Question

当您await Task时，默认情况下继续在同一个线程上运行。您实际需要的唯一时间是，如果您在UI线程上，并且延续也需要在UI线程上运行。

您可以使用ConfigureAwait来控制此项，例如：

await SomeMethodAsync().ConfigureAwait(false);

...这对于从不需要在那里运行的UI线程卸载工作非常有用。（但请参阅Stephen Cleary的评论。）

现在考虑这段代码：

try
{
    await ThrowingMethodAsync().ConfigureAwait(false);
}
catch (Exception e)
{
    // Which thread am I on now?
}

这是怎么回事？

try
{
    await NonThrowingMethodAsync().ConfigureAwait(false);

    // At this point we *may* be on a different thread

    await ThrowingMethodAsync().ConfigureAwait(false);
}
catch (Exception e)
{
    // Which thread am I on now?
}

Answer 1

除非没有异常，否则异常将在继续发生的任何线程上。

try
{
    await ThrowingMethodAsync().ConfigureAwait(false);
}
catch (Exception e)
{
    // Which thread am I on now?
    //A: Likely a Thread pool thread unless ThrowingMethodAsync threw 
    //   synchronously (without a await happening first) then it would be on the same
    //   thread that the function was called on.
}

try
{
    await NonThrowingMethodAsync().ConfigureAwait(false);

    // At this point we *may* be on a different thread

    await ThrowingMethodAsync().ConfigureAwait(false);
}
catch (Exception e)
{
    // Which thread am I on now?
    //A: Likely a Thread pool thread unless ThrowingMethodAsync threw 
    //   synchronously (without a await happening first) then it would be on the same
    //   thread that the function was called on.
}

更清晰一点：

private async Task ThrowingMethodAsync()
{
    throw new Exception(); //This would cause the exception to be thrown and observed on 
                           // the calling thread even if ConfigureAwait(false) was used.
                           // on the calling method.
}

private async Task ThrowingMethodAsync2()
{
    await Task.Delay(1000);
    throw new Exception(); //This would cause the exception to be thrown on the SynchronizationContext
                           // thread (UI) but observed on the thread determined by ConfigureAwait
                           // being true or false in the calling method.
}

private async Task ThrowingMethodAsync3()
{
    await Task.Delay(1000).ConfigureAwait(false);
    throw new Exception(); //This would cause the exception to be thrown on the threadpool
                           // thread but observed on the thread determined by ConfigureAwait
                           // being true or false in the calling method.
}

ConfigureAwait：处理异常的线程在哪？

1 个答案: