Refactoring inheritance to composition using Vbex2005

Question

I have a class that I wrote fairly early on in my vb.net programming experience which inherited from another class it really should have composed. The base class is a relatively generic nested dictionary-based collection; let's call the descendant class a "Car".

Right now there's a lot of code that does things like 'MyCar!Color.st = "Red"' (I use the generic collection rather than real properties to facilitate data interchange with code written in VB6, and also to facilitate comparisons of cars; given three cars X, Y, Z, I can e.g. detect any changes between X and Y and apply those changes to Z).

Is there any nice way to refactor the code to use composition rather than inheritance? Which properties/methods should the "Car" object wrap, and which ones should be accessed through a data-object property? Should a widening conversion be defined between a car and the collection object? Are there any gotchas when doing such refactoring?

Answer 1

You could start by saying Car has a function (or method; not sure of the vb.net terminology) to get its collection - and that function would initially return this (or self, or whatever vb calls it).

Now replace all direct references to Car-as-Collection with Car.getCollection(), both within the Car class and outside.

Finally, make the change: create a member variable, initialize it, return it from getCollection(), and stop inheriting from Collection. If you missed any references in step 2, they'll show up as compile errors at this point. Fix them and your refactoring is complete.