Do you inject your dependencies?

Injecting your dependency gives you:

• Loosely coupled classes
• Increased code reusing
• Maintainable code
• Testable methods
• All dependencies are specified in one place
• Class dependencies are clearly visible in the constructor

Figure: Bad Example – A solution where each layer depends on static classes is not maintainable or testable

Figure: Good Example – Dependencies in each layer should only be interfaces. This allows dependencies to be easily interchanged and unit tests to be written against mock/fake objects

Figure: Bad Example – Classes should not include dependencies on database classes or business objects. Both of these classes may contain dependencies on external services like web services or databases

Figure: Good Example – The dependencies are injected into the class. This enables alternative classes to be injected. For example, a DHLShippingCalculator should be easily substituted for a FedexShippingCalculator. A MockShippingCalculator and MockProductRepository could be injected if we wanted to run unit tests