\nA customer ran into a crashing bug in their shell extension.\nThe shell extension wants to change the thread’s preferred UI language\ntemporarily, so that it can load its resources from a specific language.\nYou’d think this would be easy:\n<\/p>\n
\n\/\/ error checking elided for simplicity\n\/\/ There is a bug in this code - read on\n\/\/ Get the current thread preferred UI languages\nULONG cLanguages;\nPZZWSTR pszzPrevLanguages;\nULONG cchPrevLanguages = 0;\nGetThreadPreferredUILanguages(MUI_LANGUAGE_NAME,\n &cLanguages, NULL,\n &cchPrevLanguages);\npszzPrevLanguages = new WCHAR[cchPrevLanguages];\nGetThreadPreferredUILanguages(MUI_LANGUAGE_NAME,\n &cLanguages,\n pszzPrevLanguages,\n &cchPrevLanguages);\n... change the thread preferred UI languages ...\n... load resources ...\n\/\/ Restore the original thread preferred UI languages\nSetThreadPreferredUILanguages(MUI_LANGUAGE_NAME,\n pszzPrevLanguages,\n &cLanguages);\ndelete[]<\/a> pszzPrevLanguages;<\/i>\n<\/pre>\n\nApproximately ten seconds after this code runs, Explorer crashes\nwith the exception
STATUS_CALLBACK_RETURNED_LANG<\/code>\nwhose description is\n“A threadpool worker thread enter a callback,\nwhich left with preferred languages set.\nThis is unexpected, indicating that the callback missed clearing them.”\n(Just before Explorer crashes, the message\n“ThreadPool: callback 77180274(05B67430) returned with preferred languages set”\nappears on the debugger, which says basically the same thing as the\nstatus code.)\n<\/p>\n\nExercise<\/b>: Why does it take ten seconds before the crash occurs?\n<\/p>\n
\nThis crash is puzzling, because it’s claiming that the callback didn’t\nreset the thread preferred languages, but you can see us doing it\nright there in the code when we call\n
SetThreadPreferredUILanguages<\/code>!\nSomebody’s on crack, but who?\n<\/p>\n\nA closer reading of the error message indicates that the callback\nneeds to “clear” the thread preferred languages, not merely reset them\nto their original values, and the documentation for\n
SetThreadPreferredUILanguages<\/code> says,\n“To clear the thread preferred UI languages list,\nthe application can set this parameter to a null string\nor an empty double null-terminated string.”\nOkay, so now the question is,\n“How can I tell, when I callGetThreadPreferredUILanguages<\/code>,\nthat the list of languages I receive back represents the\nclear<\/i> state as opposed to indicating that some other\ncode calledSetThreadPreferredUILanguages<\/code> before I did?”\n<\/p>\n\nThe magic is the flag\n
MUI_THREAD_LANGUAGES<\/code>.\nIf you pass this flag when you call\nGetThreadPreferredUILanguages<\/code>,\nit will return a null string if the thread has not customized\nits preferred UI languages, indicating that the way to restore\nthe thread’s preferred UI language state is to clear it\nrather than setting it.\nFortunately, this lines up nicely with the way you’re supposed to\nclear the state,\nso at the end of the day there is no special case.\n<\/p>\n\nThe fix to the above code, then, is to make the following simple\nchange:\n<\/p>\n
\n\/\/ error checking elided for simplicity\n\/\/ Get the current thread preferred UI languages\nULONG cLanguages;\nPZZWSTR pszzPrevLanguages;\nULONG cchPrevLanguages = 0;\nGetThreadPreferredUILanguages(MUI_LANGUAGE_NAME |\n MUI_THREAD_LANGUAGES<\/font>,\n &cLanguages, NULL,\n &cchPrevLanguages);\npszzPrevLanguages = new WCHAR[cchPrevLanguages];\nGetThreadPreferredUILanguages(MUI_LANGUAGE_NAME |\n MUI_THREAD_LANGUAGES<\/font>,\n &cLanguages,\n pszzPrevLanguages,\n &cchPrevLanguages);\n... change the thread preferred UI languages ...\n... load resources ...\n\/\/ Restore the original thread preferred UI languages\nSetThreadPreferredUILanguages(MUI_LANGUAGE_NAME,\n pszzPrevLanguages,\n &cLanguages);\ndelete[]<\/a> pszzPrevLanguages;\n<\/pre>\n\nAs you might expect,\n\nMichael Kaplan has his own thoughts on the\n
SetThreadPreferredUILanguages<\/code> function<\/a>.\nIn fact, he has\n\nseveral such thoughts<\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":"A customer ran into a crashing bug in their shell extension. The shell extension wants to change the thread’s preferred UI language temporarily, so that it can load its resources from a specific language. You’d think this would be easy: \/\/ error checking elided for simplicity \/\/ There is a bug in this code – […]<\/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-11933","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
A customer ran into a crashing bug in their shell extension. The shell extension wants to change the thread’s preferred UI language temporarily, so that it can load its resources from a specific language. You’d think this would be easy: \/\/ error checking elided for simplicity \/\/ There is a bug in this code – […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/11933","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=11933"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/11933\/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=11933"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=11933"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=11933"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}