{"id":112573,"date":"2026-07-30T07:00:00","date_gmt":"2026-07-30T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=112573"},"modified":"2026-07-30T21:58:58","modified_gmt":"2026-07-31T04:58:58","slug":"20260730-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20260730-00\/?p=112573","title":{"rendered":"Making an agile version of a Windows Runtime delegate in C++\/WinRT, part 9"},"content":{"rendered":"

Over half of the time we spent trying to make an agile version of a Windows Runtime delegate in C++\/WinRT was dealing with the case of a delegate that declares non-marshalability<\/a>. But how much does it matter?<\/p>\n

I looked at the three major C++ implementations of the Windows Runtime: C++\/WinRT, C++\/CX, and WRL.<\/p>\n

The C++\/WinRT implementation has an optimization for IAgile\u00adObject<\/code><\/a>, but for objects that aren’t agile, it just goes directly to agile_ref<\/code><\/a> without checking for INoMarshal<\/code>. This means that a delegate that declares non-marshability will always be rejected by C++\/WinRT when used as an event handler.<\/p>\n

The C++\/CX implementation lazy-creates the agile reference to the original delegate when the wrapper is used from a different apartment<\/a>. If the original delegate is non-marshalable, it means that the CO_E_NOT\u00adSUPPORTED<\/code> is produced only when the wrapper is used in a way that requires a marshalable delegate.<\/p>\n

The WRL implementation does not have an optimization for IAgile\u00adObject<\/code>, although it mentions it as a possible optimization<\/a>. It always creates the agile reference eagerly, which means that if the original delegate is non-marshalable, it cannot be added to an agile event source.<\/p>\n

Okay, so let’s summarize in a table.<\/p>\n\n\n\n\n\n\n\n\n
Event source<\/th>\nC++\/WinRT<\/th>\nC++\/CX<\/th>\nWRL<\/th>\nOur version<\/th>\n<\/tr>\n
single-threaded<\/th>\nmulti-threaded<\/th>\n<\/tr>\n
Optimize agile delegates<\/td>\nYes<\/td>\nYes<\/td>\nN\/A<\/td>\nNo<\/td>\nYes<\/td>\n<\/tr>\n
Avoid wrapping agile delegates<\/td>\nYes<\/td>\nNo<\/td>\nNever wraps<\/td>\nNo<\/td>\nYes<\/td>\n<\/tr>\n
Agile reference creation<\/td>\nEager<\/td>\nLazy<\/td>\nNever<\/td>\nEager<\/td>\nEager<\/td>\n<\/tr>\n
Non-marshalable delegates<\/td>\nRejected<\/td>\nAllowed if used
\nnon-agile-ly<\/td>\n		Allowed (always
\nused non-agile-ly)<\/td>\n		Rejected<\/td>\nAllowed if used
\nnon-agile-ly<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Now, maybe you think we are working too hard. (Maybe we are.) In which case you can remove support for whatever cases you feel you don’t need.<\/p>\n","protected":false},"excerpt":{"rendered":"

Maybe it doesn’t matter.<\/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-112573","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"

Maybe it doesn’t matter.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/112573","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=112573"}],"version-history":[{"count":1,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/112573\/revisions"}],"predecessor-version":[{"id":112575,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/112573\/revisions\/112575"}],"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=112573"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=112573"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=112573"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}