# NSUniqueIDSpecifier A specifier for an object in a collection (or container) by unique ID. ``` class NSUniqueIDSpecifier ``` ## Overview This specifier works only for objects that have an ID property. The unique ID object passed to an instance of `NSUniqueIDSpecifier` must be either an `NSNumber` object or an `NSString` object. The exact type should match the scripting dictionary declaration of the ID attribute for the relevant scripting class. You can expect that the ID property will be *read only* for any object that supports it. Therefore a scripter can obtain the unique ID for an object and refer to the object by the ID, but cannot set the unique ID. You don’t normally subclass `NSUniqueIDSpecifier`. The evaluation of `NSUniqueIDSpecifier` objects follows these steps until the specified object is found: 1. If the container implements a method whose selector matches the relevant `valueInWithUniqueID:` pattern established by scripting key-value coding, the method is invoked. This method can potentially be very fast, and it may be relatively easy to implement. 2. As is the case when evaluating any script object specifier, the container of the specified object is given a chance to evaluate the object specifier. If the container class implements the method, the method is invoked. This method can potentially be very fast, but it is relatively difficult to implement. 3. An [`NSWhoseSpecifier`](/documentation/Foundation/NSWhoseSpecifier) object that specifies the first object whose relevant `'ID '` attribute matches the ID is synthesized and evaluated. The `NSWhoseSpecifier` object must search through all of the keyed elements in the container, looking for a match. The search is potentially very slow. ## Topics ### Initializing a unique ID specifier [`- initWithContainerClassDescription:containerSpecifier:key:uniqueID:`](/documentation/Foundation/NSUniqueIDSpecifier/init(containerClassDescription:containerSpecifier:key:uniqueID:)) Returns an `NSUniqueIDSpecifier` object, initialized with the given arguments. ### Accessing unique ID information [`uniqueID`](/documentation/Foundation/NSUniqueIDSpecifier/uniqueID) Returns the ID encapsulated by the receiver. ## Relationships ### Conforms To [`SendableMetatype`](/documentation/Swift/SendableMetatype) [`Hashable`](/documentation/Swift/Hashable) [`CVarArg`](/documentation/Swift/CVarArg) [`CustomDebugStringConvertible`](/documentation/Swift/CustomDebugStringConvertible) [`NSCoding`](/documentation/Foundation/NSCoding) [`Equatable`](/documentation/Swift/Equatable) [`Sendable`](/documentation/Swift/Sendable) [`CustomStringConvertible`](/documentation/Swift/CustomStringConvertible) [`NSObjectProtocol`](/documentation/ObjectiveC/NSObjectProtocol) ### Inherits From [`NSScriptObjectSpecifier`](/documentation/Foundation/NSScriptObjectSpecifier) --- 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)