Marek Knápek

Yes, that is correct. We were using C++ and COM and didn’t protect ourselves against accidental exception being thrown across the COM boundary. But it never happened. Except the one case CString::Load (or whatever the function name was) failed because of Office visual style had bug in it.

The main language of the app was English. The user could change it to German, French, Russian, Chinese, Spanish, etc. But sometimes the translation was not complete. Sometimes some plug-in was not translated. We used string IDs to identify various texts. Sometimes the ID pointed to the translated text, sometimes not, thus English version of that text was used instead.

Also I reported the bug in the visual style to Microsoft. They refused to fix it and said something like “don’t do that”.

Win32 MFC desktop application, should be able to run continuously unsupervised for days or weeks. When I log in via RDP it crashed. Yes, just in that moment. In the end I figured it out.

MFC supports multiple visual styles, such as classic Windows, few Office versions and few more. We used feature for loading (possibly translated) strings (resource) from various DLLs. First from a currently active plug-in DLL, if the string was not there, then from a sub-system DLL, if it was not there then from main EXE. Plus there was some logic about translation and fallback to English in case of missing or not up-to-date translation. The cause was in one of the MFC visual styles, it changed a global variable related to that string loading thing for a short duration of time, then changed it back. It was only in one of the styles and there was no need to change it, the code could just load the value and use it as a parameter to a function that needed it. Our code in a different thread failed to load a string and threw an exception. Cross a COM interface boundary. Every COM method is decorated by a bunch of macros our senior engineer didn’t understand and was not willing to learn. One of the macros was noexcept. Thus an exception being thrown cross an noexcept boundary crashed our app.