Why would I be unable to convert an instance of a class to an instance of the class's subclass in AS3?

I'm using a library that has a function that returns an instance of some class Engin开发者_Python百科e.

I'd like to tack on some interfaces to Engine, so I subclass it class InterfacedEngine extends Engine implements AwesomeInterface.

But when I change the code that uses the classes from this:

var engine:Engine = generateEngine();

to this:

var interfacedEngine:InterfacedEngine = generateEngine();

It gives me a runtime error (elision mine):

TypeError: Error #1034: Type Coercion failed: cannot convert ...::Engine@1bc2bf11 to ....InterfacedEngine.

What about AS3 classes am I misunderstanding?

One solution to this kind of problem is the Proxy design pattern. You can read up on it here: http://www.oodesign.com/proxy-pattern.html

Basically you create a placeholder for another class. This is done through composition instead of inheritance though.

InterfaceEngine should accept an object of type Engine as a constructor parameter. AwesomeINterface defines all the Engine methods you need. InterfaceEngine just passes the method call to the corresponding meths of the Engine object it's holding onto.

If B is the base class and D extends B then a D is a B, but not the opposite. That means a reference of type B can refer to both B and D. But a reference of type D can only refer to D, not B. InterfacedEngine can only refer InterfacedEngine, not it's base Engine. But a Engine type can refer to both Engine and InterfacedEngine.

var eng1:Engine = new Engine(); // valid
var eng2:Engine = new InterfacedEngine(); // valid
var eng3:InterfacedEngine = new InterfacedEngine(); // valid
var eng4:InterfacedEngine = new Engine(); // not valid