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• The Windows.Storage.DownloadsFolder object provides a createFolderAsync method through
which you can obtain a StorageFolder in that location. It also provides a createFileAsync
method to create a StorageFile directly. You would use this API if your app manages
downloaded files directly. Note that DownloadsFolder itself provides only these two methods; it
is not a StorageFolder in its own right.
• The static method Windows.Storage.StorageFolder.getFolderFromPathAsync returns a
StorageFolder for a given pathname if and only if your app already has permissions to access it;
otherwise, you’ll get an Access Denied exception. A similar static method exists for files called
Windows.Storage.StorageFile.getFileFromPathAsync, with the same restrictions;
Windows.Storage.StorageFile.getFileFromApplicationUriAsync opens files with
ms-appx:// (package) and ms-appdata:/// URIs. Other schemas are not supported.
• Once a folder or file object is obtained, it can be stored in the Windows.Storage.-AccessCache
that allows an app to retrieve it sometime in the future with the same programmatic
permissions. This is primarily needed for folders or files selected through the pickers because
permission to access the storage item is granted only for the lifetime of that in-memory object.
You should always use this API, as demonstrated in Scenario 6 of the File access sample, where
you’d normally think to save a file path. Again, StorageFolder.getFolder-FromPathAsync and
StorageFile.getFileFromPathAsync will throw Access Denied exceptions if they refer to any
locations where you don’t already have permissions. Pathnames also will not work for files
provided by another app through the file picker, because the StorageFile object might not, in
fact, refer to anything that actually exists on the file system.
Once you have a StorageFolder in hand, you can do the kinds of operations you’d expect: obtain
folder properties (including a thumbnail), create and/or open files and folders, and enumerate the
folder’s contents. With the latter, the API provides for obtaining a list folder contents, of course (see the
getItemsAsync method), but what you want more often is a partial list of those contents according to
certain criteria, along with thumbnails and other indexed file metadata (music album and track info,
picture titles and tags, etc.) that you can use to group and organize the files. This is the purpose of file,
folder, and item (file + folder) queries, which you manage through the methods
createFileQuery-[WithOptions], createFolderQuery[WithOptions], and
createItemQuery[WithOptions]. We already saw a little of this with the FlipView app we built using the
Pictures Library in Chapter 5, “Collection and Collection Controls” and we’ll return to the subject in the
context of user data, the primary scenario for queries, at the end of this chapter. 41
41 Obtaining folder properties happens through a storage item’s getBasicPropertiesAsync method (linked here for
StorageFolder but also available on StorageFile.) This provides a
Windows.Storage.FileProperties.-BasicPropertiesClass, which then has a retrievePropertiesAsync method.
Through this you can retrieve any number of Windows properties. A property like System.FreeSpace will actually give you
the free space on the drive where the StorageFolder lives.
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