Windows universal C++ concurrency - concurrency

Question

I have the following pattern:
task<Param1^> MyClass::MethodA(String^ s)
{
return create_task([this, s]
{
....
IAsyncOperation<Param1^>^ result = SystemMethod1ReturningParam1(...);
return result;
});
}
IAsyncOperation<Param2^>^ MyClass::MethodB(String^ s)
{
return create_async([this, s]
{
return MethodA(s).then([this](task<Param1^> t)
{
Param1^ p1 = t.get();
....
Param2^ result = SystemMethod2ReturningParam2(p1);
return result ;
});
});
}
I call public MethodB which calls private MethodA. Param1 is standard enum that I cannot change. MethodA create_task my need to return error condition. I cannot return null. MethodB also needs return error condition.
What's the best way to return errors from both methods?
SystemMethod1ReturningParam1 and SystemMethod2ReturningParam2 which are framework system methods can throw exceptions. Where do I catch them?

Answer 1

If you are worried about exceptions, you should always use task-based continuations and wrap the call to task.get() in a try/catch.
Note that the code below (and other C++ code) throws std::exception, but if you are interfacing with WinRT components and you want the exception to propagate, you must re-throw as a Platform::Exception value.
concurrency::task<int> throw_async()
{
return concurrency::create_task([]
{
throw std::exception("oops");
return -1;
});
}
void test_value()
{
// This will crash inside PPL runtime due to unobserved exception
throw_async().then([](int x)
{
OutputDebugString(L"You'll never see this\r\n");
});
}
void test_task()
{
// This will handle the exception correctly
throw_async().then([](concurrency::task<int>& t)
{
OutputDebugString(L"You'll see this\r\n");
try
{
auto val = t.get();
OutputDebugString(L"You'll never see this\r\n");
}
catch (std::exception& ex)
{
auto error = ex.what(); // "oops"
// Need to re-throw if this crosses the WinRT ABI boundary.
// Conversion of 'error' to Platform::String^ omitted for brevity
throw ref new Platform::FailureException(/* error */);
}
});
}