Last time, we looked at how task cancellation is projected in C++/CX with PPL and co_await with tasks. But PPL also supports awaiting directly on IAsyncAction^ and IAsyncOperation^ objects. Let’s try it.

auto picker = ref new FileOpenPicker();
picker->FileTypeFilter.Append(L".txt");

auto pickerOp = picker->PickSingleFileAsync();
cancellation_token_source cts;
call<bool> do_cancel([pickerOp](bool) { pickerOp.Cancel(); });
timer<bool> delayed_cancel(3000U, false, &do_cancel);
delayed_cancel.start();

StorageFile^ file;
try {
    file = co_await pickerOp;
} catch (OperationCanceledException^) {
    file = nullptr;
}

if (file != nullptr) {
    DoSomething(file);
}

Observe that awaiting directly on an IAsyncAction^ and IAsyncOperation^ object throws a different exception from awaiting on a task.

You can see this in the await_resume for the IAsyncInfo^ awaiter ind in pplawait.h:

template <typename _TaskTy, typename _HandlerTy>
struct _IAsync_awaiter {
    ...

    auto await_resume() {
        _VerifyStateForResultsCall(_Task->Status);
        return _Task->GetResults();
    }
};

void _VerifyStateForResultsCall(
    Windows::Foundation::AsyncStatus _Status)
{
    if (_Status == AsyncStatus::Canceled) {
        throw ::Platform::Exception::CreateException(E_ABORT);
    }
}

The PPL framework checks whether the status of the async operation is Canceled, and if so, it throws E_ABORT, which is represented in C++/CX as Operation­Canceled­Exception.

So far, all of the cancellation exceptions have generated by the framework. That’ll change soon. Next time, we’ll look at C++/WinRT.