{"id":27643,"date":"2007-03-13T10:00:00","date_gmt":"2007-03-13T10:00:00","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/oldnewthing\/2007\/03\/13\/the-only-thing-you-can-do-with-display-names-is-display-them\/"},"modified":"2007-03-13T10:00:00","modified_gmt":"2007-03-13T10:00:00","slug":"the-only-thing-you-can-do-with-display-names-is-display-them","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20070313-00\/?p=27643","title":{"rendered":"The only thing you can do with display names is display them"},"content":{"rendered":"

\nThere are many functions that return strings called “display names”.\nAnd the only thing you can do with display names is display them.\nDon’t assume that the string actually means anything,\nbecause it doesn’t.\nTheoretically, a function like SHGetFileInfo<\/code>\ncould be implemented as\n<\/p>\n

\n...\n if (uFlags & SHGFI_DISPLAYNAME) {\n  StringCchCopy(psfi->szDisplayName, MAX_PATH, TEXT(\"Booga!\"));\n }\n...\n<\/pre>\n
\nand your program should still work.\n<\/p>\n
\n(Of course, this is merely a ground rule.\nSpecific functions may have exceptions.\nFor example, the IShellFolder::GetDisplayNameOf<\/code>\nhas a special flag SHGDN_FORPARSING<\/code> flag which explicitly\nindicates that the string returned is designed to be parsed.)\n<\/p>\n
\nThe purpose of a “display name” is to be a string suitable for\ndisplaying to the user.\nThe display name for a file, for example, might be the file name,\nor it might have the extension removed, or the name might even\nbe translated into the user’s preferred language!\nFor example, on an English system with the German multilanguage\npack installed and active, asking for the display name of\nC:\\Documents and Settings\\Raymond\\My Documents<\/code>\nwill return Eigene Dateien<\/code> because that’s the\nname for My Documents in German.\n<\/p>\n
\nIf your program assumed that the display name of\nC:\\Documents and Settings\\Raymond\\My Documents<\/code>\nwould be something like “My Documents”,\nyour program is in for a big surprise when I run it.\n<\/p>\n
\nOne of my colleagues was investigating a bug reported against\na program that wouldn’t run.\nIt claimed that the CD-ROM was not in the drive,\neven though it was.\nWhy can’t the program find its CD-ROM?\n<\/p>\n
\nAfter a few days’ investigation, my colleague found the reason.\nThe program wanted to find its CD-ROM,\nso it walked through\nall 26 drive letters and called\nSHGetFileInfo<\/code> passing the SHGFI_DISPLAYNAME<\/code> flag.\nSomething like this:\n<\/p>\n
\n\/\/ The actual code was much, much more convoluted than this.\nchar LookForCD(LPCTSTR pszVolumeLabel)\n{\n for (TCHAR chDrive = TEXT('A'); chDrive <= TEXT('Z'); chDrive++) {\n  TCHAR szRoot[4];\n  wsprintf(szRoot, TEXT("%c:\\\\"), chDrive);\n  SHFILEINFO sfi;\n  if (SHGetFileInfo(szRoot, 0, &sfi, sizeof(sfi), SHGFI_DISPLAYNAME)) {\n   TCHAR szExpected[MAX_PATH];\n   wsprintf(szExpected, TEXT("%s (%c:)"), pszVolumeLabel, chDrive);\n   if (strcmp(szExpected, sfi.szDisplayName) == 0) {\n    return chDrive; \/\/ Found it!\n   }\n  }\n }\n return 0; \/\/ not found\n}\n<\/pre>\n
\nThe program asked for the display name of each drive and\nlooked for one whose display name was of the form\nLABEL (D:)<\/code>,\nwhere LABEL<\/code> is the volume label they’re looking for\nand D:<\/code> is the drive letter.\n<\/p>\n
\nIn othe words, they were trying to interpret the display name.\n<\/p>\n
\nDon’t do that.\nThere is no guarantee that the display name for a CD-ROM\nwill be of any particular form.\nThe default in Windows XP happens to be LABEL (D:)<\/code>,\nbut there\nare a variety of system policies\nthat can be used to change this,\nand if any of those policies is in effect, the program can’t find\nits CD-ROM and refuses to run.\n<\/p>\n
\n(For the record, the correct way of doing this would be to pass\neach drive letter to the\nGetDriveType<\/code> function to see if it is a\nDRIVE_CDROM<\/code>;\nfor each such drive,\ncall GetVolumeInformation<\/code>\nto get the CD-ROM’s volume label.)\n<\/p>\n
\nSo remember,\ndisplay names are just for display purposes.†\n<\/p>\n
\nNitpicker’s corner<\/b>\n<\/p>\n
\n†See parenthetical remark at the top of this entry for clarification.\nThe sentence intentionally overlooked the exceptions in order to provide\na punchier ending to the story.\nIt’s called writing style<\/i> and is a valid literary technique.<\/p>\n","protected":false},"excerpt":{"rendered":"
There are many functions that return strings called “display names”. And the only thing you can do with display names is display them. Don’t assume that the string actually means anything, because it doesn’t. Theoretically, a function like SHGetFileInfo could be implemented as … if (uFlags & SHGFI_DISPLAYNAME) { StringCchCopy(psfi->szDisplayName, MAX_PATH, TEXT(“Booga!”)); } … and […]<\/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-27643","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
There are many functions that return strings called “display names”. And the only thing you can do with display names is display them. Don’t assume that the string actually means anything, because it doesn’t. Theoretically, a function like SHGetFileInfo could be implemented as … if (uFlags & SHGFI_DISPLAYNAME) { StringCchCopy(psfi->szDisplayName, MAX_PATH, TEXT(“Booga!”)); } … and […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/27643","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=27643"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/27643\/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=27643"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=27643"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=27643"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}