ConfigureAwait: in which thread is the exception handled?

Question

When you are awaita Task, the continuation by default is executed in a single thread. The only time you ever really need this is that you are running in a user interface thread, and the sequel must also be run in the user interface thread.

You can control this using ConfigureAwait, for example:

await SomeMethodAsync().ConfigureAwait(false);

... which may be useful for offloading work from a user interface thread that does not need to be run there. (But see Stephen Cleary's comment below.)

Now consider this bit of code:

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

And how about this?

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

, , .

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.
}

:

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.
}