Dependency Injection vs Factory Pattern

Question

Most of the examples quoted for usage of Dependency Injection  we can solve using the factory pattern as well  Looks like when it comes to usage design the difference between dependency injection and factory is blurred or thin   Once someone told me that its how you use it that makes a difference    I once used StructureMap a DI container to solve a problem  later on I redesigned it to work with a simple factory and removed references to StructureMap   Can anyone tell me what is the difference between them and where to use what  whats the best practice here

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My thoughts   Dependecy Injection  pass collaborators as parameters to the constructors  Dependency Injection Framework  a generic and configurable factory to create the objects to pass as parameters to the constructors

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I believe  3 important aspects govern objects and their usage  1  Instantiation  of a class together with initialisation if any   2  Injection  of the instance so created  where it s required  3  Life cycle management  of the instance so created    Using Factory pattern  the first aspect  instantiation  is achieved but the remaining two is questionable  The class that uses other instances must hardcode the factories  instead of instances being created  which hinders loose coupling abilities  Moreover  life cycle management of instances becomes a challenge in a large application where a factory is used in multiple places  particularly  if the factory doesn t manage the life cycle of the instance it returns  it gets ugly     Using a DI  of IoC pattern  on the other hand  all the 3 are abstracted outside the code  to the DI container  and the managed bean needs nothing about this complexity  Loose Coupling  a very important architectural goal can be achieved quiet comfortably  Another important architectural goal  the separation of concerns can be achieved much better than factories    Whereas the Factories may be suitable for small applications  large ones would be better to chose DI over factories

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Theory  There are two important points to consider    Who creates objects    Factory   You have to write HOW object should be created  You have separate Factory class which contains creation logic   Dependency Injection   In practical cases are done by external frameworks  for example in Java that would be spring ejb guice   Injection happens  magically  without explicite creation of new objects  What kind of objects it manages     Factory   Usually responsible for creation of stateful objects  Dependency Injections  More likely to create stateless objects      Practical example how to use both factory and dependency injection in single project   What we want to build   Application module for creating order which contains multiple entries called orderline    Architecture   Let s assume we want to create following layer architecture     Domain objects may be objects stored inside database  Repository  DAO  helps with retrievar of objects from database  Service provides API to other modules  Alows for operations on order module   Domain Layer and usage of factories   Entities which will be in the database are Order and OrderLine  Order can have multiple OrderLines    Now comes important design part  Should modules outside this one create and manage OrderLines on their own  No  Order Line should exist only when you have Order associated with it  It would be best if you could hide internal implementaiton to outside classes   But how to create Order without knowledge about OrderLines   Factory  Someone who wants to create new order used OrderFactory  which will hide details about the fact how we create Order      Thats how it will look inside IDE  Classes outside domain package will use OrderFactory instead of constructor inside Order   Dependency Injection Dependency injection is more commonly used with stateless layers such as repository and service    OrderRepository and OrderService are managed by dependency injection framework  Repository is responsible for managing CRUD operations on database  Service injects Repository and uses it to save find correct domain classes

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In simple terms Dependency Injection vs Factory method implies push vs pull mechanism respectively   Pull mechanism   class indirectly have dependency on Factory Method which in turn have dependency on concrete classes   Push mechanism   Root component can be configured with all dependent components in a single location and thus promoting high maintenance and loose coupling   With Factory method responsibility still lies with class  though indirectly  to create new object where as with dependency injection that responsibility is outsourced  at the cost of leaking abstraction though

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I know this question is old but i would like to add my five cents   I think that dependency injection  DI  is in many ways like a configurable Factory Pattern  FP   and in that sense anything that you could do with DI you will be able to do it with such factory   Actually  if you use spring for example  you have the option of autowiring resources  DI  or doing something like this   MyBean mb   ctx getBean  myBean      And then use that  mb  instance to do anything  Isn t that a call to a factory that will return you an instance     The only real difference I notice between most of the FP examples is that you can configure what  myBean  is in an xml or in another class  and a framework will work as the factory  but other than that is the same thing  and you can have a certainly have a Factory that reads a config file or gets the implementation as it needs   And if you ask me for my opinion  And I know you didn t   I believe that DI does the same thing but just adds more complexity to the development  why   well  for one thing  for you to know what is the implementation being used for any bean you autowire with DI  you have to go to the configuration itself   but    what about that promise that you will not have to know the implementation of the object you are using  pfft  seriously  when you use an approach like this    aren t you the same that writes the implementation   and even if you don t  arent you almost all the time looking at how the implementation does what it is supposed to do     and for one last thing  it doesn t matter how much a DI framework promises you that you will build things decoupled from it  with no dependencies to their classes  if you are using a framework you build everything aroud it  if you have to change the approach or the framework it will not be an easy task    EVER     but  since you buil everything around that particular framework instead of worrying of whats the best solution for your business  then you will face a biiig problen when doing that   In fact  the only real business application for a FP or DI approach that I can see is if you need to change the implementations being used at runtime  but at least the frameworks I know do not allow you to do that  you have to leave everything perfect in the configuration at development time an if you need that use another approach   So  if I have a class that performs differently in two scopes in the same application  lets say  two companies of a holding  I have to configure the framework to create two different beans  and adapt my code to use each  Isn t that the same as if I would just write something like this   MyBean mb   MyBeanForEntreprise1      In the classes of the first enterprise MyBean mb   MyBeanForEntreprise2      In the classes of the second enterprise   the same as this    Autowired MyBean mbForEnterprise1    In the classes of the first enterprise  Autowired MyBean mbForEnterprise2    In the classes of the second enterprise   And this   MyBean mb    MyBean MyFactory get  myBeanForEntreprise1      In the classes of the first enterprise MyBean mb    MyBean MyFactory get  myBeanForEntreprise2      In the classes of the second enterprise   In any case you will have to change something in your application  whether classes or configuration files  but you will have to do it an redeploy it   Wouldn t it be nice to do just something like this   MyBean mb    MyBean MyFactory get  mb       And that way  you set the code of the factory to get the right implementation at runtime depending on the logged user enterprise   Now THAT would be helpful  You could just add a new jar with the new classes and set the rules maybe even also at runtime  or add a new config file if you leave this option open   no changes to existing classes  This would be a Dynamic factory   wouldn t that be more helpful than having to write two configurations for each enterprise  and maybe even having two different applications for each    You can tell me  I don t need to do the switch at runtime ever  so I configure the app  and if I inherit the class or use another implementation I just change the config and redeploy  Ok  that can also be done with a factory  And be honest  how many times do you do this  maybe only when you have an app that s going to be used somewhere else in your company  and you are going to pass the code to another team  and they will do things like this  But hey  that can also be done with the factory  and would be even better with a dynamic factory    Anyway  the comment section if open for you to kill me

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DI gives you a composition root  which is a single centralized location for wiring up your object graph  This tends to make object dependencies very explicit  because objects ask for exactly what they need and there is only one place to get it  A composition root is a clean and straightforward separation of concerns  Objects being injected should have no dependency on the DI mechanism  whether that be a third-party container or a DIY DI  The DI should be invisible  Factories tend to be more distributed  Different objects use different factories and the factories represent an additional layer of indirection between the objects and their actual dependencies  This additional layer adds its own dependencies to the object graph  Factories are not invisible  A Factory is a middleman  For this reason  updating factories is more problematic  since factories are a dependency of the business logic  modifying them can have a ripple effect  A composition root is not a dependency of the business logic  so it can be modified in isolation  The GoF mentions the difficulty of updating Abstract Factories  Part of their explanation is quoted in an answer here  Contrasting DI with Factories also has a lot in common with the question  Is ServiceLocator an anti-pattern  Ultimately  the answer of which to choose may be opinionated  but I think it boils down to a Factory being a middleman  The question is whether that middleman pulls its weight by adding additional value beyond just supplying a product  Because if you can get that same product without the middleman  then why not cut the middleman out  A diagram helps to illustrate the difference

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Life cycle management is one of the responsibilities dependency containers assume in addition to instantiation and injection  The fact that the container sometimes keep a reference to the components after instantiation is the reason it is called a  container   and not a factory  Dependency injection containers usually only keep a reference to objects it needs to manage life cycles for  or that are reused for future injections  like singletons or flyweights  When configured to create new instances of some components for each call to the container  the container usually just forgets about the created object    From  http   tutorials jenkov com dependency-injection dependency-injection-containers html

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When using a factory your code is still actually responsible for creating objects  By DI you outsource that responsibility to another class or a framework  which is separate from your code

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I would suggest to keep the concepts plain and simple  Dependency Injection is more of a architectural pattern for loosely coupling software components  Factory pattern is just one way to separate the responsibility of creating objects of other classes to another entity  Factory pattern can be called as a tool to implement DI  Dependency injection can be implemented in many ways like DI using constructors  using mapping xml files etc

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I had the same question as soon as I read about DI and ended up at this post  So finally this is what I understood but please correct me if am wrong    Long ago there were little kingdoms with their own governing bodies controlling and taking decisions based on their own written rules  Later on formed a big government eliminating all these little governing bodies which has one set of rules constitution  and are implemented through courts   The little kingdoms  governing bodies are  Factories   The big government is the  Dependency Injector

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One disadvantage of DI is that it can not initialize objects with logic  For example  when I need to create a character that has random name and age  DI is not the choice over factory pattern  With factories  we can easily encapsulate the random algorithm from object creation  which supports one of the design patterns called  Encapsulate what varies

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You can have a look at this link for a comparison of the two  and others  approaches in a real example   Basically  when requirements change  you end up modifying more code if you use factories instead of DI   This is also valid with manual DI  i e  when there isn t an external framework that provides the dependencies to your objects  but you pass them in each constructor

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With dependency injection the client does not need to get its dependencies on its own  its all prepared beforehand   With factories  someone has to call those to get the generated objects over to the place where they are needed   The difference lies mostly in this one line where calling the factory and fetching the constructed object is done   But with factories you have to write this 1 line everywhere you need such an object  With DI you just have to create the wiring  relation between usage and created object  once and just rely on the presence of the object afterward everywhere  On the other side  DI often requires a bit more  how much depends on the framework  work on the preparation side

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With a factory you can group related interfaces  So If the parameters passed can be grouped in a factory then its also a good solution for constructor overinjection look at this code      public AddressModelFactory IAddressAttributeService addressAttributeService          IAddressAttributeParser addressAttributeParser          ILocalizationService localizationService          IStateProvinceService stateProvinceService          IAddressAttributeFormatter addressAttributeFormatter                this  addressAttributeService   addressAttributeService          this  addressAttributeParser   addressAttributeParser          this  localizationService   localizationService          this  stateProvinceService   stateProvinceService          this  addressAttributeFormatter   addressAttributeFormatter          Look at the constructor  you only have to pass the IAddressModelFactory there  so less parameters       public CustomerController IAddressModelFactory addressModelFactory          ICustomerModelFactory customerModelFactory          IAuthenticationService authenticationService          DateTimeSettings dateTimeSettings          TaxSettings taxSettings          ILocalizationService localizationService          IWorkContext workContext          IStoreContext storeContext          ICustomerService customerService          ICustomerAttributeParser customerAttributeParser          ICustomerAttributeService customerAttributeService          IGenericAttributeService genericAttributeService          ICustomerRegistrationService customerRegistrationService          ITaxService taxService          CustomerSettings customerSettings          AddressSettings addressSettings       You see in CustomerController a lot of parameters passed  Yes you can see this as constructor overinjection but this is how DI works  And no nothing is wrong with the CustomerController       Code is from nopCommerce

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Factory Design Pattern   The factory design pattern is characterized by   An Interface Implementation classes A factory   You can observe few things when you question yourself as below   When will the factory create object for the implementation classes - run time or compile      time  What if you want to switch the implementation at run time  - Not possible   These are handled by Dependency injection   Dependency injection  You can have different ways in which you can inject dependency  For simplicity lets go with Interface Injection  In DI  container creates the needed instances  and  injects  them into the object   Thus eliminates the static instantiation   Example   public class MyClass     MyInterface find  null       Constructor- During the object instantiation    public MyClass MyInterface myInterface             find   myInterface          public void myMethod            find doSomething

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Binoj   I don t think you have to choose one over the other    The act of moving a dependent class or interface to a class constructor or setter follows the DI pattern  The object you pass to the constructor or set can be implemented with Factory    When to use  Use the pattern or patterns that are in your developer wheelhouse  What do they feel the most comfortable with and find easiest to understand

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There are problems which are easy to solve with dependency injection which are not so easily solved with a suite of factories   Some of the difference between  on the one hand  inversion of control and dependency injection  IOC DI   and  on the other hand  a service locator or a suite of factories  factory   is   IOC DI is a complete ecosystem of domain objects and services in and of itself  It sets everything up for you in the way you specify  Your domain objects and services are constructed by the container  and do not construct themselves  they therefore do not have any dependencies on the container or on any factories  IOC DI permits an extremely high degree of configurability  with all the configuration in a single place  construction of the container  at the topmost layer of your application  the GUI  the Web front-end    Factory abstracts away some of the construction of your domain objects and services  But domain objects and services are still responsible for figuring out how to construct themselves and how to get all the things they depend on  All these  active  dependencies filter all the way through all the layers in your application  There is no single place to go to configure everything

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From a face value they look same  In very simple terms  Factory Pattern  a Creational Pattern helps to create us an object -  Define an interface for creating an object   If we have a key value sort of object pool  e g  Dictionary   passing the key to the Factory  I am referring to the Simple Factory Pattern  you can resolve the Type  Job done   Dependency Injection Framework  such as Structure Map  Ninject  Unity    etc  on the other hand seems to be doing the same thing   But     Don t reinvent the wheel   From a architectural perspective its a binding layer and  Don t reinvent the wheel    For an enterprise grade application  concept of DI is more of a architectural layer which defines dependencies  To simplify this further you can think of this as a separate classlibrary project  which does dependency resolving  The main application depends on this project where Dependency resolver refers to other concrete implementations and to the dependency resolving    Inaddition to  GetType Create  from a Factory  most often than not we need more features  ability to use XML to define dependencies  mocking and unit testing etc    Since you referred to Structure Map  look at the Structure Map feature list  It s clearly more than simply resolving simple object Mapping  Don t reinvent the wheel   If all you have is a hammer  everything looks like a nail  Depending on your requirements and what type of application you build you need to make a choice  If it has just few projects  may be one or two    and involves few dependencies  you can pick a simpler approach  It s like using ADO  Net data access over using Entity Framework for a simple 1 or 2 database calls  where introducing EF is an overkill in that scenario    But for a larger project or if your project gets bigger  I would highly recommend to have a DI layer with a framework and make room to change the DI framework you use  Use a Facade in the Main App  Web App  Web Api  Desktop  etc

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You use dependency injection when you exactly know what type of objects you require at this point of time  While in case of factory pattern  you just delegate the process of creating objects to factory as you are unclear of what exact type of objects you require

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I believe DI is a type of abstraction layer on factories  but they also provide benefits beyond abstraction   A true factory knows how to instantiate a single type and configure it   A good DI layer provides the ability  through configuration  to instantiate and configure many types   Obviously  for a project with a few simple types that requires relatively stable business logic in their construction  the factory pattern is simple to understand  implement  and works well   OTOH  if you have a project containing numerous types whose implementations you expect to change often  DI gives you the flexibility through its configuration to do this at runtime without having to recompile your factories

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Dependency Injection  Instead of instantiating the parts itself a car asks for the parts it needs to function   class Car       private Engine engine      private SteeringWheel wheel      private Tires tires       public Car Engine engine  SteeringWheel wheel  Tires tires                this engine   engine          this wheel   wheel          this tires   tires            Factory  Puts the pieces together to make a complete object and hides the concrete type from the caller   static class CarFactory       public ICar BuildCar                 Engine engine   new Engine            SteeringWheel steeringWheel   new SteeringWheel            Tires tires   new Tires            ICar car   new RaceCar engine  steeringWheel  tires           return car               Result  As you can see  Factories and DI complement each other   static void Main          ICar car   CarFactory BuildCar            use car       Do you remember goldilocks and the three bears  Well  dependency injection is kind of like that  Here are three ways to do the same thing   void RaceCar      example  1       ICar car   CarFactory BuildCar        car Race       void RaceCar ICarFactory carFactory     example  2       ICar car   carFactory BuildCar        car Race       void RaceCar ICar car     example  3       car Race        Example  1 - This is the worst because it completely hides the dependency  If you looked at the method as a black box you would have no idea it required a car   Example  2 - This is a little better because now we know we need a car since we pass in a car factory  But this time we are passing too much since all the method actually needs is a car  We are passing in a factory just to build the car when the car could be built outside the method and passed in   Example  3 - This is ideal because the method asks for exactly what it needs  Not too much or too little  I don t have to write a MockCarFactory just to create MockCars  I can pass the mock straight in  It is direct and the interface doesn t lie   This Google Tech Talk by Misko Hevery is amazing and is the basis of what I derived my example from  http   www youtube com watch v XcT4yYu TTs

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I think these are orthogonal and can be used together  Let me show you an example I recently came across at work   We were using the Spring framework in Java for DI  A singleton class  Parent  had to instantiate new objects of another class  Child   and those had complex collaborators    Component class Parent                   Autowired     Parent Dep1 dep1  Dep2 dep2       DepN depN            this dep1   dep1          this dep2   dep2             void method int p            Child c   new Child dep1  dep2       depN  p                            In this example  Parent has to receive DepX instances only to pass them to the Child constructor  Problems with this    Parent has more knowledge of Child than it should Parent has more collaborators than it should Adding dependencies to Child involves changing Parent   This is when I realized a Factory would fit here perfectly    It hides all but the true parameters of the Child class  as seen by Parent It encapsulates the knowledge of creating a Child  which can be centralized in the DI configuration    This is the simplified Parent class and the ChildFactory class    Component class Parent                   Autowired     Parent ChildFactory childFactory            this childFactory   childFactory             void method int p            Child c   childFactory newChild p                            Component class ChildFactory                   Autowired     Parent Dep1 dep1  Dep2 dep2       DepN depN            this dep1   dep1          this dep2   dep2                         this depN   depN             Child newChild int p            return new Child dep1  dep2       depN  p

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IOC is a concept which is implemented by two ways  Dependency creation and dependency injection  Factory Abstract factory are the example of dependency creation  Dependency injection is constructor  setter and interface  The core of IOC is to not depend upon the concrete classes  but  define the abstract of methods say an Interface abstract class  and use that abstract to call method of concrete class  Like Factory pattern return the base class or interface  Similariliy dependency injection use base class interface to set value for objects

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Using dependency injection is much better in my opinion if you are   1  deploying your code in small partition  because it handles well in decoupling of one big code  2  testability is one of the case DI is ok to use because you can mock easily the non decoupled objects  with the use of interfaces you can easily mock and test each objects  3  you can simultaneously revised each part of the program without needing to code the other part of it since its loosely decoupled

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i believe that DI is a way of configurings or instantianting a bean  The DI can be done in many ways like constructor  setter-getter etc   Factory pattern is just another way of instantiating beans  this pattern will be used mainly when you have to create objects using factory design pattern because while using this pattern you dont configure the properties of a bean  only instantiate the object   Check this link  Dependency Injection

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The reason Dependency Injection  DI  and Factory Patterns are similar is because they are two implementations of Inversion of Control  IoC  which is a software architecture  Put simply they are two solutions to the same problem   So to answer the question the main difference between the Factory pattern and DI is how the object reference is obtained  With dependency injection as the name implies the reference is injected or given to your code  With Factory pattern your code must request the reference so your code fetches the object  Both implementations remove or decouple the linkage between the code and the underlying class or type of the object reference being used by the code   It s worth noting that Factory patterns  or indeed Abstract Factory patterns which are factories that return new factories that return object references  can be written to dynamically choose or link to the type or class of object being requested at run time  This makes them very similar  even more so than DI  to Service Locator pattern which is another implementation of the IoC   The Factory design pattern is quite old  in terms of Software  and has been around for a while  Since the recent popularity of the architectural pattern IoC it is having a resurgence   I guess when it comes to IoC design patterns  injectors be injecting  locators be locating and the factories have been refactored

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Most answers here explain conceptual difference and implementation details of both  However I was unable to find explanation about difference in application which IMO is the most important and the OP asked about  So let me reopen this topic        Once someone told me that its how you use it that makes a difference    Exactly  In 90  cases you can obtain object reference using either Factory or DI and usually you end up with latter  In another 10  cases using Factory is only correct way  These cases include obtaining objects by variable at runtime parameters  Like this   IWebClient client   factoryWithCache GetWebClient url   stackoverflow com           useCookies  false  connectionTimeout  120     In this case getting client from DI is not possible  or at least requires some ugly workaround   So as a general rule to make decision  if a dependency can be obtained without any runtime calculated parameters then DI is preferred  otherwise use Factory

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An Injection Framework is an implementation of the Factory Pattern     It all depends upon your requirements   If you have need to implement the factory pattern in an application  it s highly likely your requirements will be met by one of the myriad of injection framework implementations out there     You should only roll out your own solution if your requirements cannot be met by any of the 3rd party frameworks  The more code you write  the more you code you have to maintain  Code is a liability not an asset   Arguments over which implementation you should use is not as fundamentally important as understanding the architectural needs of your application

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I use both to create an Inversion Of Control strategy with more readability for developers who need to maintain it after me    I use a Factory to create my different Layer objects  Business  Data Access    ICarBusiness carBusiness   BusinessFactory CreateCarBusiness      Another developer will see this and when creating an Business Layer object he looks in BusinessFactory and Intellisense gives the developer all the possible Business Layers to create  Doesn t have to play the game  find the Interface I want to create   This structure is already Inversion Of Control  I am no longer responsible for creating the specific object  But you still need to ensure Dependency Injection to be able to change things easily   Creating your own custom Dependency Injection is ridiculous  so I use Unity  Within the CreateCarBusiness   I ask Unity to Resolve which class belongs to this and it   s lifetime   So my code Factory Dependency Injection structure is   public static class BusinessFactory       public static ICarBusiness CreateCarBusiness                return Container Resolve lt ICarBusiness gt               Now I have the benefit of both  My code is also more readable for other developers as towards scopes of my objects I use  instead of Constructor Dependency Injection which just says every object is available when the class is created    I use this to change my Database Data Access to a custom coded Data Access layer when I create Unit Tests  I don   t want my Unit Tests to communicate with databases  webservers  e-mail servers etc  They need to test my Business Layer because that   s where the intelligence is

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