How to access private members of an Objective-C class from a Swift extension?

I believe that there is no way to access Objective-C instance variables from Swift. Only Objective-C properties get mapped to Swift properties.

I came across this question while trying to find a way to access a private variable of a class from one of the SDKs we use. Since we don't have or control the source code we can't change the variables to properties. I did find that the following solution works for this case:

extension ObjcClass {
    func getPrivateVariable() -> String? {
        return value(forKey: "privateVariable") as? String
    }

    open override func value(forUndefinedKey key: String) -> Any? {
        if key == "privateVariable" {
            return nil
        }
        return super.value(forUndefinedKey: key)
    }
}

Overriding value(forUndefinedKey:) is optional. value(forKey:) will crash if the private variable doesn't exist on the class unless you override value(forUndefinedKey:) and provide a default value.