async/await with ConfigureAwait的continueOnCapturedContext参数和SynchronizationContext用于异步延续

Question

我想先把代码放在一边,然后解释一下情况并根据这个问题提出我的问题:

public partial class MainWindow : Window {

    public MainWindow() {
        InitializeComponent();
    }

    private async void Button_Click_2(object sender, RoutedEventArgs e) {

        var result = await GetValuesAsync();
        Foo.Text += result;
    }

    public async Task<string> GetValuesAsync() {           

        using (var httpClient = new HttpClient()) {

            var response = await httpClient
                .GetAsync("http://www.google.com")
                .ConfigureAwait(continueOnCapturedContext: false);


            // This is the continuation for the httpClient.GetAsync method.
            // We shouldn't get back to sync context here
            // Cuz the continueOnCapturedContext is set to *false*
            // for the Task which is returned from httpClient.GetAsync method
            var html = await GetStringAsync();

            // This is the continuation for the GetStringAsync method.
            // Should I get back to sync context here?
            // Cuz the continueOnCapturedContext is set to *true*
            // for the Task which is returned from GetStringAsync 

            // However, GetStringAsync may be executed in another thread
            // which has no knowledge for the sync context 
            // because the continueOnCapturedContext is set to *false*
            // for the Task which is returned from httpClient.GetAsync method.

            // But, on the other hand, GetStringAsync method also has a 
            // chance to be executed in the UI thread but we shouldn't be
            // relying on that. 
            html += "Hey...";
            Foo.Text = html;

            return html;
        }
    }

    public async Task<string> GetStringAsync() {

        await Task.Delay(1000);
        return "Done...";
    }
}

这是一个相当简单的WPF示例,它运行在.NET 4.5上,可能没有多大意义,但这可以帮助我解释我的情况.

我在屏幕上有一个按钮,它有一个异步点击事件.查看GetValuesAsync代码时,您将看到await关键字的使用两次.在第一次使用时,我将方法的continueOnCapturedContext参数设置Task.ConfigureAwait为false.所以,这表明我不一定希望我的继续在内部执行SynchronizationContext.Current.到现在为止还挺好.

在第二次await使用(使用GetStringAsync方法)时,我没有调用该ConfigureAwait方法.所以,我基本上表示我想回到当前的同步上下文继续该GetStringAsync方法.所以,正如你所看到的,我尝试TextBlock.Text在continuation中设置(属于UI线程)属性.

当我运行应用程序并单击按钮时,我得到一个异常,给我以下消息:

调用线程无法访问此对象,因为另一个线程拥有它.

起初,这对我没有任何意义,我认为我发现了一个错误,但后来,我意识到GetStringAsync可能会在另一个线程(很可能)中执行,这个线程与UI线程不同,并且不知道同步上下文,因为continueOnCapturedContext设置false为Task从httpClient.GetAsync方法返回的.

这是这种情况吗？此外,在这种情况下,是否有机GetStringAsync会将方法发回UI线程,可以在UI线程内执行httpClient.GetAsync方法延续？

我在代码中也有一些注释.鉴于我的问题和代码中的注释,我在这里遗漏了什么吗？

Answer 1

当你调用ConfigureAwait(false),该方法的其余部分将在一个线程池线程执行,除非在Task你await荷兰国际集团已经完成.

因为GetAsync几乎肯定会以异步方式运行,所以我希望GetStringAsync在线程池线程上运行.

public async Task<string> GetValuesAsync() {           

    using (var httpClient = new HttpClient()) {

        var response = await httpClient
            .GetAsync("http://www.google.com")
            .ConfigureAwait(continueOnCapturedContext: false);

        // And now we're on the thread pool thread.

        // This "await" will capture the current SynchronizationContext...
        var html = await GetStringAsync();
        // ... and resume it here.

        // But it's not the UI SynchronizationContext.
        // It's the ThreadPool SynchronizationContext.
        // So we're back on a thread pool thread here.

        // So this will raise an exception.
        html += "Hey...";
        Foo.Text = html;

        return html;
    }
}

此外,在这种情况下,是否有机会将GetStringAsync方法发布回UI线程,可以在UI线程内执行httpClient.GetAsync方法延续？

GetStringAsync在UI线程上运行的唯一方法是GetAsync在实际await编辑之前完成.极不可能.

出于这个原因,我更喜欢使用ConfigureAwait(false)的每 await一次上下文不再需要.