# keyForChildFileWrapper(_:) Returns the dictionary key used by a directory to identify a given file wrapper. ``` func keyForChildFileWrapper(_ child: FileWrapper) -> String? ``` ## Parameters `child` The child file wrapper for which you want the key. ## Return Value Dictionary key used to store the file wrapper in the directory’s list of file wrappers. The dictionary key is a unique filename, which may not be the same as the passed-in file wrapper’s preferred filename if more than one file wrapper in the directory’s dictionary of children has the same preferred filename. See [Accessing File Wrapper Identities](https://developer.apple.com/library/archive/documentation/FileManagement/Conceptual/FileSystemProgrammingGuide/FileWrappers/FileWrappers.html#//apple_ref/doc/uid/TP40010672-CH13-SW1) in [File System Programming Guide](https://developer.apple.com/library/archive/documentation/FileManagement/Conceptual/FileSystemProgrammingGuide/Introduction/Introduction.html#//apple_ref/doc/uid/TP40010672) for more information about the file-wrapper list structure. Returns `nil` if the file wrapper specified in `child` is not a child of the directory. ## Discussion This method raises `NSInternalInconsistencyException` if the receiver is not a directory file wrapper. ## See Also [`filename`](/documentation/Foundation/FileWrapper/filename) The filename of the file wrapper object --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)