Cancelling a Windows Runtime asynchronous operation, part 5: C++/WinRT

This post has been republished via RSS; it originally appeared at: Microsoft Developer Blogs.

Last time, we looked at how task cancellation is projected in C++/CX with PPL and co_await with IAsyncAction^ and IAsyncOperation^ objects. Our next investigation is C++/WinRT. Canceling the operation after a delay is slightly tricky because we need to pass the pickerOp as a parameter to a captureless lambda, rather than capturing it into the lambda. We discussed the reason for this some time ago. The exception that comes out of awaiting for a canceled task is sometimes an hresult_canceled, and sometimes an hresult_illegal_method_call. The reason is that C++/WinRT defers to the IAsyncAction/IAsyncOperation to decide what exception to raise.¹ In other words, C++/WinRT just takes the ABI result and propagates it. It doesn't try to impose its will upon the result. You can see this in the await_resume for asynchronous operations: Whatever exception comes out of Get­Results() is the exception that comes out of the co_await. Next time, we'll look at what happens if the IAsyncAction or IAsyncOperation was implemented by the C++/WinRT library itself. Bonus chatter: This entire article is already obsolete. We'll learn more about it when this series wraps up. ¹ Therefore, in principle, it could be utterly anything since it's up to the ABI to generate the HRESULT that turns into an exception at the projection.