Question

I am sure that I am somewhat lost in this area… my understanding is that Dependency Injection means initializing something that is required by a class..so for instance.
If my controller is going to need a service and I want to be able to test it then I should define two Constructor methods for it… so, my question is… why do people use Frameworks to achieve this?? Im lost

public class CompaniesController : Controller
    { 
        private ICompaniesService _service;

        public CompaniesController()
        {
            _service = new CompaniesService();
        }

        public CompaniesController(ICompaniesService service)
        {
            _service = service;
        }

Answer 1

People don’t use a Dependency Injection Framework to generate the code that you provided in your example. That’s still the work of the developer.

The Dependency Injection Framework is used when somebody calls the constructor. The Framework will Inject the concrete implementation of the ICompaniesService rather than the developer explicitly calling the constructor.

While it is a specific product, the nInject Homepage actually has some really good examples.