# hierarchyInconsistencyException Raised if the view controller hierarchy is inconsistent with the view hierarchy. ``` class let hierarchyInconsistencyException: NSExceptionName ``` ## Discussion When a view controller’s view is added to the view hierarchy, the system walks up the view hierarchy to find the first parent view that has a view controller. That view controller must be the parent of the view controller whose view is being added. Otherwise, this exception is raised. This consistency check is also performed when a view controller is added as a child by calling the [`addChild(_:)`](/documentation/UIKit/UIViewController/addChild(_:)) method. It is also allowed for a view controller that has no parent to add its view to the view hierarchy. This is generally not recommended, but is useful in some special cases. --- 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)