C++/WinRT throws winrt::hresult_error to represent COM exceptions. The wil framework throws Result­Exception for this purpose. How do they interact, and what diagnostic information do they provide?

C++/WinRT hresult_error uses Ro­Originate­Error to generate a stowed exception which records, among other things, a stack trace for the current thread. This stack trace is stored as part of the hresult_error in the form of a IRestrictedErrorInfo.

The wil framework by default does not use Ro­Originate­Error, so there is no captured stack trace. However, it does capture the file name and line number in the Failure­Info that is stored in the Result­Exception. The Failure­Info also contains information to let you correlate multiple failures and see which ones are manifestations of the same underlying failure.

Here’s a little table of what we have so far:

Things get more complicated if you include wil/result_originate.h: This tells wil to call Ro­Originate­Error before throwing the exception, thereby capturing a stack trace. The stack trace is not explicitly saved in the exception object, however. It is stored in a thread-local object that can be retrieved via GetErrorInfo(), and many parts of the system (including C++/WinRT) understand how to retrieve and preserve this extended information, though determining whether any specific scenario preserves the extended information requires investigation.

So now we have this:

But wait, we’re not done yet. There’s another header file that affects how wil throws exceptions, and that’s wil/cppwinrt.h. This header file enables various C++/WinRT+wil interop features, including exception handling. Exceptions propagated by the C++/WinRT library (for example, by check_hresult()) are filtered through wil, which logs them through its own error logging channel. However, since the file and line number were generated from the __FILE__ and __LINE__ preprocessor symbols captured by the THROW_IF_FAILED macro, C++/WinRT cannot capture file and line number information about the origination point, so you don’t get line number information in your wil trace log. But you still get a stack trace in the hresult_error object.

Exceptions that are thrown explicitly via throw hresult_error() do not go through wil filtering.

But wait, we’re not finished yet. The wil framework alters its behavior if C++/CX is enabled. If so, then it throws a Platform::Exception^ instead of a wil::Result­Exception. The Platform::Exception^ captures a stack trace but not file/line number information.