# exceptionDuringOperation(_:error:leftOperand:rightOperand:) Specifies what an `NSDecimalNumber` object will do when it encounters an error. ``` func exceptionDuringOperation(_ operation: Selector, error: NSDecimalNumber.CalculationError, leftOperand: NSDecimalNumber, rightOperand: NSDecimalNumber?) -> NSDecimalNumber? ``` ## Parameters `operation` The method that was being executed when the error occurred. `error` The type of error that was generated. `leftOperand` The left operand. `rightOperand` The right operand. ## Discussion There are four possible values for `error`, described in [`NSDecimalNumber.CalculationError`](/documentation/Foundation/NSDecimalNumber/CalculationError). The first three have to do with limits on the ability of `NSDecimalNumber` to represent decimal numbers. An `NSDecimalNumber` object can represent any number that can be expressed as mantissa x 10^exponent, where mantissa is a decimal integer up to 38 digits long, and exponent is between –256 and 256. The fourth results from the caller trying to divide by `0`. In implementing [`exceptionDuringOperation(_:error:leftOperand:rightOperand:)`](/documentation/Foundation/NSDecimalNumberBehaviors/exceptionDuringOperation(_:error:leftOperand:rightOperand:)), you can handle each of these errors in several ways: - Raise an exception. For an explanation of exceptions, see [Exception Programming Topics](https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/Exceptions/Exceptions.html#//apple_ref/doc/uid/10000012i). - Return `nil`. The calling method will return its value as though no error had occurred. If `error` is `NSCalculationLossOfPrecision`, `operation` will return an imprecise value—that is, one constrained to 38 significant digits. If `error` is `NSCalculationUnderflow` or `NSCalculationOverflow`, `operation` will return `NSDecimalNumber`‘s `notANumber`. You shouldn’t return `nil` if `error` is `NSDivideByZero`. - Correct the error and return a valid `NSDecimalNumber` object. The calling method will use this as its own return value. --- 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)