{"id":106778,"date":"2022-06-23T07:00:00","date_gmt":"2022-06-23T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=106778"},"modified":"2022-06-23T06:22:50","modified_gmt":"2022-06-23T13:22:50","slug":"20220623-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20220623-00\/?p=106778","title":{"rendered":"Writing a marshal-by-value marshaler, part 2"},"content":{"rendered":"

When we last left our marshal-by-value marshaler, we had it marshal by value for all cases that didn’t leave the computer. But it turns out that this is actually more work than necessary: If the object is immutable (as marshal-by-value objects generally are) and is staying inside the same process, then we can just use the free-threaded marshaler, assuming the immutable state does not have thread affinity.<\/p>\n

The idea here is that immutable objects are pretty much free-threaded already, seeing as they have no mutable state that requires synchronization. So there’s no need to create a copy when marshaling between apartments within the same process; we can just have all the apartments access the object directly.<\/p>\n

Old and busted:<\/p>\n\n\n\n\n\n
client<\/td>\n\u00a0<\/td>\n\u00a0<\/td>\nclient<\/td>\n<\/tr>\n
\u2193<\/td>\n\u2193<\/td>\n<\/tr>\n
data<\/span><\/td>\ndata<\/span><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

New hotness:<\/p>\n\n\n\n\n\n
client<\/td>\n\u00a0<\/td>\n <\/td>\nclient<\/td>\n<\/tr>\n
\u2193<\/td>\n\u2199\ufe0e<\/td>\n <\/td>\n<\/tr>\n
data<\/span><\/td>\n <\/td>\n <\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Clients within the same process don’t need a separate copy of the object. They can just share the object.<\/p>\n

Of course, clients from other processes can take advantage of having their own copy, since that would avoid inter-process calls to access the data. However, we permit this only for other processes on the same computer, because processes on other computers may not have our custom marshaler installed.<\/p>\n

This means that we want to treat each of the three categories differently:<\/p>\n\n\n\n\n\n\n
Category<\/th>\nDesired marshaler<\/th>\n<\/tr>\n
Same-process<\/td>\nFree-threaded marshaler<\/td>\n<\/tr>\n
Same-machine<\/td>\nMarshal-by-value marshaler<\/td>\n<\/tr>\n
Cross-machine<\/td>\nStandard (marshal-by-reference) marshaler<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

It sounds like we’re going to have three cases to deal with, but we can collapse it down to two by realizing that the free-threaded marshaler falls back to the standard marshaler when the destination context is cross-machine.<\/p>\n

bool ShouldMarshalByValue(DWORD dwDestContext)\r\n{\r\n    return \/\/ dwDestContext == MSHCTX_CROSSCTX || dwDestContext == MSHCTX_INPROC ||<\/span><\/span>\r\n           dwDestContext == MSHCTX_LOCAL || dwDestContext == MSHCTX_NOSHAREDMEM;\r\n}\r\n<\/pre>\n
We no longer marshal by value in the same-process case, letting the free-threaded marshaler take care of that and the cross-machine case.<\/p>\n
None of our marshal-by-value business logic needs to change. What changes is our fallback marshaler. Instead of falling back to the standard marshaler, we fall back to the free-threaded marshaler.<\/p>\n
    STDMETHODIMP GetUnmarshalClass(\r\n        REFIID riid, void* pv, DWORD dwDestContext,\r\n        void* pvDestContext, DWORD mshlflags,\r\n        CLSID *clsid)\r\n    {\r\n        if (ShouldMarshalByValue(dwDestContext)) {\r\n            *clsid = CLSID_MyClass;\r\n            return S_OK;\r\n        }\r\n\r\n        ComPtr<IMarshal> marshal;\r\n        RETURN_IF_FAILED(CoCreateFreeThreadedMarshaler(nullptr, &marshal)<\/span>);\r\n        RETURN_IF_FAILED(marshal->GetUnmarshalClass(riid, pv, dwDestContext,\r\n                                              pvDestContext, mshlflags, clsid));\r\n        return S_OK;\r\n    }\r\n<\/pre>\n
Identical one-line changes apply to Get\u00adMarshal\u00adSize\u00adMax<\/code> and Marshal\u00adInterface<\/code>; I won’t write them out.<\/p>\n
Next time, we’ll apply all that we learned to diagnosing a reference counting bug related to marshaling.<\/p>\n","protected":false},"excerpt":{"rendered":"
Maybe you don’t need a copy on the other side.<\/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-106778","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Maybe you don’t need a copy on the other side.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/106778","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=106778"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/106778\/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=106778"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=106778"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=106778"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}