# addFileWrapper(_:) Adds a child file wrapper to the receiver, which must be a directory file wrapper. ``` func addFileWrapper(_ child: FileWrapper) -> String ``` ## Parameters `child` File wrapper to add to the directory. ## Return Value Dictionary key used to store `fileWrapper` in the directory’s list of file wrappers. The dictionary key is a unique filename, which is the same as the passed-in file wrapper’s preferred filename unless that name is already in use as a key in the directory’s dictionary of children. 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. ## Discussion Use this method to add an existing file wrapper as a child of a directory file wrapper. If the file wrapper does not have a preferred filename, set the [`preferredFilename`](/documentation/Foundation/FileWrapper/preferredFilename) property to give it one before calling [`addFileWrapper(_:)`](/documentation/Foundation/FileWrapper/addFileWrapper(_:)). To create a new file wrapper and add it to a directory, use the [`addRegularFile(withContents:preferredFilename:)`](/documentation/Foundation/FileWrapper/addRegularFile(withContents:preferredFilename:)) method. ### Special Considerations This method raises `NSInternalInconsistencyException` if the receiver is not a directory file wrapper. This method raises `NSInvalidArgumentException` if the child file wrapper doesn’t have a preferred name. ## See Also [`preferredFilename`](/documentation/Foundation/FileWrapper/preferredFilename) The preferred filename for 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)