A customer had what they thought was a problem with C++\/WinRT coroutines. “We are catching and handling all exceptions, but sometimes our program still crashes with an unhandled exception.”<\/p>\n
winrt::fire_and_forget MyClass::DoSomethingAsync()\r\n{\r\n auto lifetime = get_strong();\r\n try {\r\n auto name = co_await m_user.GetNameAsync();\r\n m_label.Text(name);\r\n } catch (...) {\r\n m_label.Text(L\"unknown\");\r\n }\r\n}\r\n<\/pre>\nThe C++\/WinRT knowledge you need to know here is that a coroutine that returns fire_And you can see the unhandled exception in the stack trace:<\/p>\n
KERNELBASE!RaiseFailFastException+0x15c\r\ncombase!RoFailFastWithErrorContextInternal2+0x43a\r\ncontoso!winrt::terminate+0x28\r\ncontoso!std::experimental::coroutine_traits<winrt::fire_and_forget>::promise_type::unhandled_exception+0x9\r\ncontoso!`MyClass::DoSomethingAsync$_ResumeCoro$1'::`1'::catch$2+0x1f\r\nVCRUNTIME140_1+0x1080\r\nVCRUNTIME140_1!_NLG_Return2+0x1555\r\nntdll!RcFrameConsolidation+0x6\r\ncontoso!MyClass::DoSomethingAsync$_ResumeCoro$1+0xa9\r\ncontoso!std::experimental::coroutine_handle<void>::resume+0x5\r\ncontoso!std::experimental::coroutine_handle<void>::operator()+0x5\r\ncontoso!winrt::impl::resume_apartment_callback+0x9\r\n...\r\n<\/pre>\nBut we did a catch (...)<\/code>, which catches all exceptions. How did we get an unhandled exception?<\/p>\nThe catch (...)<\/code> catches all exceptions thrown in the preceding try<\/code> block. But that’s not where the unhandled exception is coming from.<\/p>\nThe coroutine was just a red herring. Let’s take coroutines out of the picture and make this a non-coroutine function.<\/p>\n
void MyClass::DoSomething()\r\n{\r\n try {\r\n auto name = m_user.GetName();\r\n m_label.Text(name);\r\n } catch (...) {\r\n m_label.Text(L\"unknown\");\r\n }\r\n}\r\n<\/pre>\nIf an exception occurs in the try<\/code> block, it is caught and handled by the catch (...)<\/code> block. But if an exception occurs at the m_label.Text(L\"unknown\")<\/code>, there’s nobody around to catch the second<\/i> exception<\/a>.<\/p>\nYou thought you caught the exception, but instead you merely caught an<\/i> exception. If you don’t want any exceptions to escape your function, you have to play Pok\u00e9mon and catch them all<\/a>.<\/p>\nwinrt::fire_and_forget MyClass::DoSomethingAsync()\r\n{\r\n auto lifetime = get_strong();\r\n try {\r\n auto name = co_await m_user.GetNameAsync();\r\n m_label.Text(name);\r\n } catch (...) {\r\n try {<\/span>\r\n m_label.Text(L\"unknown\");\r\n } catch (...) { <\/span>\r\n LOG_CAUGHT_EXCEPTION();<\/span>\r\n } <\/span>\r\n }\r\n}\r\n<\/pre>\n