What do programmers mean when they say, “Code against an interface, not an object.”?

Question

I\'ve started the very long and arduous quest to learn and apply TDD to my workflow. I\'m under the impression that TDD fits in very well with IoC principle

Answer 1

This screencast explains agile development and TDD in practice for c#.

By coding against an interface means that in your test, you can use a mock object instead of the real object. By using a good mock framework, you can do in your mock object whatever you like.