# init(block:arguments:) Creates an expression object that uses the block for evaluating objects. ``` init(block: @escaping (Any?, [NSExpression], NSMutableDictionary?) -> Any, arguments: [NSExpression]?) ``` ## Parameters `block` The Block is applied to the object to be evaluated. The Block takes three arguments and returns a value: - evaluatedObject: The object to be evaluated. - expressions: An array of predicate expressions that evaluates to a collection. - context: A dictionary that the expression can use to store temporary state for one predicate evaluation. Note that `context` is mutable, and that it can only be accessed during the evaluation of the expression. You must not attempt to retain it for use elsewhere. ] The Block returns the `evaluatedObject` . `arguments` An array containing `NSExpression` objects that will be used as parameters during the invocation of selector. For a selector taking no parameters, the array should be empty. For a selector taking one or more parameters, the array should contain one `NSExpression` object which will evaluate to an instance of the appropriate type for each parameter. If there is a mismatch between the number of parameters expected and the number you provide during evaluation, an exception may be raised or missing parameters may simply be replaced by `nil` (which occurs depends on how many parameters are provided, and whether you have over- or underflow). See [`init(forFunction:arguments:)`](/documentation/Foundation/NSExpression/init(forFunction:arguments:)) for a complete list of arguments. ## Return Value An expression that filters a collection using the specified Block. ## See Also [`expressionBlock`](/documentation/Foundation/NSExpression/expressionBlock) The block that executes to evaluate the expression. --- 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)