Why do I need an IoC container as opposed to straightforward DI code

Question

I ve been using Dependency Injection  DI  for a while  injecting either in a constructor  property  or method   I ve never felt a need to use an Inversion of Control  IoC  container   However  the more I read  the more pressure I feel from the community to use an IoC container   I played with  NET containers like StructureMap  NInject  Unity  and Funq   I still fail to see how an IoC container is going to benefit   improve my code   I m also afraid to start using a container at work because many of my co-workers will see code which they don t understand  Many of them may be reluctant to learn new technology   Please  convince me that I need to use an IoC container   I m going to use these arguments when I talk to my fellow developers at work

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Dittos about Unity  Get too big  and you can hear the creaking in the rafters    It never surprises me when folks start to spout off about how clean IoC code looks are the same sorts of folks who at one time spoke about how templates in C   were the elegant way to go back in the 90 s  yet nowadays will decry them as arcane  Bah

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In my opinion the number one benefit of an IoC is the ability to centralize the configuration of your dependencies   If you re currently using Dependency injection your code might look like this  public class CustomerPresenter     public CustomerPresenter     this new CustomerView    new CustomerService            public CustomerPresenter ICustomerView view  ICustomerService service             init view service fields          readonly view service fields     If you used a static IoC class  as opposed to the  IMHO the more confusing  configuration files  you could have something like this   public class CustomerPresenter     public CustomerPresenter     this IoC Resolve lt ICustomerView gt     IoC Resolve lt ICustomerService gt             public CustomerPresenter ICustomerView view  ICustomerService service             init view service fields          readonly view service fields     Then  your Static IoC class would look like this  I m using Unity here   public static IoC      private static readonly IUnityContainer  container     static IoC             InitializeIoC             static void InitializeIoC               container   new UnityContainer           container RegisterType lt ICustomerView  CustomerView gt            container RegisterType lt ICustomerService  CustomerService gt              all other RegisterTypes and RegisterInstances can go here in one file           one place to change dependencies is good

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Using a container is mostly about changing from an imperative scripted style of initialization and configuration to a declarative one   This may have a few different beneficial effects    Reducing hairball main-program startup routines  Enabling fairly deep deployment-time reconfiguration capabilities  Making dependency-injectable style the path of least resistance for new work    Of course  there may be difficulties    Code that requires complex startup shutdown lifecycle management may not be easily adapted to a container  You will probably have to navigate any personal  process and team culture issues -- but then  that s why you asked    Some of the toolkits are fast becoming heavyweight themselves  encouraging the sort of deep dependency that many DI containers started off as a backlash against

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IoC Containers are also good for loading deeply nested class dependencies   For example if you had the following code using Depedency Injection   public void GetPresenter         var presenter   new CustomerPresenter new CustomerService new CustomerRepository new DB          class CustomerPresenter       private readonly ICustomerService service      public CustomerPresenter ICustomerService service                this service   service           class CustomerService       private readonly IRespository lt Customer gt  repository      public CustomerService IRespository lt Customer gt  repository                this repository   repository           class CustomerRepository   IRespository lt Customer gt        private readonly DB db      public CustomerRepository DB db                this db   db           class DB       If you had all of these dependencies loaded into and IoC container you could Resolve the CustomerService and the all the child dependencies will automatically get resolved   For example   public static IoC      private IUnityContainer  container     static IoC               InitializeIoC             static void InitializeIoC               container   new UnityContainer           container RegisterType lt ICustomerService  CustomerService gt            container RegisterType lt IRepository lt Customer gt   CustomerRepository gt              static T Resolve lt T gt               return  container Resolve lt T gt             public void GetPresenter        var presenter   IoC Resolve lt CustomerPresenter gt           presenter is loaded and all of its nested child dependencies        are automatically injected       -       Also  note that only the Interfaces need to be registered       the concrete types like DB and CustomerPresenter will automatically        resolve

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I just so happen to be in the process of yanking out home grown DI code and replacing it with an IOC  I have probably removed well over 200 lines of code and replaced it with about 10  Yes  I had to do a little bit of learning on how to use the container  Winsor   but I m an engineer working on internet technologies in the 21st century so I m used to that  I probably spent about 20 minutes looking over the how tos  This was well worth my time

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You would need an IoC container if you needed to centralize the configuration of your dependencies so that they may be easily swapped out en mass   This makes the most sense in TDD  where many dependencies are swapped out  but where there is little interdependence between the dependencies   This is done at the cost of obfuscating the flow of control of object construction to some degree  so having a well organized and reasonably documented configuration is important   It is also good to have a reason to do this  otherwise  it is mere abstraction gold-plating   I have seen it done so poorly that it was dragged down to being the equivalent to a goto statement for constructors

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Honestly I don t find there to be many cases where IoC containers are needed  and most of the time  they just add unneeded complexity   If you are using it just for making construction of an object simpler  I d have to ask  are you instantiating this object in more than one location  Would a singleton not suit your needs  Are you changing the configuration at runtime   Switching data source types  etc    If yes  then you might need an IoC container  If not  then you re just moving the initialization away from where the developer can easily see it   Who said that an interface is better than inheritance anyway  Say you re testing a Service  Why not use constructor DI  and create mocks of the dependencies using inheritance  Most services I use only have a few dependencies  Doing unit testing this way prevents maintaining a ton of useless interfaces and means you don t have to use Resharper to quickly find the declaration of a method   I believe that for most implementations  saying that IoC Containers remove unneeded code is a myth   First  there s setting up the container in the first place  Then you still have to define each object that needs to be initialized  So you don t save code in initialization  you move it  unless your object is used more than once  Is it better as a Singleton    Then  for each object you ve initialized in this way  you have to create and maintain an interface   Anyone have any thoughts on this

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I m with you  Vadim  IoC containers take a simple  elegant  and useful concept  and make it something you have to study for two days with a 200-page manual   I personally am perplexed at how the IoC community took a beautiful  elegant article by Martin Fowler and turned it into a bunch of complex frameworks typically with 200-300 page manuals   I try not to be judgemental  HAHA    but I think that people who use IoC containers are  A  very smart and  B  lacking in empathy for people who aren t as smart as they are  Everything makes perfect sense to them  so they have trouble understanding that many ordinary programmers will find the concepts confusing  It s the curse of knowledge  The people who understand IoC containers have trouble believing that there are people who don t understand it   The most valuable benefit of using an IoC container is that you can have a configuration switch in one place which lets you change between  say  test mode and production mode  For example  suppose you have two versions of your database access classes    one version which logged aggressively and did a lot of validation  which you used during development  and another version without logging or validation that was screamingly fast for production  It is nice to be able to switch between them in one place  On the other hand  this is a fairly trivial problem easily handled in a simpler way without the complexity of IoC containers   I believe that if you use IoC containers  your code becomes  frankly  a lot harder to read  The number of places you have to look at to figure out what the code is trying to do goes up by at least one  And somewhere in heaven an angel cries out

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I will try to find why IOC might not be good for from my perspective   As with everything else  IOC container  or as Einstein would put it I OC 2  is a concept you have to decide for yourself if you need it or not in your code  Recent fashion outcry about IOC is only that  fashion  Don t fall for fashion  that is first  There are myriads of concepts out there you could implement in your code  First of all  I am using dependency injection since I have started programming  and learned the term itself when it was popularized under that name  Dependency control is a very old subject and it was addressed so far in trillions of ways  depending on what was decoupling from what  Decoupling everything from everything is a nonsense  The problem with IOC container is that it tries to be as useful as Entity Framework or NHibernate  While writing an object-relational mapper is simply a must as soon as you have to couple any database with your system  IOC container is not always necessary  So when IOC container is useful    When you have a situation with many dependencies you want to organize When you do not care about coupling your code with third-party product When your developers want to learn how to work with a new tool   1  It is not that often that you have so many dependencies in your code  or that you are aware of them early in design  Abstract thinking is useful when abstract thinking is due   2  Coupling your code with third-party code is a HuGe problem  I was working with code that is 10  years old and that was following at that time fancy and advanced concepts ATL  COM  COM  and so on  There is nothing you can do with that code now  What I am saying is that an advanced concept gives an apparent advantage  yet this is cancelled on long run with the outdated advantage itself  It just had made all of it more expensive   3  Software development is hard enough  You can extend it to unrecognizable levels if you allow some advanced concept to crop into your code  There is a problem with IOC2  Although it is decoupling dependencies  it is decoupling the logic flow as well  Imagine you have found a bug and you need to set a break to examine the situation  IOC2  as any other advanced concept  is making that more difficult  Fixing a bug within a concept is more difficult than fixing a bug in a plainer code  because when you fix a bug a concept must be obeyed again   Just to give you an example  C    NET is constantly changing the syntax so much that you need to think hard before you refactor some older version of  NET   So what is the problem with IOC  The problem is in resolving dependencies  The logic for resolving is commonly hidden in the IOC2 itself  written maybe in uncommon way that you need to learn and maintain  Will your third-party product be there in 5 years  Microsoft s was not    We know how  syndrome is written all over the place regarding IOC2  This is similar to automation testing  Fancy term and perfect solution at first glance  you simply put all your tests to execute over night and see the results in the morning  It is really painful to explain company after company what automated testing really means  Automated testing is definitely not a quick way of reducing the number of bugs which you can introduce overnight to increase the quality of your product  But  fashion is making that notion annoyingly dominant  IOC2 suffers the same syndrome  It is believed that you need to implement it in order your software to be good  EvErY recent interview I was asked if I am implementing IOC2 and automation  That is a sign of fashion  the company had some part of code written in MFC they will not abandon   You need to learn IOC2 as any other concept in software  The decision if IOC2 needs to be used is within the team and the company  However  at least ALL above arguments must be mentioned before the decision is made  Only if you see that plus side outweighs negative side  you can make a positive decision   There is nothing wrong with IOC2 except that it does solve only the problems it solves and introduces the problems it introduces  Nothing else  However  going against the fashion is very difficult  they have sweat mouth  the followers of anything  It is strange how none of them is there when the problem with their fanciness becomes apparent  Many concepts in software industry have been defended because they create profit  books are written  conferences held  new products made  That is fashion  usually short lived  As soon as people find something else they abandon it completely  IOC2 is useful but it shows the same signs as many other vanished concepts I have seen  I do not know if it will survive  There is no rule for that  You think if it is useful  it will survive  No  it does not go that way  One big rich company is enough and the concept can die within few weeks  We ll see  NHibernate survived  EF came second  Maybe IOC2 will survive too  Do not forget that most concepts in software development are about nothing special  they are very logical  simple and obvious  and sometimes it is more difficult to remember the current naming convention than to understand the concept itself  Does the knowledge of IOC2 make a developer a better developer  No  because if a developer was not able to come up with a concept similar in nature to IOC2 then it will be difficult for him or her to understand which problem IOC2 is solving  using it will look artificial and he or she may start using it for sake of being some sort of politically correct

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Dependency Injection in an ASP NET project can be accomplished with a few lines of code  I suppose there is some advantage to using a container when you have an app that uses multiple front ends and needs unit tests

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I m a recovering IOC addict   I m finding it hard to justify using IOC for DI in most cases these days   IOC containers sacrifice compile time checking and supposedly in return give you  easy  setup  complex lifetime management and on the fly discovering of dependencies at run time    I find the loss of compile time checking and resulting run time magic exceptions  is not worth the bells and whistles in the vast majority of cases  In large enterprise applications they can make it very difficult to follow what is going on   I don t buy the centralization argument because you can centralize static setup very easily as well by using an abstract factory for your application and religiously deferring object creation to the abstract factory i e  do proper DI   Why not do static magic-free DI like this   interface IServiceA     interface IServiceB     class ServiceA   IServiceA     class ServiceB   IServiceB      class StubServiceA   IServiceA     class StubServiceB   IServiceB      interface IRoot   IMiddle Middle   get  set      interface IMiddle   ILeaf Leaf   get  set      interface ILeaf      class Root   IRoot       public IMiddle Middle   get  set         public Root IMiddle middle                Middle   middle            class Middle   IMiddle       public ILeaf Leaf   get  set         public Middle ILeaf leaf                Leaf   leaf           class Leaf   ILeaf       IServiceA ServiceA   get  set        IServiceB ServiceB   get  set         public Leaf IServiceA serviceA  IServiceB serviceB                ServiceA   serviceA          ServiceB   serviceB            interface IApplicationFactory       IRoot CreateRoot       abstract class ApplicationAbstractFactory   IApplicationFactory       protected abstract IServiceA ServiceA   get        protected abstract IServiceB ServiceB   get         protected IMiddle CreateMiddle                 return new Middle CreateLeaf                protected ILeaf CreateLeaf                 return new Leaf ServiceA ServiceB               public IRoot CreateRoot                 return new Root CreateMiddle              class ProductionApplication   ApplicationAbstractFactory       protected override IServiceA ServiceA               get   return new ServiceA                 protected override IServiceB ServiceB               get   return new ServiceB               class FunctionalTestsApplication   ApplicationAbstractFactory       protected override IServiceA ServiceA               get   return new StubServiceA                 protected override IServiceB ServiceB               get   return new StubServiceB                namespace ConsoleApplication5       class Program               static void Main string   args                        var factory   new ProductionApplication                var root   factory CreateRoot                             TestFixture      class FunctionalTests                  Test          public void Test                         var factory   new FunctionalTestsApplication                var root   factory CreateRoot                        Your container configuration is your abstract factory implementation   your registrations are implementations of abstract members   If you need a new singleton dependency  just add another abstract property to the abstract factory   If you need a transient dependency  just add another method and inject it as a Func lt      Advantages    All setup and object creation configuration is centralized    Configuration is just code  Compile time checking makes it easy to maintain as you cannot forget to update your registrations    No run-time reflection magic   I recommend sceptics to give it a go next green field project and honestly ask yourself at which point you need the container  It s easy to factor in an IOC container later on as you re just replacing a factory implementation with a IOC Container configuration module

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In the  NET world AOP isn t too popular  so for DI a framework is your only real option  whether you write one yourself or use another framework   If you used AOP you can inject when you compile your application  which is more common in Java   There are many benefits to DI  such as reduced coupling so unit testing is easier  but how will you implement it   Do you want to use reflection to do it yourself

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So  it s been almost 3 years  eh     50  of those who commented in favor of IoC frameworks don t understand the difference between IoC and IoC Frameworks  I doubt they know that you can write code without being deployed to an app server If we take most popular Java Spring framework  it is IoC configuration moved from XMl into the code and now look like this     Configuration public class AppConfig    public  Bean TransferService transferService         return new TransferServiceImpl accountRepository        public  Bean AccountRepository accountRepository         return new InMemoryAccountRepository             And we need a framework to do this why exactly

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The biggest benefit of using IoC containers for me  personally  I use Ninject  has been to eliminate the passing around of settings and other sorts of global state objects     I don t program for the web  mine is a console application and in many places deep in the object tree I need to have access to the settings or metadata specified by the user that are created on a completely separate branch of the object tree   With IoC I simply tell Ninject to treat the Settings as a singleton  because there is always only one instance of them   request the Settings or Dictionary in the constructor and presto     they magically appear when I need them     Without using an IoC container I would have to pass the settings and or metadata down through 2  3       n objects before it was actually used by the object that needed it   There are many other benefits to DI IoC containers as other people have detailed here and moving from the idea of creating objects to requesting objects can be mind-bending  but using DI was very helpful for me and my team so maybe you can add it to your arsenal

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It sounds to me like you already built your own IoC container  using the various patterns which were described by Martin Fowler  and are asking why someone else s implementation is better than yours   So  you have a bunch of code that already works  And are wondering why you would want to replace it with someone else s implementation   Pros for considering a third-party IoC container   You get bugs fixed for free The library design may be better than yours People may be already familiar with the particular library The library may be faster than yours It may have some features you wish you implemented but never had time to  do you have a service locater     Cons   You get bugs introduced  for free    The library design may be worse than yours You have to learn a new API Too many features you will never use It s usually harder to debug code you did not write Migrating from a previous IoC container may be tedious   So  weigh your pros against your cons and make a decision

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IOC CONTAINER SOLVES A PROBLEM YOU MIGHT NOT HAVE BUT IT S A NICE PROBLEM TO HAVE  http   kozmic net 2012 10 23 ioc-container-solves-a-problem-you-might-not-have-but-its-a-nice-problem-to-have

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I ve found that correctly implementing Dependency Injection tends to force programmers to use a variety of other programming practices that help to improve the testability  flexibility  maintainability  and scalability of code  practices like the Single Responsibility Principle  Separations of Concerns  and coding against APIs  It feels like I m being compelled to write more modular  bite-sized classes and methods  which makes the code easier to read  because it can be taken in bite-sized chunks    But it also tends to create rather large dependency trees  which are far more easily managed via a framework  especially if you use conventions  than by hand  Today I wanted to test something really quickly in LINQPad  and I figured it d be too much bother to create a kernel and load in my modules  and I ended up writing this by hand   var merger   new SimpleWorkflowInstanceMerger      new BitFactoryLog typeof SimpleWorkflowInstanceMerger  FullName        new WorkflowAnswerRowUtil          new WorkflowFieldAnswerEntMapper            new ActivityFormFieldDisplayInfoEntMapper            new FieldEntMapper         new AnswerRowMergeInfoRepository       In retrospect  it would have been quicker to use the IoC framework  since the modules define pretty much all of this stuff by convention   Having spent some time studying the answers and comments on this question  I am convinced that the people who are opposed to using an IoC container aren t practicing true dependency injection  The examples I ve seen are of practices that are commonly confused with dependency injection  Some people are complaining about difficulty  reading  the code  If done correctly  the vast majority of your code should be identical when using DI by hand as when using an IoC container  The difference should reside entirely in a few  launching points  within the application   In other words  if you don t like IoC containers  you probably aren t doing Dependency Injection the way it s supposed to be done   Another point  Dependency Injection really can t be done by hand if you use reflection anywhere  While I hate what reflection does to code navigation  you have to recognize that there are certain areas where it really can t be avoided  ASP NET MVC  for example  attempts to instantiate the controller via reflection on each request  To do dependency injection by hand  you would have to make every controller a  context root   like so   public class MyController   Controller       private readonly ISimpleWorkflowInstanceMerger  simpleMerger      public MyController                  simpleMerger   new SimpleWorkflowInstanceMerger              new BitFactoryLog typeof SimpleWorkflowInstanceMerger  FullName                new WorkflowAnswerRowUtil                  new WorkflowFieldAnswerEntMapper                    new ActivityFormFieldDisplayInfoEntMapper                    new FieldEntMapper                 new AnswerRowMergeInfoRepository                      Now compare this with allowing a DI framework to do it for you   public MyController   Controller       private readonly ISimpleWorkflowInstanceMerger  simpleMerger      public MyController ISimpleWorkflowInstanceMerger simpleMerger                 simpleMerger   simpleMerger                    Using a DI framework  note that    I can unit-test this class  By creating a mock ISimpleWorkflowInstanceMerger  I can test that it gets used the way I anticipate  without the need for a database connection or anything  I use far less code  and the code is much easier to read  If one of my dependency s dependency s changes  I don t have to make any changes to the controller  This is especially nice when you consider that multiple controllers are likely to use some of the same dependencies  I never explicitly reference classes from my data layer  My web application can just include a reference to the project containing the ISimpleWorkflowInstanceMerger interface  This allows me to break the application up into separate modules  and maintain a true multi-tier architecture  which in turn makes things much more flexible    A typical web application will have quite a few controllers  All of the pain of doing DI by hand in each controller will really add up as your application grows  If you have an application with only one context root  which never tries to instantiate a service by reflection  then this isn t as big a problem  Nevertheless  any application that uses Dependency Injection will become extremely expensive to manage once it reaches a certain size  unless you use a framework of some kind to manage the dependency graph

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Presumably no one is forcing you to use a DI container framework  You re already using DI to decouple your classes and improve testability  so you re getting many of the benefits  In short  you re favoring simplicity  which is generally a good thing   If your system reaches a level of complexity where manual DI becomes a chore  that is  increases maintenance   weigh that against the team learning curve of a DI container framework   If you need more control over dependency lifetime management  that is  if you feel the need to implement the Singleton pattern   look at DI containers   If you use a DI container  use only the features you need  Skip the XML configuration file and configure it in code if that is sufficient  Stick to constructor injection  The basics of Unity or StructureMap can be condensed down to a couple of pages   There s a great blog post by Mark Seemann on this  When to use a DI Container

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I think most of the value of an IoC is garnered by using DI  Since you are already doing that  the rest of the benefit is incremental   The value you get will depend on the type of application you are working on    For multi-tenant  the IoC container can take care of some of the infrastructure code for loading different client resources  When you need a component that is client specific  use a custom selector to do handle the logic and don t worry about it from your client code  You can certainly build this yourself but here s an example of how an IoC can help  With many points of extensibility  the IoC can be used to load components from configuration  This is a common thing to build but tools are provided by the container  If you want to use AOP for some cross-cutting concerns  the IoC provides hooks to intercept method invocations  This is less commonly done ad-hoc on projects but the IoC makes it easier    I ve written functionality like this before but if I need any of these features now I would rather use a pre-built and tested tool if it fits my architecture   As mentioned by others  you can also centrally configure which classes you want to use  Although this can be a good thing  it comes at a cost of misdirection and complication  The core components for most applications aren t replaced much so the trade-off is a little harder to make   I use an IoC container and appreciate the functionality but have to admit that I ve noticed the trade-off  My code becomes more clear at the class level and less clear at the application level  i e  visualizing control flow

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IoC frameworks are excellent if you want to         throw away type safety  Many  all   IoC frameworks forces you to execute the code if you want to be certain everything is hooked up correctly   Hey  Hope I got everything set up so my initializations of these 100 classes won t fail in production  throwing null-pointer exceptions      litter your code with globals  IoC frameworks are all about changing global states      write crappy code with unclear dependencies that s hard to refactor since you ll never know what depends on what    The problem with IoC is that the people who uses them used to write code like this  public class Foo       public Bar Apa  get set       Foo             Apa   new Bar              which is obviously flawed since the dependency between Foo and Bar is hard-wired  Then they realized it would be better to write code like  public class Foo       public IBar Apa  get set       Foo             Apa   IoC lt IBar gt               which is also flawed  but less obviously so  In Haskell the type of Foo   would be IO Foo but you really don t want the IO-part and is should be a warning sign that something is wrong with your design if you got it   To get rid of it  the IO-part   get all advantages of IoC-frameworks and none of it s drawbacks you could instead use an abstract factory   The correct solution would be something like  data Foo   Foo   apa    Bar     or maybe  data Foo   forall b   IBar b    gt  Foo   apa    b     and inject  but I wouldn t call it inject  Bar   Also  see this video with Erik Meijer  inventor of LINQ  where he says that DI is for people who don t know math  and I couldn t agree more   http   www youtube com watch v 8Mttjyf-8P4  Unlike Mr  Spolsky I don t believe that people who use IoC-frameworks are very smart - I simply believe they don t know math

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I m a fan of declarative programming  look at how many SQL questions I answer   but the IoC containers I ve looked at seem too arcane for their own good        or perhaps the developers of IoC containers are incapable of writing clear documentation      or else both are true to one degree or another   I don t think the concept of an IoC container is bad   But the implementation has to be both powerful  that is  flexible  enough to be useful in a wide variety of applications  yet simple and easily understood   It s probably six of one and half a dozen of the other   A real application  not a toy or demo  is bound to be complex  accounting for many corner cases and exceptions-to-the-rules   Either you encapsulate that complexity in imperative code  or else in declarative code   But you have to represent it somewhere

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Here is why  The project is called IOC-with-Ninject  You can download and run it with Visual Studio  This example uses Ninject but ALL the  new  statements are in one location and you can completely change how your application runs by changing which bind module to use  The example is set up so you can bind to a mocked version of the services or the real version   In small projects that may not matter  but in big projects it s a big deal   Just to be clear  advantages as I see them  1  ALL new statements in one location at root of code  2  Totally re-factor code with one change  3  Extra points for  cool factor   cause it s    well  cool   p

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you do not need a framework to achieve dependency injection  You can do this by core java concepts as well   http   en wikipedia org wiki Dependency injection Code illustration using Java

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Wow  can t believe that Joel would favor this   var svc   new ShippingService new ProductLocator        new PricingService    new InventoryService        new TrackingRepository new ConfigProvider         new Logger new EmailLogger new ConfigProvider         over this   var svc   IoC Resolve lt IShippingService gt       Many folks don t realize that your dependencies chain can become nested  and it quickly becomes unwieldy to wire them up manually   Even with factories  the duplication of your code is just not worth it   IoC containers can be complex  yes   But for this simple case I ve shown it s incredibly easy     Okay  let s justify this even more   Let s say you have some entities or model objects that you want to bind to a smart UI   This smart UI  we ll call it Shindows Morms  wants you to implement INotifyPropertyChanged so that it can do change tracking  amp  update the UI accordingly    OK  that doesn t sound so hard  so you start writing   You start with this     public class Customer       public string FirstName   get  set        public string LastName   get  set        public DateTime CustomerSince   get  set        public string Status   get  set          and end up with this   public class UglyCustomer   INotifyPropertyChanged       private string  firstName      public string FirstName               get   return  firstName            set                       string oldValue    firstName               firstName   value              if oldValue    value                  OnPropertyChanged  FirstName                         private string  lastName      public string LastName               get   return  lastName            set                       string oldValue    lastName               lastName   value              if oldValue    value                  OnPropertyChanged  LastName                         private DateTime  customerSince      public DateTime CustomerSince               get   return  customerSince            set                       DateTime oldValue    customerSince               customerSince   value              if oldValue    value                  OnPropertyChanged  CustomerSince                         private string  status      public string Status               get   return  status            set                       string oldValue    status               status   value              if oldValue    value                  OnPropertyChanged  Status                         protected virtual void OnPropertyChanged string property                var propertyChanged   PropertyChanged           if propertyChanged    null              propertyChanged this  new PropertyChangedEventArgs property               public event PropertyChangedEventHandler PropertyChanged      That s disgusting plumbing code  and I maintain that if you re writing code like that by hand you re stealing from your client   There are better  smarter way of working   Ever hear that term  work smarter  not harder   Well imagine some smart guy on your team came up and said    Here s an easier way   If you make your properties virtual  calm down  it s not that big of a deal  then we can weave in that property behavior automatically    This is called AOP  but don t worry about the name  focus on what it s going to do for you   Depending on which IoC tool you re using  you could do something that looks like this   var bindingFriendlyInstance   IoC Resolve lt Customer gt  new NotifyPropertyChangedWrapper       Poof   All of that manual INotifyPropertyChanged BS is now automatically generated for you  on every virtual property setter of the object in question   Is this magic   YES   If you can trust the fact that this code does its job  then you can safely skip all of that property wrapping mumbo-jumbo   You ve got business problems to solve   Some other interesting uses of an IoC tool to do AOP    Declarative  amp  nested database transactions Declarative  amp  nested Unit of work Logging Pre Post conditions  Design by Contract

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I realize this is a rather old post  but it seems to still be reasonably active  and I thought I d contribute a couple of points that have not yet been mentioned in other answers   I will say that I agree with the benefits of dependency injection  but I do prefer to construct and manage the objects myself  using a pattern not unlike that outlined by Maxm007 in his answer  I have found two main problems with using 3rd party containers     1  Having a 3rd party library manage the lifetime of your objects  automagically  can lend itself to unexpected results   We have found that especially in large projects  you can have vastly more copies of an object than you expect  and more than you would if you were manually managing the lifecycles   I m sure this varies depending on the framework used  but the problem exists nonetheless  This can also be problematic if your object holds resources  data connections  etc   since the object can sometimes live longer than you expect   So inevitably  IoC containers tend to increase the resource utilization and memory footprint of an application   2  IoC containers  in my opinion  are a form of  black box programming    I have found that in particular  our less experienced developers tend to abuse them   It allows the programmer to not have to think about how objects should relate to each other or how to decouple them  because it provides them with a mechanism in which they can simply grab any object they want out of thin air   Eg  there may be a good design reason that ObjectA should never know about ObjectB directly  but rather than creating a factory or bridge or service locator  an inexperienced programmer will simply say  no problem  I ll just grab ObjectB from the IoC container    This can actually lead to increased object coupling  which is what IoC is supposed to help prevent

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Whenever you use the  new  keyword  you are creating a concrete class dependency and a little alarm bell should go off in your head  It becomes harder to test this object in isolation  The solution is to program to interfaces and inject the dependency so that the object can be unit tested with anything that implements that interface  eg  mocks    The trouble is you have to construct objects somewhere  A Factory pattern is one way to shift the coupling out of your POXOs  Plain Old  insert your OO language here  Objects   If you and your co-workers are all writing code like this then an IoC container is the next  Incremental Improvement  you can make to your codebase  It ll shift all that nasty Factory boilerplate code out of your clean objects and business logic  They ll get it and love it  Heck  give a company talk on why you love it and get everyone enthused   If your co-workers aren t doing DI yet  then I d suggest you focus on that first  Spread the word on how to write clean code that is easily testable  Clean DI code is the hard part  once you re there  shifting the object wiring logic from Factory classes to an IoC container should be relatively trivial

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As you continue to decouple your classes and invert your dependencies  the classes continue to stay small and the  dependency graph  continues to grow in size   This isn t bad   Using basic features of an IoC container makes wiring up all these objects trivial  but doing it manually can get very burdensome  For example  what if I want to create a new instance of  Foo  but it needs a  Bar   And a  Bar  needs an  A    B   and  C   And each of those need 3 other things  etc etc   yes  I can t come up with good fake names        Using an IoC container to build your object graph for you reduces complexity a ton and pushes it out into one-time configuration  I simply say  create me a  Foo   and it figures out what s needed to build one   Some people use the IoC containers for much more infrastructure  which is fine for advanced scenarios but in those cases I agree it can obfuscate and make code hard to read and debug for new devs

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You don t need an IoC container   But if you re rigorously following a DI pattern  you ll find that having one will remove a ton of redundant  boring code   That s often the best time to use a library framework  anyway - when you understand what it s doing and could do it without the library

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Because all the dependencies are clearly visible  it promotes creating components which are loosely coupled and at the same time easily accessible and reusable across the application

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Personally  I use IoC as some sort of structure map of my application  Yeah  I also prefer StructureMap       It makes it easy to substitute my ussual interface implementations with Moq implementations during tests  Creating a test setup can be as easy as making a new init-call to my IoC-framework  substituting whichever class is my test-boundary with a mock   This is probably not what IoC is there for  but it s what I find myself using it for the most

[dependency-injection] Why do I need an IoC container as opposed to straightforward DI code?

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