会传递到 ProcessUrlAsync 方法中，并且代码将返回 Task<TResult>，其中 TResult 是一个整数：
int total = 0;
foreach (string url in s_urlList)
{
int contentLength = await ProcessUrlAsync(url, s_client, s_cts.Token);
total += contentLength;
}
添加进程方法
在 SumPageSizesAsync 方法下添加以下 ProcessUrlAsync 方法：
static async Task<int> ProcessUrlAsync(string url, HttpClient client, CancellationToken token)
{
HttpResponseMessage response = await client.GetAsync(url, token);
byte[] content = await response.Content.ReadAsByteArrayAsync(token);
Console.WriteLine($"{url,-60} {content.Length,10:#,#}");
return content.Length;
}
对于任何给定的 URL，该方法都将使用提供的 client 实例以 byte[] 形式来获取响应。 CancellationToken 实例会传递到 HttpClient.GetAsync(String, CancellationToken) 和 HttpContent.ReadAsByteArrayAsync() 方法中。 token 用于注册请求取消。 将 URL 和长度写入控制台后会返回该长度。
示例应用程序输出
Application started.
Press the ENTER key to cancel...
https://learn.microsoft.com 37,357
https://learn.microsoft.com/aspnet/core 85,589
https://learn.microsoft.com/azure 398,939
https://learn.microsoft.com/azure/devops 73,663
https://learn.microsoft.com/dotnet 67,452
https://learn.microsoft.com/dynamics365 48,582
https://learn.microsoft.com/education 22,924
ENTER key pressed: cancelling downloads.
Application ending.
完整示例
下列代码是示例的 Program.cs 文件的完整文本。
using System.Diagnostics;
class Program
{
static readonly CancellationTokenSource s_cts = new CancellationTokenSource();
static readonly HttpClient s_client = new HttpClient
{
MaxResponseContentBufferSize = 1_000_000
};
static readonly IEnumerable<string> s_urlList = new string[]
{
"https://learn.microsoft.com",
"https://learn.microsoft.com/aspnet/core",
"https://learn.microsoft.com/azure",
"https://learn.microsoft.com/azure/devops",
"https://learn.microsoft.com/dotnet",
"https://learn.microsoft.com/dynamics365",
"https://learn.microsoft.com/education",
"https://learn.microsoft.com/enterprise-mobility-security",
"https://learn.microsoft.com/gaming",
"https://learn.microsoft.com/graph",
"https://learn.microsoft.com/microsoft-365",
"https://learn.microsoft.com/office",
"https://learn.microsoft.com/powershell",
"https://learn.microsoft.com/sql",
"https://learn.microsoft.com/surface",
"https://learn.microsoft.com/system-center",
"https://learn.microsoft.com/visualstudio",
"https://learn.microsoft.com/windows",
"https://learn.microsoft.com/maui"
};
static async Task Main()
{
Console.WriteLine("Application started.");
Console.WriteLine("Press the ENTER key to cancel...\n");
Task cancelTask = Task.Run(() =>
{
while (Console.ReadKey().Key != ConsoleKey.Enter)
{
Console.WriteLine("Press the ENTER key to cancel...");
}
Console.WriteLine("\nENTER key pressed: cancelling downloads.\n");
s_cts.Cancel();
});
Task sumPageSizesTask = SumPageSizesAsync();
Task finishedTask = await Task.WhenAny(new[] { cancelTask, sumPageSizesTask });
if (finishedTask == cancelTask)
{
// wait for the cancellation to take place:
try
{
await sumPageSizesTask;
Console.WriteLine("Download task completed before cancel request was processed.");
}
catch (OperationCanceledException)
{
Console.WriteLine("Download task has been cancelled.");
}
}
Console.WriteLine("Application ending.");
}
static async Task SumPageSizesAsync()
{
var stopwatch = Stopwatch.StartNew();
int total = 0;
foreach (string url in s_urlList)
{
int contentLength = await ProcessUrlAsync(url, s_client, s_cts.Token);
total += contentLength;
}
stopwatch.Stop();
Console.WriteLine($"\nTotal bytes returned: {total:#,#}");
Console.WriteLine($"Elapsed time: {stopwatch.Elapsed}\n");
}
static async Task<int> ProcessUrlAsync(string url, HttpClient client, CancellationToken token)
{
HttpResponseMessage response = await client.GetAsync(url, token);
byte[] content = await response.Content.ReadAsByteArrayAsync(token);
Console.WriteLine($"{url,-60} {content.Length,10:#,#}");
return content.Length;
}
}
