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;
}
A major reason is to better support unit testing and mocking out objects to create controlled tests.
By not specifying the implementation inside the class, you can 'inject' an implementation at run time. (In your example, the ICompaniesService interface).
At runtime, using an inversion of control/dependency injection container such as StructureMap, Unity or Castle Windsor, you can say "hey, anytime someone wants an instance of ICompaniesService give them a new CompaniesService object".
To unit test this class, you can mock our a ICompaniesService and supply it yourself to the constructor. This allows you to setup controlled methods on the mock object. If you couldn't do this, your unit tests for CompaniesController would be limited to using only the one implementation of your companies service, which could hit a live database etc, making your unit tests both slow and inconsistent.
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.
From Wikipedia:
Without the concept of dependency
injection, a consumer who needs a
particular service "ICompaniesService" in order to
accomplish a certain task would be
responsible for handling the
life-cycle (instantiating, opening and
closing streams, disposing, etc.) of
that service. Using the concept of
dependency injection, however, the
life-cycle of a service is handled by
a dependency provider/framework (typically a
container) rather than the consumer.
The consumer would thus only need a
reference to an implementation of the
service "ICompaniesService" that it needed in order to
accomplish the necessary task.
Read this one too:
What is dependency injection?
People use Dependency Injection frameworks because otherwise you have to write a metric ton of boring, repetitive factory classes (if using dependency injection, that is).
It can be done, it's just very, very annoying.
I think your understanding is only partly correct. Dependency Injection is "injecting"
a dependency of a component into it. Its a more specific form of inversion of control. During
this process some dependencies can also be initialized before injecting.
With DI, the onus of looking up the dependency is not on the component
(as in ServiceLoacator pattern) but upto the container in which the component is
running. The pattern has following advantages:
Dependency lookup code can be eliminated from the component.
Some frameworks providing auto-wiring, injecting saving you from manually creating your component
hierarchies (Answers one of your questions)
Allows you to interchange implementations of your dependencies. (without using Factories)
Testing (replacing dependencies with mock implementations)
In your code example, a dependency can be injected by a DI container at runtime via the second
constructor. Other forms of injections are also possible (depending on the DI container). e.g.
Field injection, setter injection, etc.
There's a good article by martin fowler who coined the DI term
You dont have to have a DI framework, but at some point in your codebase a concrete implementation is going to need to be instantiated and injected into your constructors/properties. It can get very messy if you dont have a DI framework, I recommend looking at Castle Windsor although as mentioned there are others that will perform the same functionality. Or if you could role your own....:)
I'll pitch in:
You can accomplish dependency injection simply by having a parameterized function definition.
However, in order to make that work consistently, everyone has to actually do that. Many find that's its easier to enforce the convention by using a factory design pattern.
Dependency injection frameworks solve the problem of reducing the boilerplate of writing those factories.
I would say that dependency injection through a factory, is non-ideal. In my opinion factories add an extra layer of indirection and in a sense make deterministic functions in-deterministic in the sense that they are now a function of the inputs, plus state from the rest of the program (your di setup) which you cannot see from the function definition alone. Factories make code harder as they add an extra layer of indirection I'd argue that in many cases it's really not too hard to follow the convention and manually inject classes via the function arguments. Again though, for a large codebase, it's probably easier to enforce the rule through a factory.
..that being said sometimes I do wonder if these large codebases would even be this large if they didn't write so many things which try to preemptively solve problems that they don't have in the first place.
Related
I have used Unity for my last project and was generally pleased. But benchmarks have me thinking I may go with Simple Injector for my next project.
However, Simple Injector does not seem to have an interface for its Container class. This means that anytime I want to use the container in a method, I cannot mock the container for unit testing.
I am confused how a tool that really functions based of interfaces, would not itself make an interface to the container. I know that the classic methods of dependency injection do not need the container for anywhere more than the startup. (The rest uses constructor injection.) But I have found that when the rubber hits the road that cannot always be true. Sometimes you just need the container in order to do a "resolve" in the code.
If I go with Simple Injector then that code seems to gets harder to unit test.
Am I right? Or am I missing something?
Simple Injector does not contain an IContainer abstraction, because:
It would be useless for Simple Injector to define it,
because in case of depending on IContainer instead of Container, your code would in that case still depend on Simple Injector, and this causes a vendor lock-in, which Simple Injector tries to prevent.
Any code you write, apart from the application's Composition Root, should not depend on the container, nor on an abstraction over the container. Both are implementations of the Service Locator anti-pattern.
You should NOT use a DI library when unit testing. When unit testing, you should manually inject all fake or mock objects in the class under test. Using a container only complicates things. Perhaps you are using a container, because manually creating those classes is too cumbersome for you. This might indicate problems with your code (you might be violating the Single Responsibility Principle) or your tests (you might be missing a factory method to create the class under test).
You might use the container for your integration tests, but you
shouldn't have that many integration tests in the first place. The focus should be on unit tests and this should be easy when applying the dependency injection pattern. On top of that, there are better ways of hiding the container from your integration tests, compared to depending on a very wide library-defined interface.
It is trivial to define such interface (plus an adapter) yourself, which justifies not having it in the library. It is your job as application developer to define the right abstractions for your application as stated by the Dependency Inversion Principle. Libraries and frameworks that tend to do this will fail most of the time in providing an abstraction that works for everyone.
The library itself does not use that abstraction and a library should, according to the Framework Design Guidelines, in that case not define such abstraction for you. As stated in the previous point, Simple Injector would get the abstraction wrong anyway.
Last but not least, the Simple Injector container does actually implement System.IServiceProvider which is defined in mscorlib.dll and can be used for retrieving service objects.
I think the answer given here is entirely founded upon accepting that ServiceLocator is an anti-pattern, which in turn I don't believe is globally accepted as true. See Windows Workflow Foundation's Extensions support.
The anti-pattern link (and its two updates) may also be weak... the latest update claims violation of encapsulation ("relieving you of the burden of having to understand every implementation detail of every piece of code in your code base.") while then at the same time claiming that up-front knowledge of dependencies is somehow different for that claim than discovering them via unit tests. Either way, you're going to need to know what to give it.
All in all, if you want to follow the Locator pattern, either leverage its IServiceProvider, or simplify your container population (to a singleton) and create a static wrapper for it.
I have some questions about DI containers (Unity in particular) and how they actually aid in DI.
I believe I understand IoC/DI and have been using constructor based DI for some years. Usually with my use of DI it involved simply having a constructor on my class, say MyClassX that takes an interface as an argument say IDataService and then using the new operator to create an instance of an IDataService implementing class and pass it into MyClassX's constructor. This way MyClassX doesn't need to know the exact type of the IDataService it is using decoupling it from a specific type. Now correct me if I am wrong, but that's what I understand DI to be...though it does not have to be constructor based.
Now I have seen a stack of examples of Unity on the net, but I am finding it difficult to not only understand everything it does (it seems like a magic object factory to me) but also how it exactly aids in DI as I understand it. To me Unity seems more like a Factory implementation (or a Mock framework?) rather than anything to do specifically with DI. I think I have really missed something though and am waiting for an "ah ha" moment. I have done alot of Googling but examples don't help... I need an theoretical explanation.
Can someone explain to me what Unity is exactly for...the broad points of what it does and how it is related to DI as I understand it.
Your understanding of basic Dependency Injection is correct. Constructor injection is the most common pattern.
Some other DI Unity does:
Lifetime management – instance creation can be singleton, one per
thread, and other advanced models.
Handles dependency graphs - request a root object and Unity creates all its dependency’s
dependencies.
Enables method and property injection – requires a
Unity attribute in your business code (which I prefer to avoid)
Service locator pattern – generally considered an anti-pattern
1 and 2 are nice when you need them. I think #3 and #4 are to be avoided when possible because it adds dependencies in your code to your Unity container.
The big bang that you are missing is Aspect Oriented Programing enabled by Interception with Unity. This allows the implementation of cross cutting concerns. Logging is the classic example. If you want more, start reading all the Enterprise Library Call Handlers for exception handling, validation, etc. or just start searching the web for AOP.
When you combine constructor injection of dependencies with external implementation of cross cutting concerns, you can get very close to business objects that only contain business logic. In a large Enterprise development team, that’s a very big bang.
When the object graph is simple you may not see the obvious advantages of using a DI container.
Say you have a class MyClassX which depends on IExampleA, IExampleB. Now the Implementation of IExampleA and IExampleB may depend on some other classes and so on. Now, this kind of object graph is complex to handle manually when it comes to materialized/instantiate it.
This is where DI comes into play. Once registered(class and its depended classes), All you need to do is,
var myclassX = _container.Resolve<MyClassX>()
And, don't get me wrong here, DI can provide much more than just resolving dependency. For example managing the LifeStyle and LifeCycle of objects etc.
There are some cases in which unit test don't work for the project.
I'm studing the Inversion of Control and Dependency Injection utilities and I would like to know if there are good reasons for use it than for make unit tests easier.
--update
Ok, let's analysis 1 of the cited advanteges: less coupling.
You take out the coupling from the child type, and you add coupling to the handler type who need to create the objects to inject.
Without unit testing, what's the advantage in this coupling transfer (not coupling eliminate).
IOC/DI bring some very important features to your application
Plugability: with DI you can inject dependency into the code without explicitly knowing how the functionality is actually working.
For example: your class might get a ILog interface injected so that it can write logs. Since the class works with the ILog interface, it would be possible to implement a FileLog, MemoryLog or a DatabaseLog & inject this into your class. Any of these implementation will work fine as long as they implement the ILog interface
Testability: With DI in your class, you can inject mock objects to test the behaviour of your class without actually needing the concrete implementation.
For example: consider a Controller class which needs a Repository to perform data operations. In this case, the repository can be DI for the controller. If you need to write tests on the Controller class, you can pass a DI'd mock version of the repository without having to work with the actual repository class
Configurability: Some of the common DI frameworks like Castle Windor, Unity, Spring etc., allow doing DI along with lifetime management of the object created. This is a very powerful feature & allow you to manage the dependencies & their lifetime via configuration. For example consider your application needs a ICache dependency. Via the configuration for lifetime & object management, you will be able to configure the cache to be a Per-application or per-session or per-request etc. without having to explicitly bake the implementation in your code.
HTH
IoC reduces coupling, which has a correlation with defect rates in some studies. (If that really long link doesn't work, it's to Software Engineering Quality Practices by Ronald Kirk Kandt.)
Sure, here are a few reasons:
Dynamic generation of proxies for remoting and transactons
Aspect oriented programming
Layering using interfaces and separation of implementation
Enough?
From the IoC wikipedia article:
There is a decoupling of the execution of a certain task from implementation.
Every system can focus on what it is designed for.
Every system does not make assumptions about what other systems do or should do.
Replacing systems will have no side effect on other systems.
While I would call the above feature list a bit vague, you see most of the above benefits, even without testing.
If I had to say it in a nutshell, I would say that IoC significantly improves separation of concerns which is a valuable goal in software development.
Yes, dependency injection helps you make your classes more focused, clearer*, and easier to change, because it makes it easier to adhere to the single-responsibility principle.
It also makes it easier to vary parts of your application independently of one another.
When you use constructor injection in particular, it's easier to tell what your code needs to do its job. If the WeatherUpdater class requires an IWeatherRepository in its constructor no one is surprised that it uses a database.
* Again, constructor injection only.
In most samples I have seen on the web, DI in MVC Controllers is done like this
public ProductController(IProductRepository Rep)
{
this._rep = Rep;
}
A custom ControllerFactory is used and it utilizes the DI framework of choice and the repository is injected.
Why is the above considered better than
public ProuctController()
{
this._rep = ObjectFactory.GetInstance<IProductRepository>();
}
This will get the same results but doesn't require a custom controller factory.
As far as testing is concerned the Test App can have a separate BootStrapper. That way when the controllers are being tested they can get the fake repositories and when they are used for real they will get the real ones.
Constructor injection (the first approach) is better than the service locator pattern (the second approach) for several reasons.
First, service locator hides dependencies. In your second example, looking at the public interface alone, there's no way to know that ProductControllers need repositories.
What's more, I've got to echo OdeToCode. I think
IProductRepository repository = Mockery.NewMock<IProductRepository>();
IProductController controller = new ProductController(repository);
is clearer than
ObjectFactory.SetFactory(IProductRepository, new MockRepositoryFactory())
IProductController controller = new ProductController();
Especially if the ObjectFactory is configured in a test fixture's SetUp method.
Finally, the service locator pattern is demonstrably sub-optimal in at least one particular case: when you're writing code that will be consumed by people writing applications outside of your control. I wager that people generally prefer constructor injection (or one of the other DI methods) because it's applicable for every scenario. Why not use the method that covers all cases?
(Martin Fowler offers a much more thorough analysis in "Inversion of Control Containers and the Dependency Injection Pattern", particularly the section "Service Locator vs Dependency Injection").
The primary drawback to the second constructor is now your IoC container has to be properly configured for each test. This setup can become a real burden as the code base grows and the test scenarios become more varied. The tests are generally easier to read and maintain when you explicitly pass in a test double.
Another concern is coupling a huge number of classes to a specific DI/IoC framework. There are ways to abstract it away, of course, but you still have code littered throughout your classes to retrieve dependencies. Since all the good frameworks can figure out what dependencies you need by looking at the constructor, it’s a lot of wasted effort and duplicated code.
When you use the second approach, disadvantages are:
Huge and unreadable test setup/context methods are needed
The container is coupled to the controller
You will need to write a lot more code
Why do you want to use an ioc container anyway when you don't want dependency injection?
I'm fairly new to the DI concept, but I have been using it to some extent in my designs - mainly by 'injecting' interfaces into constructors and having factories create my concrete classes. Okay, it's not configuration-based - but it's never NEEDED to be.
I started to look at DI frameworks such as Spring.NET and Castle Windsor, and stumbled across this blog by Ayende.
What I got from this is
A) DI frameworks are awesome, but
B) It means we don't have to worry about how our system is designed in terms of dependencies.
For me, I'm used to thinking hard about how to loosely-couple my system but at the same time have some sort of control over dependencies.
I'm a bit scared of losing this control, and it being just a free-for-all. ClassA needs ClassB = no problem, just ask and ye shall receive! Hmmm.
Or is that just the point and this is the future and I should just go with it?
Thoughts?
One basic OO principle is that you want your code to depend on interfaces and not implementations, DI is how we do that. Historically, here is how it evolved:
People initially created classes they depended upon by "new'ing" them:
IMyClass myClass = new MyClass();
Then we wanted to remove instantiation so there were static methods to create them:
IMyClass myClass = MyClass.Create();
Then we no longer depended on the lifecycle of the class, but still depended on it for instantiation, so then we used the factory:
IMyClass myClass = MyClassFactory.Create();
This moved the direct dependency from the consuming code to the factory, but we still had the dependency on MyClass indirectly, so we used the service locator pattern like this:
IMyClass myClass = (IMyClass)Context.Find("MyClass");
That way we were only dependent on an interface and a name of a class in our code. But it can be made better, why not depend simply on an interface in our code? We can with dependency injection. If you use property injection you would simply put a property setter for the interface you want in your code. You then configure what the actual dependency is outside of your code and the container manages the lifecycle of that class and your class.
I wouldn't say that you don't have to think about dependencies, but using an IoC framework allows you to change the types which fulfill the dependencies with little or no hassle, since all the wiring is done in a central place.
You still have to think about what interfaces you need and getting them right is not always a trivial matter.
I don't see how a loosely coupled system could be considered lazily designed. If you go through all the trouble of getting to know an IoC framework, you're certainly not taking the shortcut.
I think that ideally, if you already have a loosley coupled system.., using a container will only move the place where you take the dependencies out of your code making them softer and let your system depend on the container building your object graph.
In reality, attempting to use the the container will probably show you that your system is not as loosley coupled as you thought it was.. so in this way, it may help you to create a better design.
Well, i'm a newbie at this subjet.. so maybe i'm not that right.
Cheers.
I must be high, because I thought the whole point of dependency injection is that the code that does stuff simply declares its dependencies so that someone who's creating it will know what to create with it for it to operate correctly.
How dependency injection makes you lazy is maybe it forces someone else to deal with dependencies? That's the whole point! That someone else doesn't need to be really someone else; it just means the code you write doesn't need to be concerned with dependencies because it declares them upfront. And they can be managed because they are explicit.
Edit: Added the last sentence above.
Dependency injection can be a bit difficult to get used to - instead of a direct path through your code, you end up looking at seemingly unconnected objects, and a given action traces it's path through a series of these objects whose coupling seems, to be kind, abstract.
It's a paradigm shift similar to getting used to OO. The intention is that your objects are written do have a focused and single responsibility, using the dependent objects as they're declared by the interface and handled by the framework.
This not only makes loose coupling easier, it makes it almost unavoidable, or at least nearly so, which makes it much simpler to do things like run your object in a mock environment - The IOC container is taking the place of the run environment.
I would disagree and say they lead to better design in many cases. Too often devs create components that do too much and have too many dependencies. With IOC developers i find tend to migrate to a better way of thinking and produce smaller simpler components that can be assembled together into an app.s
If they follow the spirit and do tests, they will further refine your components. Both exercises force you to write better testable components which fits very well with how IOC containers work.
You still have to worry. My team use Castle Windsor in our current project. It annoys me that it delays dependency lookup from compile time to runtime.
Without Castle Windsor, you write code and if you haven't sorted your dependencies out. Bang, the compiler will complain. With Castle Windsor you configure the dependencies in an xml file. They're still there, just separated out from the bulk of your code. The problem is, your code can compile fine if you make a mess of defining the dependencies. But, at runtime, Castle Windsor looks up a concrete classes to service requests for an interface by using reflection. If the dependency can't be found, you get an error at runtime.
I think Castle Windsor does check the dependencies exist when its initialized so that it can throw an error pretty quick. But, it's still annoying when using a strongly typed language that this fuss can't be sorted out at runtime.
So... anyway. Dependencies still seriously matter. You'll all most certainly pay more attention to them using DI than before.
We wrote custom DI framework, thought it took some time getting it right but it all worth the effort. We have divided the who systems into layers and the dependency injection in each layer is bound by rules. E.g. In the Log layer, CUD and BO interfaces cannot be injected.
We are still contemplating over the rules and some of these change every week while the others are remain the same.