{"id":9363,"date":"2011-10-17T07:00:00","date_gmt":"2011-10-17T07:00:00","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/oldnewthing\/2011\/10\/17\/why-is-there-a-csidl_desktop-value-if-you-need-the-desktop-in-order-to-get-it-anyway\/"},"modified":"2011-10-17T07:00:00","modified_gmt":"2011-10-17T07:00:00","slug":"why-is-there-a-csidl_desktop-value-if-you-need-the-desktop-in-order-to-get-it-anyway","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20111017-00\/?p=9363","title":{"rendered":"Why is there a CSIDL_DESKTOP value if you need the desktop in order to get it anyway?"},"content":{"rendered":"

\nJohn asks\n\nwhy there is a special constant CSIDL_DESKTOP<\/code>\ndefined for the desktop<\/a>.\nAfter all, in order to use CSIDL_DESKTOP<\/code>,\nyou need to call SHGet­Desktop­Folder<\/code> and then\nbind to it.\nWhat’s the point of having an ITEM­ID­LIST<\/code> that\nrepresents the desktop if, in order to use it,\nyou first need to get the desktop?\n<\/p>\n

\nIt’s like asking why the file system uses\n.<\/code> (dot)\nto refer to the current directory.\nYou’re already in<\/i> the current directory.\nIn order to resolve .<\/code> (dot), you already need to have\nthe current directory, so why bother with the dot at all?\n<\/p>\n

\nBecause it is often convenient to give a name to your starting point.\n<\/p>\n

\nSuppose somebody wants to save a file to the desktop.\nHow would you represent this as an ITEM­ID­LIST<\/code>?\nIf the only thing you can do is fill in the blank in the\nsentence,\n“Start with the desktop folder, then go to ________, then\nsave the file there,”\nthen you need a way to say “where you are now.”\n<\/p>\n

\nAnd that’s what CSIDL_DESKTOP<\/code> gives you.\nAn ITEM­ID­LIST<\/code> that says “Where you are now.”\n<\/p>\n

\nBesides, if CSIDL_DESKTOP<\/code> weren’t defined,\nsomebody would have invented it.\nSay your program has a list of directories it wants to operate on,\nsay, the Documents folder, the Music folder,\nand the Shared Documents folder.\nGreat, so let me write a function:\n<\/p>\n

\nvoid DoItIn(HWND hwnd, int csidl)\n{\n PIDLIST_ABSOLUTE pidl;\n if (SUCCEEDED(SHGetSpecialFolderLocation(\n                     hwnd, csidl, &pidl))) {\n  IShellFolder *psf;\n  if (SUCCEEDED(SHBindToObject(NULL, pidl, NULL,\n                    IID_PPV_ARGS(&psf)))) {\n   ...\n   psf->Release();\n  }\n  CoTaskMemFree(pidl);\n }\n}\nvoid DoItInStandardPlaces(HWND hwnd)\n{\n const static int csidls[] = {\n  CSIDL_MYDOCUMENTS,\n  CSIDL_MYMUSIC,\n  CSIDL_COMMON_DOCUMENTS,\n };\n for (int i = 0; i < ARRAYSIZE(csidls); i++) {\n  DoItIn(hwnd, csidls[i]);\n }\n}\n<\/pre>\n
\nNow you want to add the desktop folder.\nOh wait, there is no CSIDL<\/code> value for the desktop,\nso I’ll have to make one up.\n<\/p>\n
\n\/\/ Our custom CSIDLs use the high word. None of the CSIDLs we use\n\/\/ set any bits in the high word, so we can use the high word to\n\/\/ detect whether we have a standard CSIDL or a custom CSIDL.\n#define CUSTOMCSIDL_DESKTOP 0x00010000\n#define ISCUSTOMCSIDL(csidl) HIWORD(csidl)\n#define STANDARDCSIDLOF(csidl) LOWORD(csidl)\nHRESULT MyGetSpecialFolderLocation(\n    HWND hwnd, int csidl, PIDLIST_ABSOLUTE *ppidl)\n{\n HRESULT hr;\n if (ISCUSTOMCSIDL(csidl)) {\n  *ppidl = (PIDLIST_ABSOLUTE)CoTaskMemAlloc(sizeof(WORD));\n  if (*ppidl) {\n   ppidl->mkid.cb = 0;\n   hr = S_OK;\n  } else {\n   hr = E_OUTOFMEMORY;\n  }\n } else {\n  hr = SHGetSpecialFolderLocation(hwnd, STANDARDCSIDLOF(csidl), ppidl);\n }\n return hr;\n}\n<\/pre>\n
\nOkay, cool, now I can add\n<\/p>\n
\n const static int csidls[] = {\n  CSIDL_MYDOCUMENTS,\n  CSIDL_MYMUSIC,\n  CSIDL_COMMON_DOCUMENTS,\n  CUSTOMCSIDL_DESKTOP,<\/font>\n };\n<\/pre>\n
\nOh wait, I also have to have a custom version of\nSHBind­To­Object<\/code> that knows how to bind to\nthis special new type of pidl that means “where you are now.”\n<\/p>\n
\nHRESULT MyBindToObject(IShellFolder *psf, PCUIDLIST_RELATIVE pidl,\n  IBindCtx *pbc, REFIID riid, void **ppv)\n{\n HRESULT hr;\n if (pidl->mkid.cb == 0) {\n  *ppv = NULL;\n  if (psf == NULL) {\n   hr = SHGetDesktopFolder(&psf);\n   if (SUCCEEDED(hr)) {\n    hr = psf->QueryInterface(riid, ppv);\n    psf->Release();\n   }\n  } else {\n   hr = psf->QueryInterface(riid, ppv);\n  }\n } else {\n  hr = SHBindToObject(psf, pidl, pbc, riid, ppv);\n }\n return hr;\n}\n<\/pre>\n
\nCongratulations, you just reinvented CSIDL_DESKTOP<\/code>.\n<\/p>\n
\nIt can be\n\nvery convenient to have a name for the null action<\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":"
John asks why there is a special constant CSIDL_DESKTOP defined for the desktop. After all, in order to use CSIDL_DESKTOP, you need to call SHGet­Desktop­Folder and then bind to it. What’s the point of having an ITEM­ID­LIST that represents the desktop if, in order to use it, you first need to get the desktop? It’s […]<\/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":[26],"class_list":["post-9363","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-other"],"acf":[],"blog_post_summary":"
John asks why there is a special constant CSIDL_DESKTOP defined for the desktop. After all, in order to use CSIDL_DESKTOP, you need to call SHGet­Desktop­Folder and then bind to it. What’s the point of having an ITEM­ID­LIST that represents the desktop if, in order to use it, you first need to get the desktop? It’s […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/9363","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=9363"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/9363\/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=9363"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=9363"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=9363"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}