# doesNotRecognizeSelector(_:) Handles messages the receiver doesn’t recognize. ``` func doesNotRecognizeSelector(_ aSelector: Selector!) ``` ## Parameters `aSelector` A [Selector](https://developer.apple.com/library/archive/documentation/General/Conceptual/DevPedia-CocoaCore/Selector.html#//apple_ref/doc/uid/TP40008195-CH48) that identifies a method not implemented or recognized by the receiver. ## Discussion The runtime system invokes this method whenever an object receives an `aSelector` message it can’t respond to or forward. This method, in turn, raises an `NSInvalidArgumentException`, and generates an error message. Any [`doesNotRecognizeSelector(_:)`](/documentation/ObjectiveC/NSObject-swift.class/doesNotRecognizeSelector(_:)) messages are generally sent only by the runtime system. However, they can be used in program code to prevent a method from being inherited. For example, an `NSObject` subclass might renounce the [`copy()`](/documentation/ObjectiveC/NSObject-swift.class/copy()) or [`init()`](/documentation/ObjectiveC/NSObject-swift.class/init()) method by re-implementing it to include a [`doesNotRecognizeSelector(_:)`](/documentation/ObjectiveC/NSObject-swift.class/doesNotRecognizeSelector(_:)) message as follows: ```objc - (id)copy { [self doesNotRecognizeSelector:_cmd]; } ``` The `_cmd` variable is a hidden argument passed to every method that is the current selector; in this example, it identifies the selector for the `copy` method. This code prevents instances of the subclass from responding to `copy` messages or superclasses from forwarding `copy` messages—although [`responds(to:)`](/documentation/ObjectiveC/NSObjectProtocol/responds(to:)) will still report that the receiver has access to a `copy` method. If you override this method, you must call `super` or raise an exception at the end of your implementation. In other words, this method must not return normally; it must always result in an exception being thrown. ## See Also [`forwardInvocation:`](/documentation/ObjectiveC/NSObject-swift.class/forwardInvocation:) Overridden by subclasses to forward messages to other objects. --- 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)