How to check if Error is subclass of NSError

Question

I want a function which, for any given Error, will give me some description of it

protocol CustomError { }

func customDescription(_ error: Error) -> String {
    switch error {
    case let customError as CustomError:
        return "custom error"
    case ???:
        return "not subclass of NSError"
    case let nsError as NSError:
        return "subclass of NSError"
    }
}

Above is not the real code, and I don't want a String description, but a Dictionary, but this is not important in context of the question.

The problem is I don't know how to distinguish Errors which is subclass of NSError and which is not because any swift error could be bridged to NSError. Is it possible in swift?

Answer 1

As you already noticed, any type conforming to Error can be bridged to NSError, therefore error is NSError is always true, and a cast error as NSError does always succeed.

What you can do is to check the dynamic type of the value with type(of:):

type(of: error) is NSError.Type

evaluates to true if error is an instance of NSError or a subclass.

Answer 2

private protocol _NSError: Error { // private, so _NSError is equal to NSError
}

extension NSError: _NSError {

}

public func customDescription(_ error: Error) -> String {
    switch error {
    case let nsError as _NSError:
        print(nsError as! NSError)
        return "NSError"
    default:
        return "others"
    }
}