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Inversion of Control < Dependency Injection
I always read IoC(Inversion of Control) and DI(Dependency Injection) in the same context. What is exactly the difference between IoC and DI? How does IoC differ from DI?
In common usage, the terms have become somewhat synonymous. The original idea of IoC -- Inversion of Control -- was very much related to the "Hollywood Principle:" Don't Call Us, We'll Call You.
In traditional applications, developers would write business code and framework code. The business code would then call the framework code to accomplish tasks. Under an IoC model, you "invert" that model and create a framework that accepts business modules and calls them to accomplish tasks. This principle is exhibited in several development frameworks including the old Smart Client Software Factory and the newer Prism (or Composite Applications for WPF/Silverlight). In these models, the UI Modules are registered with an IoC Container and loaded as needed based on configuration and user actions. While powerful, these models also tend to have a very steep learning curve.
Dependency Injection is a technique (hard to call it a pattern, really) of removing internal dependencies from implementations by allowing dependent objects to be injected into the class/method by an external caller. IoC frameworks use dependency injection to supply user modules and other dependent code to framework routines that "glue it all together." Dependency injection is used heavily by IoC frameworks because that is the mechanism that allows them to "Call You."
IoC Containers, such as Castle Windsor and Structure Map help with dependency injection by providing automatic instantiation and lifecycle management of classes you register -- including automatic instantiation and injection of parameters required by registered classes. All of this makes it easier to use dependency injection, but is not required.
Dependency Injection is a mechanism for flexibility that maximizes dependency on Interfaces while minimizing dependency on specific implementation. Consequently, systems that use dependency injection can support "pluggable" implementation classes that can be used depending on circumstances. A very big benefit of this "pluggability" is that it makes creating Unit Tests much easier. You can mock an object to the needed interface, then inject it into your test object.
So, IoC is really the broader principle and DI is a fundamental technique within. IoC (Hollywood Principle) systems tend to be pretty complex, can be hard to understand, and thus have steep learning curves. DI, on the other hand is a good practice for everyday developers. I tend to prefer understandable and clear over cool, but complex.
IoC is the ability to vary the implementation of a contract.
DI is the ability to supply the implementation.
Related
first, I'm new to the software world. so my apologies if this is a simple or bad question.
I've been reading about DI and IOC and I understood DI is one way of achieving IOC ( there are other ways as well). so it kept me thinking, does DI always result in Inversion of Control? does it depend on how you call the methods or classes that benefit from DI? in other words, the flow of your application will decide whether or not DI will result in IOC, is this right?
I think this quote from Dependency Injection Principles, Practices, and Patterns (DIPP&P) pretty much sums it up:
Dependency Injection or Inversion of Control?
The term Inversion of Control originally meant any sort of programming style where an overall framework or runtime controlled the program flow. According to that definition, most software developed on the .NET Framework uses IoC. When you write an ASP.NET Core MVC application, for instance, you create controller classes with action methods, but it’s ASP.NET Core that will be calling your action methods. This means you aren’t in control — the framework is.
These days, we’re so used to working with frameworks that we don’t consider this to be special, but it’s a different model from being in full control of your code. This can still happen for a .NET application, most notably for command-line executables. As soon as Main is invoked, your code is in full control. It controls program flow, lifetime — everything. No special events are being raised and no overridden members are being invoked.
Before DI had a name, people started to refer to libraries that manage Dependencies as Inversion of Control Containers, and soon, the meaning of IoC gradually drifted towards that particular meaning: Inversion of Control over Dependencies. Always the taxonomist, Martin Fowler introduced the term Dependency Injection to specifically refer to IoC in the context of dependency management. Dependency Injection has since been widely accepted as the most correct terminology. In short, IoC is a much broader term that includes, but isn’t limited to, DI.
[source: Section 1.4.1, page 29. Online version]
This quote is written in the context of .NET, but if you filter out the .NET and ASP.NET, it is applicable broadly. In Martin Fowler's original definition, IoC was about frameworks, while DI in itself can be applied without any frameworks, for instance using 'simple' (UI-less) Console applications. This means that, from Fowler's original definition, it is possible to practice DI without applying IoC.
Over the years the term IoC, however, has "drifted" and we now generally see DI as as specialized form of IoC. A "sub type" or "implementation" so to speak, where IoC is the concept or "abstraction." This means that you can't apply DI without IoC, because practicing DI means practicing IoC. You can, however, practice IoC, without practicing DI as there are other IoC implementations, such as Service Locator.
I have been searching about the dependency injection and read a few articles.
But still I am unable to find out the difference between MEF and other IoC's.
So, my question is this: In which situation should I prefer to use a MEF or IoC container?
Why is it good to use MEF with PRISM for (WPF & Silverlight) or for desktop applications?
Whereas in web application people use IoC containers.
So, what is the criteria to decide which dependency technique I should use?
I have been through the article http://devlicio.us/blogs/casey/archive/2009/12/18/what-is-the-difference-between-an-ioc-container-and-mef.aspx
, but I could not determine anything.
Eventually what I have concluded about the MEF vs IoC container is as follows:
MEF is preferred to be used when one has to deal with unknown types or a plugin based architecture.
IoC containers are preferred to be used with known types.
Moreover, MEF is an architectural solution for dependency injection
Whereas, IoC containers are code-level solutions for dependency injection.
IoC containers are just dependency injection techniques which populates the instance of a class and if the constructor of those classes requires objects of other classes, then IoC also injects the required objects. But MEF does more than just dependency injection. Although, MEF also uses an IoC-based approach for dependency injection, but MEF does so many other things apart from dependency injection.
MEF has got two components:
Catalog: Is responsible for discovering extension
Container: Provides the ability to load an extension to a running
application
MEF is more than just dependency injection techniques. It is used wherein we need a plugin-based architecture for our application, but at the same time MEF uses an IoC-based approach for dependency injection.
I am expecting more people to comment on this.
IoC is an architectural design strategy, and MEF is an implementation of the design pattern dependency injection. Dependency injection (DI) is often the implementation strategy of IoC. Often the term IoC container is used, suggesting that IoC is the technique.
No, it is otherwise. IoC is a broad concept and DI is the design pattern to implement the core of IoC. MEF is some form of DI, but it has not all fundamental features of IoC.
MEF uses composition to find out the dependencies it needs to resolve. That is much like a lot of other IoC containers, for instance Pico and Spring. But it stops there. I did not see any life cycle management nor pooling configuration. The latter two do I consider to be a fundamental part of IoC (not of DI), because the performance of the caller should not suffer because of the memory consumption used by the callee.
The IoC principle is a service to the caller and the callee by loosely coupling them. That way both functionalities can work optimized. MEF might have the problem that there are issues with optimization. For instance, when you have a call from the menu to a database, then at some time a call to the database will be made. It is always best to use pooling for that. MEF is not capable of doing that.
The type of application should be independent of the choice for a design pattern. There is not a big difference between a desktop or a web application. Both are user interfaces, and both should be able to use MEF and IoC. If the functionality is straightforward and does not need to cross optimization boundaries (like database calls), then MEF is the first choice, because it is a framework that is present when using .NET 4. Then it could be useful, but if a call crosses an optimization boundary (like the parsing or uploading of a file), then the use of an IoC container is more fruitful for performance and maintenance.
Information I used:
Hanselminutes 148 (podcast interview with Glenn Block).
Managed Extensibility Framework (MEF) (MSDN)
Cutting Edge - Application Extensibility: MEF vs. IoC
Inversion of Control Containers and the Dependency Injection pattern
Inversion of control (Wikipedia)
Why exactly isn't MEF a DI/IoC container?
I've read a number of threads explaining the difference between IoC and DI and while many of explanations contradicted each other, I think they still helped me understand the difference.
So here I'd like to ask whether my understanding is correct and also post excerpts that helped me ( though some of them contradict each other ).
I know there have been lots of threads done on the subject, but I'm hoping this thread won't get closed since I don't think any of OPs in mentioned threads also showed all the relevant posts ( from various threads ) that helped them finally understand it.
Anyways, here is how I understand it (if possible,please address/answer each question individually ):
a) When we apply DIP principle at the framework level, then we use the term IoC? And one of the mechanisms to implement DIP at framework level is DI?
b) The term IoC does not apply when we implement DIP ( using DI ) at a lower level/non-framework level, in which case we simply call it DI?
c) DI helps us to achive DIP by passing the control of the actual creation of and selection of dependencies to a 3rd party which is neutral to either of the other 2 involved?
d) When DIP is applied ( using DI ) at framework level (IoC), then three types of control get inverted:
The control of the interface. Now high level module is controlling the interface that the lower level modules need to adhere to instead of the other way around
The control of the flow. --> Now framework code ( instead of user/business code ) controls the flow of the program ( in other words - they ( ie framework ) call you ( ie business code ) )
The control of dependency creation. This inversion is passing the control of the actual creation and selection of dependencies to a 3rd party which is neutral to either of the other 2 involved.
e) When DIP is applied (using DI) at non-framework level, then two types of control get inverted:
The control of the interface. Now high level module is controlling the interface that the lower level modules need to adhere to instead of the other way around
The control of dependency creation. This inversion is passing the control of the actual creation and selection of dependencies to a 3rd party which is neutral to either of the other 2 involved.
?
Here are excerpts that helped:
Why so many terms to say the same thing? IoC and DIP
Inversion of Control is the generic term. Dependency Injection is a
specific type of IoC
...
Inversion of Control is when the framework/infrastructure invokes
application code, rather than the other way around
...
can do DI without doing IoC. If you inject aConsoleStringWriter into a
HelloWorld I don't really think of this as IoC because there is no
"framework" or "infrastructure".
Inversion of Control < Dependency Injection
If you accept Fowler's definition, Inversion of Control is a much
broader term than DI that covers allframework usage where you plug
into a framework, but the framework is still in control. Dependency
Injection is a specialization of IoC that applies IoC specifically to
manage dependencies.
Where exactly is the difference between IoC and DI
IoC is the ability to vary the implementation of a contract. DI is the
ability to supply the implementation.
...
In traditional applications, developers would write business code and
framework code. The business code would then call the framework code
to accomplish tasks. Under an IoC model, you "invert" that model and
create a framework that accepts business modules and calls them to
accomplish tasks
Dependency Injection is a technique (hard to call it a pattern,
really) of removing internal dependencies from implementations by
allowing dependent objects to be injected into the class/method by an
external caller. IoC frameworks use dependency injection to supply
user modules and other dependent code to framework routines that "glue
it all together." Dependency injection is used heavily by IoC
frameworks because that is the mechanism that allows them to "Call
You."
DIP vs. DI vs. IoC
DIP is the principle that guides us towards DI. Basically, loose
coupling is the goal, and there are at least two ways to achieve it.
• Dependency Injection • Service Locator
Does anyone have a good analogy for dependency injection?
The essence of Inversion of Control (of which Dependency Injection is
an implementation) is the separation of the use of an object from the
management thereof.
Difference between ioc and dependency injection
The terms Dependency Injection (DI) & Inversion of Control (IoC) are
generally used interchangeably to describe the same design pattern
(although not everyone agrees on that point, and some people tend to
apply them in slightly different ways). The pattern was originally
called IoC, but Martin Fowler proposed the shift to DI because all
frameworks invert control in some way and he wanted to be more
specific about which aspect of control was being inverted.
Inversion of Control vs Dependency Injection
Inversion of Control (IoC) means that objects do not create other
objects on which they rely to do their work. Instead, they get the
objects that they need from an outside source (for example, an xml
configuration file). Dependency Injection (DI) means that this is done
without the object intervention, usually by a framework component that
passes constructor parameters and set properties.
thank you
Well this is my point of view:
DIP means that you program against an abstraction. You invert the kind of a dependency from an implementation to an abstraction.
IOC means that somebody else is responsible for getting the implementation for the given abstraction. Normally the consumer would use the new keyword to get a dependency. With IoC you invert the control, so that the consumer is not responsible for creating the instance anymore.
Dependency Injection and Service Location are a part of Inversion of Control.
See also: https://stackoverflow.com/a/10053413/175399
I'm reading theory about dependency inversion and decoupling and I can't see the difference between the two.
Dependency inversion talks about decoupling functional components so that higher level components don't depend on lower level components.
Decoupling talks about the same thing and how to achieve it. But then we have IoC Containers that mess things up even further. Why aren't they rather called Dependency Inversion Containers or even better Dependency Injection Containers, because they serve runtime coupling of independent components?
Then we have Inversion of Control. It's basically the same thing as Dependency Inversion isn't it? Why are there three terms that describe the same thing? Or am I blind?
What is the difference between the three?
What does IoC have to do in IoC Containers?
Decoupling is a very general principle applicable in many fields. Dependency inversion is a specific form of decoupling where you decouple the higher levels of your system from the lower levels by separating them into libraries and using interfaces. This allows you to replace lower level parts of your system without major rework.
For example, instead of the higher level parts of the system creating concrete instances of the lower level classes, an IoC container can be used to decouple how objects are created.
Inversion of control is a design principle used by framework libraries that allow the framework to regain some control from the application. I.e., a windowing framework may call back into application code when certain user interface events occur. Martin Fowler uses the term Hollywood Principle as in Don't call us, we'll call you. Decoupling is an important part of inversion of control.
But what has an IoC container to do with inversion of control? To quote Martin Fowler:
Inversion of Control is too generic a term, and thus people find it confusing. As a result with a lot of discussion with various IoC advocates we settled on the name Dependency Injection.
(Note that Martin Fowler talks about dependency injection, not dependency inversion.)
An IoC container helps to implement dependency injection and perhaps a better term would be dependency injection container. However, the IoC container name seems to stick. Dependency injection is an important component in dependency inversion, but the use of IoC containers for dependency injection can be confusing as inversion of control is a broader and more generic principle.
You point out that the naming isn't very consistent but that shouldn't be a big surprise as these terms have been independently invented and used even though they overlap.
Dependency Injection achieves Decoupling using Inversion of Control.
I find the following explanation from DIP in the Wild article on martinfowler.com straightforward to understand (here DI = Dependency Injection, DIP = Dependency Inversion Principle, IoC = Inversion of Control):
DI is about how one object acquires a dependency. When a dependency is
provided externally, then the system is using DI. IoC is about who
initiates the call. If your code initiates a call, it is not IoC, if
the container/system/library calls back into code that you provided
it, is it IoC.
DIP, on the other hand, is about the level of the abstraction in the
messages sent from your code to the thing it is calling. (...) DI is about
wiring, IoC is about direction, and DIP is about shape [of the object
upon which the code depends].
Dependency inversion: Depend on abstractions, not on concretions.
Inversion of control: Main vs Abstraction, and how the Main is the glue of the systems.
These are some good posts talking about this:
https://coderstower.com/2019/03/26/dependency-inversion-why-you-shouldnt-avoid-it/
https://coderstower.com/2019/04/02/main-and-abstraction-the-decoupled-peers/
https://coderstower.com/2019/04/09/inversion-of-control-putting-all-together/
What is an IoC container?
What is an IoC/DI framework?
Why do we need a framework for IoC/DI?
Is there any relationship between IoC/DI and AOP?
What is Spring.net/ninject with respect to IoC and AOP?
JMSA,
James Kovacs wrote a fantastic article which covers many of your questions I would recommend reading it Here
Spring.Net, Ninject, Unity, Castle Windsor, Autofac are all IOC containers which are configurable in different ways, many of them do also support AOP.
Frameworks for IOC/DI are useful because they provide standard mechanisms, for example if you are hiring a new developer it is much easier to say, we use this framework and pass them the links to the tutorials / help guide. At the same time these frameworks are tried and tested by a large community / companies.
Let me know if any of your questions remain unanswered after reading the article and the above answes and I'll do my best to provide further assistance.
From a semantics point of view...
Dependency Injection itself implies a dependency, i.e. something that is required for construction/use (the application's "core concerns"). For example, a car is not a car without an engine.
Aspects are described as being cross-cutting to the application's core concerns. That means both separate from and non-crucial to the core concerns (you could think of them as "nice-to-haves"). Since the application can run without aspects, are they really dependencies? For example, a car is still a car even without an immobiliser.
(Of course, this is from a theoretical standpoint. In the real world matters like security are often as crucial to the existence of a marketable product as the core concerns themselves.)
So in while in practice DI can be used to implement aspects, I would not call that process true DI. This is coming from someone who uses constructor injection exclusively.
Martin Fowler has a good article here on the meaning of Inversion of Control and Dependency Injection.
Spring.NET usage AOP is described in detail here. I'm more familiar with the Java-based version of Spring, so I cannot say with absolutely certainty that Spring.NET currently only supports proxy-based AOP.
That is, a class to be advised must implement an interface. Spring will create a dynamic proxy that implements this interface and delegates to the original target instance.
Although it does state:
In a future release we will implement proxies using inheritance, which will allow you to proxy classes without interfaces as well and will remove some of the remaining raw reference issues that cannot be solved using composition-based proxies.