Task.Delay() 触发取消应用程序生命周期

Question

我遇到过 Task.Delay() 方法会在 IApplicationLifetime 中触发取消事件的情况。这是代码：

    static async Task Main(string[] args)
    {
        Console.WriteLine("Starting");

       await BuildWebHost(args)
            .RunAsync();
        
        Console.WriteLine("Press any key to exit..");
        Console.ReadKey();
    }

    private static IHost BuildWebHost(string[] args)
    {
        var hostBuilder = new HostBuilder()
            .ConfigureHostConfiguration(config =>
            {
                config.AddEnvironmentVariables();
                config.AddCommandLine(args);
            })
            .ConfigureAppConfiguration((hostContext, configApp) =>
            {
                configApp.SetBasePath(Directory.GetCurrentDirectory());
                configApp.AddCommandLine(args);
            })
            .ConfigureServices((hostContext, services) =>
            {
                services.AddHostedService<BrowserWorkerHostedService>();
                services.AddHostedService<EmailWorkerHostedService>();
            })
            .UseConsoleLifetime();

        return hostBuilder.Build();
    }

这里是异常停止的托管服务：

public class BrowserWorkerHostedService : BackgroundService
{
    private IApplicationLifetime _lifetime;
    private IHost _host;

    public BrowserWorkerHostedService(
        IApplicationLifetime lifetime,
        IHost host)
    {
        this._lifetime = lifetime;
        this._host = host;
    }

    protected override async Task ExecuteAsync(CancellationToken stopToken)
    {
        while (!_lifetime.ApplicationStarted.IsCancellationRequested
            && !_lifetime.ApplicationStopping.IsCancellationRequested
            && !stopToken.IsCancellationRequested)
        {
            Console.WriteLine($"{nameof(BrowserWorkerHostedService)} is working. {DateTime.Now.ToString()}");
            
            //lifetime.StopApplication();
            //await StopAsync(stopToken);

            await Task.Delay(1_000, stopToken);
        }

        Console.WriteLine($"End {nameof(BrowserWorkerHostedService)}");

        await _host.StopAsync(stopToken);
    }
}

public class EmailWorkerHostedService : BackgroundService
{
    private IApplicationLifetime _lifetime;
    private IHost _host = null;

    public EmailWorkerHostedService(
        IApplicationLifetime lifetime,
        IHost host)
    {
        this._lifetime = lifetime;
        this._host = host;
    }

    protected override async Task ExecuteAsync(CancellationToken stopToken)
    {
        while (!_lifetime.ApplicationStarted.IsCancellationRequested
            && !_lifetime.ApplicationStopping.IsCancellationRequested
            && !stopToken.IsCancellationRequested)
        {
            Console.WriteLine($"{nameof(EmailWorkerHostedService)} is working. {DateTime.Now.ToString()}");

            await Task.Delay(1_000, stopToken);
        }

        Console.WriteLine($"End {nameof(EmailWorkerHostedService)}");
        
        await _host.StopAsync(stopToken);
    }
}

我希望我的服务是运行，除非 lifetime.StopApplication() 被触发。但是，托管服务已停止，因为 lifetime.ApplicationStarted.IsCancellationRequested 变量在第二次迭代时设置为 true。尽管从理论上讲，我没有明确中止应用程序的代码。

日志将如下所示：

Starting BrowserWorkerHostedService is working. 09.07.2019 17:03:53

EmailWorkerHostedService is working. 09.07.2019 17:03:53

Application started. Press Ctrl+C to shut down.

Hosting environment: Production

Content root path: xxxx

End EmailWorkerHostedService

End BrowserWorkerHostedService

是否有很好的解释为什么 Task.Delay() 触发 ApplicationStarted 取消事件？

Answer 1

您在滥用 IApplicationLifetime 事件。它们的目的是让您能够将某些操作与它们相关联。例如，您希望仅在应用程序完全启动时才开始监听消息队列。你要这样做：

_applicationLifetime.ApplicationStarted. Register(StartListenMq);

我认为在这里使用 CancellationTokens 不是最好的主意，但它是实现的方式。

当您想取消 HostedService 时，您应该只检查在 ExecuteAsync 方法中收到的令牌。流程将如下所示：

IApplicationLifetime.StopApplication() => 将触发 IApplicationLifetime.ApplicationStopping => 将触发 IHostedService.StopAsync() => 将 stopToken

现在回答您的问题：为什么它会发生在 await Task.Delay()？再看BackgroundService.StartAsync()

    public virtual Task StartAsync(CancellationToken cancellationToken)
    {
        // Store the task we're executing
        _executingTask = ExecuteAsync(_stoppingCts.Token);

        // If the task is completed then return it, this will bubble cancellation and failure to the caller
        if (_executingTask.IsCompleted)
        {
            return _executingTask;
        }

        // Otherwise it's running
        return Task.CompletedTask;
    }

此代码不会等待ExecuteAsync。目前您在代码中调用异步操作 StartAsync() 将继续运行。在它的调用堆栈中的某个地方，它将触发 ApplicationStarted 并且由于您正在收听它，您将得到 _lifetime.ApplicationStarted.IsCancellationRequested = true.

Task.Delay() 触发取消应用程序生命周期

Task.Delay() triggers cancellation of application lifetime

c#

.net-core

asp.net-core-hosted-services