{"id":40033,"date":"2004-03-26T06:58:00","date_gmt":"2004-03-26T06:58:00","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/oldnewthing\/2004\/03\/26\/the-ways-people-mess-up-iunknownqueryinterface\/"},"modified":"2004-03-26T06:58:00","modified_gmt":"2004-03-26T06:58:00","slug":"the-ways-people-mess-up-iunknownqueryinterface","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20040326-00\/?p=40033","title":{"rendered":"The ways people mess up IUnknown::QueryInterface"},"content":{"rendered":"

\nWhen you’re dealing with application compatibility,\nyou discover all sorts of things that worked only by accident.\nToday, I’ll talk about some of the “creative” ways people\nmess up the\n\nIUnknown::QueryInterface method<\/a>.\n<\/p>\n

\nNow, you’d think, “This interface is so critical to COM,\nhow could anybody possible mess it up?”\n<\/p>\n

\n
Forgetting to respond to IUnknown.<\/p>\n
\n

\nSometimes you get so excited about responding to all these\ngreat interfaces that you forget to respond to IUnknown itself.\nWe have found objects where<\/p>\n

\nIShellFolder *psf = some object;\nIUnknown *punk;\npsf->QueryInterface(IID_IUnknown, (void**)&punk);\n<\/pre>\n
fails with E_NOINTERFACE!\n<\/p>\n
Forgetting to respond to your own interface.<\/p>\n
\n
\nThere are some methods which return an object with a specific\ninterface. And if you query that object for its own interface,\nits sole reason for existing, it says “Huh?”\n<\/p>\n
\nIShellFolder *psf = some object;\nIEnumIDList *peidl, *peidl2;\npsf->EnumObjects(..., &peidl);\npeidl->QueryInterface(IID_IEnumIDList, (void**)&peidl2);\n<\/pre>\n
\nThere are some objects which return E_NOINTERFACE to the QueryInterface\ncall, even though you’re asking the object for itself!\n“Sorry, I don’t exist,” it seems they’re trying to say.\n<\/p>\n
Forgetting to respond to base interfaces.<\/p>\n
\n
\nWhen you implement a derived interface, you implicitly implement\nthe base interfaces, so don’t forget to respond to them, too.<\/p>\n
\nIShellView *psv = some object;\nIOleView *pow;\npsv->QueryInterface(IID_IOleView, (void**)&pow);\n<\/pre>\n
Some objects forget and the QueryInterface fails with E_NOINTERFACE.\n<\/p>\n
Requiring a secret knock.<\/p>\n
\n
\nIn principle, the following two code fragments are equivalent:\n<\/p>\n
\nIShellFolder *psf;\nIUnknown *punk;\nCoCreateInstance(CLSID_xyz, ..., IID_IShellFolder, (void**)&psf);\npsf->QueryInterface(IID_IUnknown, (void**)&punk);\nCoCreateInstance(CLSID_xyz, ..., IID_IUnknown, (void**)&punk);\npunk->QueryInterface(IID_IShellFolder, (void**)&psf);\n<\/pre>\n
\nIn reality, some implementations mess up and fail the second call\nto CoCreateInstance. The only way to create the object successfully\nis to create it with the IShellFolder interface.\n<\/p>\n
Forgetting to say “no” properly.<\/p>\n
\n
\nOne of the rules for saying “no” is that you have to set the\noutput pointer to NULL before returning.  Some people forget to do\nthat.\n<\/p>\n
\nIMumble *pmbl;\npunk->QueryInterface(IID_IMumble, (void**)&pmbl);\n<\/pre>\n
\nIf the QueryInterface succeeds, then pmbl must be non-NULL on return.\nIf it fails, then pmbl must be NULL on return<\/strong>.\n<\/p>\n<\/dl>\n
\nThe shell has to be compatible with all these buggy objects because\nif it weren’t, customers would get upset and the press would have\na field day.  Some of the offenders\nare big-name programs. If they broke, people would report,\n“Don’t upgrade to Windows XYZ, it’s not compatible with\n<big-name program>.”\nConspiracy-minded folks would shout,\n“Microsoft intentionally broke <big-name program>!\nProof of unfair business tactics!”\n<\/p>\n
\n[Raymond is currently on vacation; this message was pre-recorded.]<\/p>\n","protected":false},"excerpt":{"rendered":"
When you’re dealing with application compatibility, you discover all sorts of things that worked only by accident. Today, I’ll talk about some of the “creative” ways people mess up the IUnknown::QueryInterface method. Now, you’d think, “This interface is so critical to COM, how could anybody possible mess it up?” Forgetting to respond to IUnknown. Sometimes […]<\/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":[2],"class_list":["post-40033","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-history"],"acf":[],"blog_post_summary":"
When you’re dealing with application compatibility, you discover all sorts of things that worked only by accident. Today, I’ll talk about some of the “creative” ways people mess up the IUnknown::QueryInterface method. Now, you’d think, “This interface is so critical to COM, how could anybody possible mess it up?” Forgetting to respond to IUnknown. Sometimes […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/40033","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=40033"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/40033\/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=40033"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=40033"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=40033"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}