未捕获异步方法中的异常 [英] Exception in async methods is not caught

问题描述

以下代码无法捕获我通过调用ct.ThrowIfCancellationRequested引发的OperationCancelException.

The following code does not catch my OperationCancelException which is thrown by calling ct.ThrowIfCancellationRequested.

public partial class TitleWindow : Window, IAsyncInitialization
{
    public Task Initialization{get; private set;}
    CancellationTokenSource cts;

    public TitleWindow()
    {
        InitializeComponent();
        cts = new CancellationTokenSource();
    }

    private void Window_Loaded(object sender, RoutedEventArgs e)
    {
        try
        {
            cts.Cancel();
            Initialization = GetCancelExceptionAsync(cts.Token);
        }
        catch (OperationCanceledException)
        {
            Console.WriteLine("Operation canceled!");
        }
    }

    public async Task GetCancelExceptionAsync(CancellationToken ct)
    {
        await Task.Delay(1000);
        ct.ThrowIfCancellationRequested();
    }
}

但是，如果我将我的Window_Loaded方法替换为以下内容(使其异步并等待异步方法的调用)，则会捕获异常.

However if i replace my Window_Loaded method with the following (making it async and await the call of my async method), the exception gets caught.

private async void Window_Loaded(object sender, RoutedEventArgs e)
{
    try
    {
        cts.Cancel();
        await GetCancelExceptionAsync(cts.Token);
    }
    catch (OperationCanceledException)
    {
        Console.WriteLine("Operation canceled!");
    }
}

为什么我的第一种方法行不通?异常是否没有正确传播到正确的同步上下文?

Why is my first approach not working? Is the exception not properly propagated to the correct synchronization context?

我正在尝试使用 Stephen中所述的The Asynchronous Initialization Pattern Clearys博客文章，以便稍后可以等待在构造函数中启动的任务(并且为了使其与我的第二个示例具有可比性，我使用了(async)Window_Loaded事件来等待在那里的方法就像在上一个问题).然后，我想提供一个选项，以取消在构造函数中启动的异步方法，该方法由于不如预期的那样无法正常工作，而目前仍停留在该方法中.

I was trying to use The Asynchronous Initialization Pattern described in Stephen Clearys blog post to be able to later on await a task which was started in a constructor (and in order to make it comparable to my second example I used the (async) Window_Loaded event to await methods there right away, like suggested to me in a previous question). Then I wanted to provide an option to cancel the async method that I started in the constructor, where i am currently stuck because the exception handling does not work as I expected.

使用我的非工作"代码，可以通过将await Initialization放在try-catch块中的某个地方来捕获异常，但是我仍然会收到其他未处理的异常.

With my "non-working" code, I can catch the exception by putting await Initialization in a try-catch block somewhere, but I still get an additional unhandled exception.

如何以一种允许我稍后等待异步方法的方式实现此方法(以确保我不会处理对象的不一致状态)，并且仍然能够取消该长期运行的Task(当然需要返回/设置默认值)?

How do I implement this in a way that allows me to await my async method later on (to ensure that I do not work with an inconsistent state of my object) and still being able to cancel that long-running Task (which would of course need to return/set default values)?

推荐答案

在您的第一个示例中，未捕获到异常，因为在离开try/catch块之前不会发生该异常.如果要在那里捕获它，则需要像在第二个示例中一样完全在其中等待/await. 如果您不等待返回的任务，则该方法将继续执行并在实际发生异常之前离开try/catch块...

In your first example the exception is not caught because it does not occure before leaving the try/catch block. If you want to catch it there you need to wait/await it there exactly like you do in the second example. If you do not await the returned task the method continues execution and leaves the try/catch block before the exception actually occures...

如果要带外"捕获异常，还可以注册到TaskScheduler.UnobservedTaskException(如果任务抛出了一个未被捕获的异常，则调用此事件)以获取所有未捕获的异常或监视任务 Exception 属性.也可以查看这答案.

If you want to catch the exception "out of band" you can also register to TaskScheduler.UnobservedTaskException (this event is called if a task is throwing an exception which is nowhere caught) to get all uncaught exceptions or monitor the tasks Exception property. May also check out THIS answer.

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