{"id":106475,"date":"2022-04-13T07:00:00","date_gmt":"2022-04-13T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=106475"},"modified":"2022-08-15T18:32:34","modified_gmt":"2022-08-16T01:32:34","slug":"20220413-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20220413-00\/?p=106475","title":{"rendered":"A survey of how implementations of Windows Runtime events deal with errors"},"content":{"rendered":"

There are three commonly-seen implementations of Windows Runtime events (one of which with four variants), and they deal with errors differently.<\/p>\n\n\n\n\n\n\n\n\n\n\n
Class<\/th>\nDisconnect
\nerrors<\/th>\n		Behavior for other errors<\/th>\n<\/tr>\n
RPC_E_DISCONNECTED<\/code> <\/span><\/td>\n RPC_S_SERVER_UNAVAILABLE<\/code> <\/span><\/td>\n RPC_E_SERVER_DIED<\/code> <\/span><\/td>\n RPC_E_SERVER_DIED_DNE<\/code> <\/span><\/td>\n JSCRIPT_E_CANTEXECUTE<\/code> <\/span><\/td>\n<\/tr>\n
\n
WRL::EventSource<<\/code><\/div>\n
FireAll><\/code><\/div>\n<\/td>\n
\u2022<\/td>\n\u2022<\/td>\n <\/td>\n <\/td>\n\u2022<\/td>\nIgnore errors.<\/td>\n<\/tr>\n
\n
WRL::EventSource<<\/code><\/div>\n
StopOnFirstError><\/code><\/div>\n<\/td>\n
\u2022<\/td>\n\u2022<\/td>\n <\/td>\n <\/td>\n\u2022<\/td>\nStop on first handler to return an error and return that error.<\/td>\n<\/tr>\n
\n
WRL::EventSource<<\/code><\/div>\n
ReportUnhandled-<\/code><\/div>\n
OnFirstError-<\/code><\/div>\n
WithWin8Quirk><\/code><\/div>\n<\/td>\n
\u2022<\/td>\n\u2022<\/td>\n\u2022<\/td>\n\u2022<\/td>\n\u2022<\/td>\nFor Win8 apps: Ignore errors.
\nOtherwise: Stop on first handler to return an error. Raise the CoreApplication.Unhandled\u00adError\u00adDetected<\/code><\/a> event. If handled, then return S_OK<\/code>, else return the original error.<\/td>\n<\/tr>\n
\n
WRL::EventSource<<\/code><\/div>\n
StopOnFirstError-<\/code><\/div>\n
WithWin8Quirk><\/code><\/div>\n<\/td>\n
\u2022<\/td>\n\u2022<\/td>\n\u2022<\/td>\n\u2022<\/td>\n\u2022<\/td>\nFor Win8 apps: Ignore errors.
\nOtherwise: Stop on first handler to return an error and return that error.<\/td>\n<\/tr>\n
C++\/CX event<\/code><\/td>\n\u2022<\/td>\n\u2022<\/td>\n <\/td>\n <\/td>\n\u2022<\/td>\nStop on first handler to return an error and return that error.<\/td>\n<\/tr>\n
winrt::event<\/code><\/td>\n\u2022<\/td>\n\u2022<\/td>\n <\/td>\n <\/td>\n\u2022<\/td>\nIgnore errors.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

The code for WRL::Event\u00adSource<\/code> is in wrl\/event.h<\/code>. The code for C++\/CX event<\/code> is in vccorlib.h<\/code>, where it is called Platform::Event\u00adSource<\/code>. And the code for C++\/WinRT winrt::event<\/code> is in winrt\/base.h<\/code>. I built the above table by reading the code. (You can do it too.)<\/p>\n

For WRL, the default for Event\u00adSource<\/code> is Fire\u00adAll<\/code> for third-party code, and Report\u00adUnhandled\u00adOn\u00adFirst\u00adError\u00adWith\u00adWin8\u00adQuirk<\/code> for Windows operating system code.<\/p>\n

In all cases, a handler that returns one of the recognized Disconnect error codes is removed as an event handler and therefore will not receive any future events. (However, it will not be considered an error to decide whether or not to stop calling further handlers.) To avoid accidentally propagating one of these secret error codes<\/a> out of your event handler, wrap your event handler in a try<\/code>\u2026catch (...)<\/code> or mark your event handlers as noexcept<\/code>.<\/p>\n

In fact, given that different event sources deal with errors differently, you probably should simply avoid returning errors from your event handler. Wrap the event handler in a try<\/code>\u2026catch (...)<\/code> so you can decide what to do in case of an error, or mark the event handler as noexcept<\/code> to say that all errors are fatal to the application.<\/p>\n","protected":false},"excerpt":{"rendered":"

All happy event delegates are alike; each unhappy event delegate is unhappy in its own way.<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[25],"class_list":["post-106475","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"

All happy event delegates are alike; each unhappy event delegate is unhappy in its own way.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/106475","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=106475"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/106475\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=106475"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=106475"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=106475"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}