Dependency Injection: Turtles all the way down?

Question

So I\'m wondering about how unit testing works in regards to dealing external dependencies. Here and elsewhere I\'ve become familiar with dependency injection, and how that

Answer 1

When you unit test a class, you should mock its dependencies, to test your class in isolation -- this is regardless of dependency injection.

The answer to your question about Bar is: yes, you should inject Foo. Once you go down the DI path, you will use it across your entire stack. If you really need a new Foo for every doSomethingWithFoo call, you might want to inject a FooFactory (which you can then mock for testing purposes), if you want a single Bar to use many Foos.