This question already has answers here:
What is dependency injection?
(37 answers)
Closed 7 years ago.
TL;DR Version
Can you explain the concept of dependency injection to an 'enthusiast programmer' with a fundamental understanding of programming. i.e. Classes, functions and variables.
What is the purpose of dependency injection, is it purely a readability / ease of programming concept or does it provide compile time benefits also?
My original more waffly version!
My coding skills are reasonably rudimentary. (it isn't my primary skill, but it does sometimes come in handy to proof of concept something) I can hack stuff together and make things work, but I'm always perfectly aware that there are a host of better / more efficient ways of doing things.
Primarily I bounce things around between functions and classes and variables! (like what I learnt on my c64 a long time ago!)
Dependency injection seems to be everywhere lately, and while I think I kind of get it, I feel like I'm missing a point (or something)
The problem seems to be when I try to google around to understand what it is at a fundamental level, it very quickly gets very complicated and my head hurts (I'm a bear of very small brain and big words confuse me!)
So I'm hoping someone can explain dependency injection to a five year old! What is it, why do I need it, how does it make my coding better. (ideally working in concepts of functions, classes and variables!)
This is largely language independent, it seems to be a thing that all languages use, but my language of choice is usually C# (typically ASP/MVC though some native Windows / Console) though I've recently started poking around with Nodejs also.
Actually it seems this is a duplicate of this question - What is dependency injection?
(which fared much better than my version! - that's what I get for waffling)
Dependency Injection allows the developer to put a class reference into a project a will without having to create it as an object.
In the case of spring, where I have most knowledge of DI, I would like the classpath in a config.xml file and I can pass this class reference to any class were I need it to be called. The class being injected is like a singleton as only that one reference is being passed without declaring it as a singleton. It is usually a service class that is injected.
Dependency injection allows a client the flexibility of being configurable. Only the client's behaviour is fixed. This can save time swapping out what is to be injected in an xml file without the need to recompile the project.
This is why I say it is more like a singleton using only one instance
public class Foo {
// Constructor
public Foo() {
// Specify a specific implementation in the constructor instead of using dependency injection
Service service1 = new ServiceExample();
}
private void RandomMethod() {
Service service2 = new ServiceExample();
}
}
Here the one service is used twice because two instances are created. I have seen projects where class files have become so big where a service class was created three times through out the one class in different methods.
public class Foo {
// Internal reference to the service
private Service service1;
// Constructor
public Foo(Service service1) {
this.service1 = service1;
}
}
The same issuse can be created in the second example but by having all dependencies in the constructor, it makes it a little more obvious to the developer looking at the code what is being used and that the service has already been created from the start.
Code Injection may have various meanings depending on context.
For example, in a security context it can mean malicious code being injected into your application (eg. sql-injection).
In other contexts (e.g aspect-oriented programming) it might mean a way to patch a method with additional code for an aspect.
Dependency Injection is something different and means a way for one part of code (e.g a class) to have access to dependencies (other parts of code, e.g other classes, it depends upon) in a modular way without them being hardcoded (so they can change or be overriden freely, or even be loaded at another time, as needed)
Related
This question already has answers here:
Is there a pattern for initializing objects created via a DI container
(5 answers)
Closed 7 years ago.
I'm working on a project in which my constructors contain - only - behavioral dependencies. i.e. I never pass values / state.
Example:
class ProductProcessor : IProductProcessor
{
public double SomeMethod(){ ... }
}
class PackageProcessor
{
private readonly IProductProcessor _productProcessor;
private double _taxRate;
public PackageProcessor(IProductProcessor productProcessor)
{
_productProcessor = productProcessor;
}
public Initialize(double taxRate)
{
_taxRate = taxRate;
return this;
}
public double ProcessPackage()
{
return _taxRate * _productProcessor.SomeMethod();
}
}
In order to pass state, it was decided to include a second step (a call to Initialize).
I know we can configure this as a named parameter in the IoC Container config class, however, we did not like the idea of creating "new namedParameter(paramvalue)'s" in the configuration file as it makes it unnecessarily unreadable and creates a future maintenance pain spot.
I've seen this pattern in more than one place.
Question: I read some consider this two step initialization an anti-pattern. If that is the consensus, wouldn't this imply a limitation / weakness of sorts in the approach of dependency injection via a IoC container?
Edit:
After looking into Mark Seeman's suggestion:
and the answers to this one, I have a few comments:
Initialize/Apply : Agree on it being an anti pattern / smell.
Yacoub Massad: I agree IoC containers are a problem when it comes to primitive dependencies. Manual (poor man's) DI, as described here sounds great for smaller or architecturally stable systems but I think it could become very hard to maintain a number of manually configured composition roots.
Options:
1)Factories as dependencies (when run time resolution is required)
2) Separate stateful object from pure services as described here.
(1): This is what I had been doing but I realized that there is a potential to incur into a another anti-pattern: the service locator.
(2): My preference for my particular case is this one about this one as I can cleanly separate both types. Pure services are a no brainer - IoC Container, whereas stateful object resolution will depend on whether they have primitive dependencies or not.
Every time I've 'had' to use dependency injection, it has been used in a dogmatic way, generally under the orders of a supervisor bent on applying DI with IoC container at any cost.
I read some consider this two step initialization an anti-pattern
The Initialize method leads to Temporal Coupling. Calling it an anti-pattern might be too strict, but it sure is a Design Smell.
How to provide this value to the component depends on what type of value it is. There are two flavors: configuration values and runtime values:
Configuration Values: If it is a constant/configuration value that won't change during the lifetime of the component, the value should be injected into the constructor directly.
Runtime values: In case the value changes during runtime (such as request specific values), the value should not be provided during initialization (neither through the constructor nor using some Initialize method). Initializing components with runtime data actually IS an anti-pattern.
I partly agree with #YacoubMassad about the configuration of primitive dependencies using DI containers. The APIs provided by containers do not enable setting those values in a maintainable way when using auto-wiring. I think this is mainly caused by limitations in C# and .NET. I struggled a long time with such API while designing and developing Simple Injector, but decided to leave out such API completely, because I didn't find a way to define an API that was both intuitive and lead to code that was easy maintainable for the user. Because of this I usually advise developers to extract the primive types into Parameter Objects and instead register and inject the Parameter Object into the consuming type. In other words, a TaxRate property can be wrapped in a ProductServiceSettings class and this Parameter Object can be injected into ProductProcessor.
But as I said, I only partly agree with Yacoub. Although it is more practical to compose some of your objects by hand (a.k.a. Pure DI), he implies that this means you should abandon DI containers completely. IMO that is too strongly put. In most of the applications I write, I batch-register about 98% of my types using the container, and I hand-wire the other two 2%, because auto-wiring them is too complex. This gives in the context of my applications the best overall result. Of course, you're mileage may vary. Not every application really benefits from using a DI container, and I don't use a container myself in all the application I write. But what I always do however, is apply the Dependency Injection pattern and the SOLID principles.
The taxRate in your example is a Primitive Dependency. And primitive dependencies should be injected normally in the constructor like the other dependencies. Here is how the constructor would look like:
public PackageProcessor(IProductProcessor productProcessor, double taxRate)
{
_productProcessor = productProcessor;
_taxRate = taxRate;
}
The fact that DI containers do not nicely/easily support primitive dependency is a problem/weakness of DI containers in my opinion.
In my opinion, it is better to use Pure DI for object composition instead of a DI container. One reason is that it supports easier injection of primitive dependencies. See this article also for another reason.
Using the Initialize method has some problems. It makes the construction of an object more complex by requiring the invocation of the Initialize method. Also, a programmer might forget to call the Initialize method, which leaves your object in an invalid state. This also means that the taxRate in this example is a hidden dependency. Programmers wouldn't know that your class depends on such primitive dependency by simply looking into the constructor.
Another problem with the Initialize method is that it might be called twice with different values. Constructors on the other hand, ensure that dependencies do not change. You would need to create a special boolean variable (e.g. isInitialized) to detect if the Initialize method has been called already. This just complicates things.
I have three layers in my solution, Presentation (winforms project), Domain (class lib) and Persistence (class lib).
I am trying to implement dependency injection to decouple them.
My application root is in my presentation layer.
I understand the only time the DI container (unity in this case) should be referenced is in my application root, otherwise I would be simply replacing class dependencies all over the place with a dependency on my DI container (which I suppose is still slightly better).
So with these foundation concepts in mind, I am really struggling with the specific implementation. Perhaps my application root should be in its own seperate project - perhaps a console application. I can then resolve the first 'overallApplication' class, listing IPresentation, IDomain and IPersistence in its constructors. I understand (assuming actual implementations have been registered) the unity framework would then recursively solve all respective sub-dependencies.
In your experience - would you be able to advise if this was a sound approach. I really understand the concept and importance of decoupling, and how this is solved by DI conceptually at a high level, but I am struggling to tie it all together in a real application solution with multiple layers (organised in VS as seperate projects).
Any help or pointers towards examples of proper implementations would be greatly appreciated.
Some thoughts for you in hope that they help.
Throughout your question you make statements such as:
"I really understand the concept and importance of decoupling, and how this is solved by DI..."
The first thing I would evaluate is the understanding that DI != IoC Container (e.g. Unity).
An IoC container is used to remove boilerplate code that results from an existing DI structure. As such I would suggest you refactor without Unity /first/. Then go back and add Unity to reduce your code.
So that:
-1. Make application Di via manual Ctor, Property, Method, Service Locator injection methods.
-2. After this is setup you should see things like:
public View() {
var controller = new Controller(new IView(), new model(), new IService(new Dal(ISession(connectionString))), new , new ILogger(), etc.){}
}
-3. Then once you have something like this in your code you can then use Unity to inject all that fun:
public View() {}
Controller Controller {get;set;} //<- Unity auto builds & populates this for you with all the "new" items normally found in your constructor (or wherever).
While not a production example, it should give an idea of some steps to refactor. Going straight to Unity will put the cart before the horse, so to speak.
This may seem obvious to most people, but I'm just trying to confirm that Dependency Injection (DI) relies on the use of Interfaces.
More specifically, in the case of a class which has a certain Interface as a parameter in its constructor or a certain Interface defined as a property (aka. Setter), the DI framework can hand over an instance of a concrete class to satisfy the needs of that Interface in that class. (Apologies if this description is not clear. I'm having trouble describing this properly because the terminology/concepts are still somewhat new to me.)
The reason I ask is that I currently have a class that has a dependency of sorts. Not so much an object dependency, but a URL. The class looks like this [C#]:
using System.Web.Services.Protocols;
public partial class SomeLibraryService : SoapHttpClientProtocol
{
public SomeLibraryService()
{
this.Url = "http://MyDomainName.com:8080/library-service/jse";
}
}
The SoapHttpClientProtocol class has a Public property called Url (which is a plain old "string") and the constructor here initializes it to a hard-coded value.
Could I possibly use a DI framework to inject a different value at construction? I'm thinking not since this.Url isn't any sort of Interface; it's a String.
[Incidentally, the code above was "auto-generated by wsdl", according to the comments in the code I'm working with. So I don't particularly want to change this code, although I don't see myself re-generating it either. So maybe changing this code is fine.]
I could see myself making an alternate constructor that takes a string as a parameter and initializes this.Url that way, but I'm not sure that's the correct approach regarding keeping loosely coupled separation of concerns. (SoC)
Any advice for this situation?
DI really just means a class wont construct it's external dependencies and will not manage the lifetime of those dependencies. Dependencies can be injected either via constructor, or via method parameter. Interfaces or abstract types are common to clarify the contract the consumer expects from its dependency, however simple types can be injected as well in some cases.
For example, a class in a library might call HttpContext.Current internally, which makes arbitrary assumptions about the application the code will be hosted in. An DI version of the library method would expect a HttpContext instance to be injected via parameter, etc.
It's not required to use interfaces -- you could use concrete types or abstract base classes. But many of the advantages of DI (such as being able to change an implementation of a dependancy) come when using interfaces.
Castle Windsor (the DI framework I know best), allows you to map objects in the IoC container to Interfaces, or to just names, which would work in your case.
Dependency Injection is a way of organizing your code. Maybe some of your confusion comes from the fact that there is not one official way to do it. It can be achieved using "regular" c# code , or by using a framework like Castle Windsor. Sometimes (often?) this involves using interfaces. No matter how it is achieved, the big picture goal of DI is usually to make your code easier to test and easier to modify later on.
If you were to inject the URL in your example via a constructor, that could be considered "manual" DI. The Wikipedia article on DI has more examples of manual vs framework DI.
I would like to answer with a focus on using interfaces in .NET applications. Polymorphism in .NET can be achieved through virtual or abstract methods, or interfaces.
In all cases, there is a method signature with no implementation at all or an implementation that can be overridden.
The 'contract' of a function (or even a property) is defined but how the method is implemented, the logical guts of the method can be different at runtime, determined by which subclass is instantiated and passed-in to the method or constructor, or set on a property (the act of 'injection').
The official .NET type design guidelines advocate using abstract base classes over interfaces since they have better options for evolving them after shipping, can include convenience overloads and are better able to self-document and communicate correct usage to implementers.
However, care must be taken not to add any logic. The temptation to do so has burned people in the past so many people use interfaces - many other people use interfaces simply because that's what the programmers sitting around them do.
It's also interesting to point out that while DI itself is rarely over-used, using a framework to perform the injection is quite often over-used to the detriment of increased complexity, a chain-reaction can take place where more and more types are needed in the container even though they are never 'switched'.
IoC frameworks should be used sparingly, usually only when you need to swap out objects at runtime, according to the environment or configuration. This usually means switching major component "seams" in the application such as the repository objects used to abstract your data layer.
For me, the real power of an IoC framework is to switch implementation in places where you have no control over creation. For example, in ASP.NET MVC, the creation of the controller class is performed by the ASP.NET framework, so injecting anything is impossible. The ASP.NET framework has some hooks that IoC frameworks can use to 'get in-between' the creation process and perform their magic.
Luke
This question already has answers here:
Closed 13 years ago.
Possible Duplicate:
What is dependency injection?
Other developers on my team keep talking about dependency injection. I have looked it up on Wikipedia but I still don't understand exactly what it is or when I would want to use it in my designs. If you have any good examples of when you have used it or when you shouldn't that would really help me out. Thanks
The basic idea is that when an object needs some other other to do it's work (say for example, a database connection), instead of creating that object internally, the object is "injected" into the object, usually either as a constructor parameter, or by a public property that is set before the object is used.
The advantage of that is that the value of the used object can be changed externally (this is especially true if the object is declared as an interface). One common use of this is to replace concrete object with mock object for unit testing.
Another term of reference which might be of assistance is "Inversion of Control".
Rather than constructing your software with dependencies and assumptions about the libraries you will continue to use with that application forever, IoC or DI allow you to specify an interface which must be satisfied by some other component, then at runtime supply a mapping of interface-satisfying components for the executing application (usually in a service container which provides the IoC satisfying service as some variety of service-resolution service).
This abstraction allows you to more readily replace an implementation which no longer meets your organization's needs with a new version, or even an entirely new backing technology, with a smaller footprint of change and thus lower risk.
Castle Windsor is one .Net implementation of an IoC container.
You might find this article useful in understanding this whole idea.
The idea is to help you decouple your clases so that each individual class can be used or tested independently.
I find the dependency injection article from Misko Hevery very explanatory including great example (java). Have a look at the series of articles around the topic.
I'm a long-time Windows developer, having cut my teeth on win32 and early COM. I've been working with .NET since 2001, so I'm pretty fluent in C# and the CLR. I'd never heard of Castle Windsor until I started participating in Stack Overflow. I've read the Castle Windsor "Getting Started" guide, but it's not clicking.
Teach this old dog new tricks, and tell me why I should be integrating Castle Windsor into my enterprise apps.
Castle Windsor is an inversion of control tool. There are others like it.
It can give you objects with pre-built and pre-wired dependencies right in there. An entire object graph created via reflection and configuration rather than the "new" operator.
Start here: http://tech.groups.yahoo.com/group/altdotnet/message/10434
Imagine you have an email sending class. EmailSender. Imagine you have another class WorkflowStepper. Inside WorkflowStepper you need to use EmailSender.
You could always say new EmailSender().Send(emailMessage);
but that - the use of new - creates a TIGHT COUPLING that is hard to change. (this is a tiny contrived example after all)
So what if, instead of newing this bad boy up inside WorkflowStepper, you just passed it into the constructor?
So then whoever called it had to new up the EmailSender.
new WorkflowStepper(emailSender).Step()
Imagine you have hundreds of these little classes that only have one responsibility (google SRP).. and you use a few of them in WorkflowStepper:
new WorkflowStepper(emailSender, alertRegistry, databaseConnection).Step()
Imagine not worrying about the details of EmailSender when you are writing WorkflowStepper or AlertRegistry
You just worry about the concern you are working with.
Imagine this whole graph (tree) of objects and dependencies gets wired up at RUN TIME, so that when you do this:
WorkflowStepper stepper = Container.Get<WorkflowStepper>();
you get a real deal WorkflowStepper with all the dependencies automatically filled in where you need them.
There is no new
It just happens - because it knows what needs what.
And you can write fewer defects with better designed, DRY code in a testable and repeatable way.
Mark Seemann wrote and excellent book on DI (Dependency Injection) which is a subset of IOC. He also compares a number of containers. I cannot recommend this book enough. The book's name is: "Dependency Injection in .Net" https://www.manning.com/books/dependency-injection-in-dot-net
I think IoC is a stepping stone in the right direction on the path towards greater productivity and enjoyment of development team (including PM, BA an BOs). It helps to establish a separation of concerns between developers and for testing. It gives peace of mind when architecting which allows for flexibility as frameworks may come in and out.
The best way to accomplish the goal that IoC (CW or Ninject etc..) takes a stab at is to eliminate politics #1 and #2 remove need for developers to put on the facade of false understanding when developing. Do these two solutions not seem related to IoC? They are :)
Castle Windsor is Dependency Injection container. It means with the help of this you can inject your dependencies and use them without creating them with the help of new keyword.
e.g. Consider you have written a repository or a service and you wish to use it at many places, you need to first register your service / repository and you can start using it after injecting it on the required place.
You can take a look at the below tutorial which I followed to learn castle windsor.
link.
Hope it will help you.
Put simply. Imagine you have some class buried in your code that needs a few simple config values to do its job. That means everything that creates an instance of that class needs to get those dependencies, so you usually end up having to refactor loads of classes along the way to just pass a bit of config down to where the instance gets created.
So either lots of classes are needlessly altered, you bunch the config values into one big config class which is also bad... or worst still go Service Locator!
IoC allows your class to get all its depencencies without that hassle, and manages lifetimes of instances more explicitly too.