What is Ninject and when do you use it?

Question

I have been helping a few friends on a project and there is a class that uses Ninject. I am fairly new to C# and I have no idea what that class is doing, which is why I need to understand Ninject. Can anyone explain what Ninject is and when does one use it(with example if possible)? Or if you can point to some links that would be great too.

I tried this question: Ninject tutorials/documentations? but it didn't really help a beginner like me.

Answer 1

Ninject is dependency injector for .NET, practical realisation of pattern Dependency Injection (form of Inversion of Control pattern).

Suppose you have two classes DbRepository and Controller:

class Controller {
   private DbRepository _repository;

   // ... some methods that uses _repository
}

class DbRepository {
   // ... some bussiness logic here ...
}

So, now you have two problems:

1. You must initialize _repository to use it. You have several options for doing this:

   1. Manually, within the constructor. But what if the constructor of DbRepository changes? You would need to rewrite your Controller class because code it's dependent upon was changed. It's not hard if you have only one Controller, but if you have a couple of classes that have a dependency on your Repository you have a real problem.

   2. You can use a service locator or factory. But now you have a dependency on your service locator. You have a global service locator and all code must use it. How you will you change the behavior of your service locator when you need to use it in one part of your code for activation logic but for something else in another part of your code? There is only one way - passing the service locator through constructors. But with more and more classes you will need to pass it more and more times. Anyway, it's a good thought but in the long run, it's a bad idea.

      class Controller {
         private DbRepository _repository;
      
         public Controller() {
           _repository = GlobalServiceLocator.Get<DbRepository>()
         }
      
         // ... some methods that uses _repository
      }
      

   3. You can use dependency injection. Look at the code:

      class Controller {
         private IRepository _repository;
      
         public Controller(IRepository repository) {
            _repository = repository;
         }
      }
      

      Now when you need your controller you write: ninjectDevKernel.Get<Controller>(); or ninjectTestKernel.Get<Controller>();. You can switch beetween dependency resolvers as fast as you want. See? It's simple, you don't need to write a lot.

2. You can't create unit tests for it. Your Controller has a dependency on DbRepository and if you want to test some method that uses repository, your code will go to the database and ask it for data. That's slow, very slow. If your code in DbRepository changes, your unit test on Controller will fall. Only integration test must warn you of 'problems' in this case. What you need in unit tests - is to isolate your classes and test only one class in one test (in ideal - only one method). If your DbRepository code fails, you will think that Controller code failed - and that's bad (even if you have tests for DbRepository and Controller - they both will fail and you can start from the wrong place). It takes a lot of time to determine where the error really is. You need to know that class A is ok, and it was class B where something failed.

3. When you want to replace DbRepository with something else in all your classes, you have to do a lot of work.

4. You can't easily control the lifetime of DbRepository. An object of this class is created on initialization of Controller and deleted when Controller is deleted. There is no sharing between different instances of the Controller class and there is no sharing between other classes. With Ninject you can simply write:

   kernel.Bind<IRepository>().To<DbRepository>().InSingletonScope();
   

A special feature of dependency injection - agile development! You describe that your controller uses a repository with interface IRepository. You don't need to write DbRepository, you can simply create a MemoryRepository class and develop Controller while another person develops DbRepository. When work on DbRepository is finished, you just rebind in your dependency resolver that default IRepository is now DbRepository. Have a lot of controllers? All of them will now use DbRepository. That's cool.

Read more:

1. Inversion of control (wiki)
2. Dependency injection (wiki)
3. Inversion of Control Containers and the Dependency Injection pattern (Martin Fowler)