Unit testing and dependency injection with deeply nested dependencies?

Let us start with your last question. Where are the dependencies injected: A common approach is to use constructor injection (as described by Fowler ). So Foo is injected with an IBar in the constructor.

The concrete implementation of IBar Bar in turn has an IBaz injected into its constructor. And finally the IBaz implementation ( Baz ) has an IDatabase (or whatever) injected. If you use a DI framework such as Castle Project you would simply ask the DI container to resolve an instance of Foo for you.

It will then use whatever you have configured to determine which implementation of IBar you are using. If it determines that your implementation of IBar is Bar it will then determine which implementation of IBaz you are using, etc.

The kind of test you described in the first part of your post (when you try all the parts together) it is usually defined as integration test. As a good practice in your solution you should have either a unit test project and an integration test project. In order to inject dependecies in your code the first and most important rule is to code using interfaces.

Assumed this, let's say your class contains an interface as a member and you want to inject/mock it: you can either expose it as a property or pass the implementation using the class constructor. I prefer to use properties to expose dependencies, this way the constructor don't become too verbose. I suggest you to use NUnit or MBunit as a testing framework and Moq as a mocking framework (more clear in it's outputs than Rhino mocks) Here's the documentation with a some examples on how to mock with Moq http://code.google.com/p/moq/wiki/QuickStart.

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