, Ion Todirel wrote

Extension methods is one of my least favorite features in C#, they are a hack to inject functionality into something that doesn't have it. Mixins can be a better choice sometimes, and you can do that in C++.

a mixin is created by creating a new class that is based on the class being extended?

http://www.jakesee.com/2009/04/c-mixins-a-powerful-concept/

I have a class library that is returning an instance of a class. How to apply the mixin pattern to an object that already exists?