I have a class that needs a dependency injecting. As the class is already an implementation of another abstraction, and its 'sibling' implementations may not share the same dependencies, I am attempting to use property injection and not constructor injection.
(All these classes/interface names are just for illustrative purposes)
My IProvider abstraction:
public interface IProvider
{
void ProviderMethod();
}
My IProvider implementation (with the IData dependency I want to inject):
public class ProviderClass : IProvider
{
// How do I inject this dependency?
[Dependency]
public IData data { get; set; }
public void ProviderMethod()
{
// Can't do this as data == null!
data.DataMethod();
}
}
Another IProvider implementation (example to show that it doesn't have the same dependencies):
public class AnotherProviderClass : IProvider
{
// No data dependency here!!
public void ProviderMethod()
{
// Do other stuff here
}
}
Example IData abstraction and implementation:
public interface IData
{
void DataMethod();
}
public class DataClass : IData
{
public void DataMethod();
}
What I need to know is: How do I successfully inject the property dependency (IData) into ProviderClass using Unity (my IOC container of choice)?
I have tried all manner of Unity registering options (RegisterType, RegisterInstance, Resolve...) but my injected property always ends up as NULL. I want to do this right and not just force random code in until it just manages to work.
Or is there a better way of injecting (optional) dependencies into 'sibling' classes?
Incidentally, my initial IProvider implementations are created via an abstract factory, so maybe that might be another area I should focus this IData dependency on(?)
You should still use constructor injection because dependencies should hardly ever be optional.
You are trying to prevent constructor over-injection in the IProviderFactory implementation, and you probably don't want to inject the container into your factory to prevent falling into the Service Locator anti-pattern.
If however you define your IProviderFactory implementation INSIDE your Composition Root, you prevent yourself from doing Service Locator, even though you inject the container, since Service Locator is not about mechanics.
So you should define your ProviderFactory implementation as close to your Unity configuration as possible and it should look something like this:
public class ProviderFactory : IProviderFactory
{
private readonly Dictionary<string, Type> providerTypes;
private readonly Container container;
public ProviderFactory(Dictionary<string, Type> providerTypes,
Container container) {
this.providerTypes = providerTypes;
this.container = container;
}
public IProvider CreateProvider(string name) {
return (IProvider)this.container.Resolve(this.providerTypes[name]);
}
}
This implementation can be registered as singleton in Unity. This saves you from having to do constructor over-injection into your factory, while staying away from Service Locator.
Related
I have the following problem. I have an interface that is implemented by 2 classes. One of these classes does the real work while the other uses the first one mentioned.
How can I tell the framework to use a specific implementation of an interface when instantiating a type of object?
I want the controller to get the facade implementation and not the real one:
public interface IDependency
{
void Method();
}
public class Real : IDependency
{
public void Method()
{
}
}
public class Facade : IDependency
{
private IDependency dependency;
Facade(IDependency dependency) //I want a Real here
{
this.dependency=dependency;
}
public void Method()=>dependency.Method();
}
public MyController : Controller
{
private IDependency dependency;
MyController(IDependency dependency) //I want the `Facade` here not the `Real`
{
this.dependency=dependency;
}
[HttpGet]
[Route("[some route]")]
public async Task CallMethod()
{
await this.dependency.Method();
}
}
As you can see in the above example, I need a Real type of IDependency injected inside my Facade one, while I need a Facade type of IDependency injected in my MyController.
How could that be done?
Register your dependency as follows:
public void ConfigureServices(IServiceCollection services)
{
services.AddControllers();
services.AddTransient<IDependency>(_ => new Facade(new Real()));
}
If you have other controllers that need a different implementation of IDependency, you'll want to register your controllers as services, allowing the registrations to be overwritten. For example, if you want most controllers to resolve with IDependency as Real, but only MyController to resolve IDependency as Facade, you could do this:
public void ConfigureServices(IServiceCollection services)
{
// Adds controllers as services, allowing their registrations to be overwritten.
services.AddMvc().AddControllersAsServices();
//services.AddControllers(); REMOVE THIS
// Makes Real the default implementation of IDependency
services.AddTransient<IDependency, Real>();
// Overwrite the default registration of MyController to instantiate using Facade.
services.AddTransient<MyController>(sp =>
new MyController(new Facade(sp.GetService<IDependency>())));
}
Microsoft.Extensions.DependencyInjection doesn't have any means to do this. All you can do is inject all implementations, i.e. via List<IDependency>. Then, you can implement whatever manual logic you'd like to pick and choose from the available options there.
If you have any control over these classes, it would be better to simply implement a different interface, to distinguish the two. For example, you could create an IFacadeDependency interface that inherits from IDependency, and then have your Facade class implement that instead. Then, you can inject IFacadeDependency and get exactly what you want.
These days I'm facing this situation often and I'm looking for an elegant solution. I have :
public abstract class TypeA
{
public abstract void AbtractMethod(IDependency dependency);
}
public class TypeB : TypeA
{
public override void AbtractMethod(ISpecializedDependencyForB dependency) { }
}
public class TypeC : TypeA
{
public override void AbtractMethod(ISpecializedDependencyForC dependency) { }
}
public interface IDependency { }
public interface ISpecializedDependencyForB : IDependency { }
public interface ISpecializedDependencyForC : IDependency { }
My objective is to make things transparent in the client perspective and to consume this code like that :
TypeA myDomainObject = database.TypeARepository.GetById(id); // The important point here is that I don't know if the object is of TypeB or TypeC when I consume it.
IDependency dependency = ? // How do I get the right dependency
myDomainObject.AbtractMethod(dependency);
So the thing is that since I don't know the concrete type of the object, I can't inject the right dependency into it.
What I'm currently doing is that I create an abstract factory, to inject the right properties. I have two problems with that, the first one is that I would end up with a lot of factories. The second one is that it makes polymorphism useless since the client actually needs to care about "managing" the underlying type (I need to inject all the possible dependencies in the factory, and to instantiate the factory on the client code).
1) Therefore I was thinking of using property injection with unity, but I can't find out if it's possible to resolve the dependencies of an object, after it's been instanciated manually. Even with this approach I think I could still meet the same problem : I'm not sure if unity would check the actual type of the object and resolve the right dependency if a syntax like this existed :
unityContainer.Resolve<TypeA>(myDomainObject)
If not, I would need to know the type in advance and would be back to the same problem.
2) I have found this article mentionning that EF provides some mechanism for DI, but it seems that it is only meant to inject the framework services (PluralizationService, etc...). Otherwise it would have been a nice way to achieve that.
3) I could also not use DI in this case... It looks like by concept DI does not fit well with polymorphism. I'm not excited by this idea though.
I'd be happy to have a solution for the property injection I'm trying to achieve, or an idea of pattern I could use. However I really don't want to create a big infrastructure and obfuscate my code just for this purpose.
Note : I don't want to you use domain events in this case.
Thank you
TL;DR
Replace the IDependency parameter of the polymorphic AbstractMethod with an implementation-specific construction dependency parameter, which is injected by the IoC container, not by the consumer.
In more detail
The original class hierarchy will need to look like more like this for inheritance polymorphicism to work, as the superclass virtual method and subclass override methods must match signatures:
public abstract class TypeA // superclass
{
public abstract void AbtractMethod(IDependency dependency);
}
public class TypeB : TypeA // subclass 1
{
public override void AbtractMethod(IDependency dependency)
{
Contract.Requires(dependency is ISpecializedDependencyForB);
// ...
}
}
public class TypeC : TypeA // subclass 2
{
public override void AbtractMethod(IDependency dependency)
{
Contract.Requires(dependency is ISpecializedDependencyForC)
// ...
}
}
However, some things don't ring true with this design:
The LSP appears to be violated, since the although AbtractMethod() advertises that it accepts the base IDependency interface, the two subclasses actually depend on a specialized subclassed dependency.
It is also unusual, and arguably inconvenient, for a caller of these methods to build up the correct dependency and pass it to the method in order for it to be invoked correctly.
So, if possible, I would adopt a more conventional approach to the arrangement of dependencies, whereby the dependency is passed to the subclass constructor, and will be available to the polymorphic method when needed. This decouples the need to supply the appropriate IDependency to the method. Leave it to the IoC container to do the appropriate dependency resolution:
Use constructor injection to create the correct dependency into Classes TypeB and TypeC
If there is a secondary requirement to Expose an IDependency on the base class TypeA to consumers, then add an additional abstract property to TypeA of type IDependency (but this seems iffy)
As per Ewan's observation, the repository would need some kind of strategy pattern in order to serve up polymorphic domain entities (B or C). In which case, couple the repository to a factory to do exactly this. The concrete factory would need to be bound to the container in order to tap into Resolve().
So putting this all together, you might wind up with something like this:
using System;
using System.Diagnostics;
using Microsoft.Practices.Unity;
namespace SO29233419
{
public interface IDependency { }
public interface ISpecializedDependencyForB : IDependency { }
public interface ISpecializedDependencyForC : IDependency { }
public class ConcreteDependencyForB : ISpecializedDependencyForB {};
public class ConcreteDependencyForC : ISpecializedDependencyForC { };
public abstract class TypeA
{
// Your polymorphic method
public abstract void AbtractMethod();
// Only exposing this for the purpose of demonstration
public abstract IDependency Dependency { get; }
}
public class TypeB : TypeA
{
private readonly ISpecializedDependencyForB _dependency;
public TypeB(ISpecializedDependencyForB dependency)
{
_dependency = dependency;
}
public override void AbtractMethod()
{
// Do stuff with ISpecializedDependencyForB without leaking the dependency to the caller
}
// You hopefully won't need this prop
public override IDependency Dependency
{
get { return _dependency; }
}
}
public class TypeC : TypeA
{
private readonly ISpecializedDependencyForC _dependency;
public TypeC(ISpecializedDependencyForC dependency)
{
_dependency = dependency;
}
public override void AbtractMethod()
{
// Do stuff with ISpecializedDependencyForC without leaking the dependency to the caller
}
public override IDependency Dependency
{
get { return _dependency; }
}
}
public interface ITypeAFactory
{
TypeA CreateInstance(Type typeOfA);
}
public class ConcreteTypeAFactory : ITypeAFactory
{
private readonly IUnityContainer _container;
public ConcreteTypeAFactory(IUnityContainer container)
{
_container = container;
}
public TypeA CreateInstance(Type typeOfA)
{
return _container.Resolve(typeOfA) as TypeA;
}
}
public class TypeARepository
{
private readonly ITypeAFactory _factory;
public TypeARepository(ITypeAFactory factory)
{
_factory = factory;
}
public TypeA GetById(int id)
{
// As per Ewan, some kind of Strategy Pattern.
// e.g. fetching a record from a database and use a discriminating column etc.
return (id%2 == 0)
? _factory.CreateInstance(typeof (TypeB))
: _factory.CreateInstance(typeof (TypeC));
// Set the properties of the TypeA from the database after creation?
}
}
class Program
{
static void Main(string[] args)
{
// Unity Bootstrapping
var myContainer = new UnityContainer();
myContainer.RegisterType<ISpecializedDependencyForB, ConcreteDependencyForB>();
myContainer.RegisterType<ISpecializedDependencyForC, ConcreteDependencyForC>();
myContainer.RegisterType(typeof(TypeB));
myContainer.RegisterType(typeof(TypeC));
var factory = new ConcreteTypeAFactory(myContainer);
myContainer.RegisterInstance(factory);
myContainer.RegisterType<TypeARepository>(new InjectionFactory(c => new TypeARepository(factory)));
// And finally, your client code.
// Obviously your actual client would use Dependency Injection, not Service Location
var repository = myContainer.Resolve<TypeARepository>();
var evenNumberIsB = repository.GetById(100);
Debug.Assert(evenNumberIsB is TypeB);
Debug.Assert(evenNumberIsB.Dependency is ISpecializedDependencyForB);
var oddNumberIsC = repository.GetById(101);
Debug.Assert(oddNumberIsC is TypeC);
Debug.Assert(oddNumberIsC.Dependency is ISpecializedDependencyForC);
}
}
}
Could whatever it is that knows about the dependencies live behind an interface IDependencyProvider which has a function
IDependency GetDependency(Type type).
This could even just return an object and the class that realises the interface needs to know all the sub types and their associated dependencies.
AbstractMethod is then changed to:
void AbstractMethod(IDependencyProvider provider);
In your sub classes you then override this and call
var dependency = provider.GetDependency(this.GetType());
Your middle tier then knows nothing about the sub types or the sub dependencies.
It's an interesting problem, what I was thinking is that your repository knows about and creates the TypeB and TypeC classes and so you can add the correct dependency at that point
public class TypeARepository
{
private ISpecializedDependencyForB depB;
private ISpecializedDependencyForC depC;
public TypeARepository(ISpecializedDependencyForB depB, ISpecializedDependencyForC depC)
{
this.depB = depB;
this.depC = depC;
}
public TypeA GetById(string id)
{
if (id == "B")
{
return new TypeB(depB);
}
else
{
return new TypeC(depC);
}
}
}
The TypeB and TypeC would then implement their abstract methods with their private ref to the dependency rather than having it passed in in the method.
I come across this problem in various forms myself from time to time and it always seems to me that if there is that hard link between the types just having it setup via an injection config or the like is wrong. As it allows the installer to potentially set a bad config
This approach also allows you to inject your dependencies with unity
Thank you a lot for your interest in my question, I came up with a solution yesterday evening. The objective is to keep things transparent for the client and to take full advantage of polymorphism by syntaxes such as baseObjectReference.AbstractMethodCall().
I finally realized that I was able to achieve what I'm after by taking advantage of the static modifier and using it for DI purposes. So I have that :
public abstract class TypeA
{
public abstract void AbtractMethod();
}
public class TypeB : TypeA
{
private ISpecializedDependencyForB SpecializedDependencyForB
{
get
{
return GetSpecializedDependencyForB.CreateSpecializedDependencyForB();
}
}
public override void AbtractMethod() { // do stuff with dependency }
}
public static class GetSpecializedDependencyForB
{
public static ISpecializedDependencyForB DependencyForB
{
return CreateSpecializedDependencyForB();
}
public delegate ISpecializedDependencyForB CreateSpecializedDependencyForBDelegate();
public static CreateSpecializedDependencyForBDelegate CreateSpecializedDependencyForB;
}
And then, in my unity container I add this code :
public static void RegisterTypes(IUnityContainer container)
{
// .... registrations are here as usual
GetSpecializedDependencyForB.CreateSpecializedDependencyForB = CreateMyDomainService;
}
Having this method in the same unity config class :
private ISpecializedDependencyForB CreateMyDomainService()
{
return container.Value.Resolve<ISpecializedDependencyForB>();
}
And finally, I can simply use my object like this :
TypeA myDomainObject = database.TypeARepository.GetById(id);
myDomainObject.AbtractMethod();
And that's it !
So four things here :
The first one is that I inject the delegate that will create and instance of the service.
Then it is thread safe because static member is only written one time at the beginning of the application. All other accesses will be read. Moreover two threads won't share the same instance of the dependency since the delegate creates a new one all the time.
Also one interesting thing is that I can rely on my existing unity container configuration, no extra code is needed. It is important because my dependency may need other dependency to be constructed.
And finally the unity container is anyway also static, so there is no memory leak.
It's basically a manual and easy to set up "DI framework" sitting beside Unity.
And more importantly it works like a charm ! I'm finally satisfied with my design. I will only use this approach for polymorphic situations since injecting the right dependency in the method is easy for other situations. However it might be interesting to fully encapsulate the domain model using this approach.
This question already has answers here:
Multiple implementations for one interface with DI
(2 answers)
Closed 6 years ago.
For the following interfaces and classes, how do I use Unity Container to Fluently (programatically) wire it up so that FooController gets an instance of ARepository and BarController gets an instance of BRepository via constructor injection?
public interface IRepository
{
}
public class ARepository : IRepository
{
}
public class BRepository : ARepository
{
}
public class FooController
{
public FooController(IService service, IRepository repository)
{
}
}
public class BarController
{
public BarController(IService service, IRepository repository)
{
}
}
You can achieve this at registration time by telling each controller registration how to resolve its constructor parameters.
container.RegisterType<FooController>(new InjectionConstructor(
new ResolvedParameter<IService>(), new ResolvedParameter<ARepository>());
container.RegisterType<BarController>(new InjectionConstructor(
new ResolvedParameter<IService>(), new ResolvedParameter<BRepository>());
I would highly recommend against creating a dedicated/local/injected UnityContainer for each type like one of the posters suggested.
This can be approached in two ways.
One is by specifically defining the resolving in registration time as TylerOhlsen suggested.
This is a good solution, though if you register later both ARepository and BRepository as implementations for IRepository, you still need to deal with the fact you have two implenetations for the same interface, and if a third class will someday require an implementation of IRepository, without defining it specifically in the registration, it will get an unpredictable instance.
The second option, which is slightly safer, is registering a factory for IRepository.
The simplest example would be using a string as the key, like this:
// Create a resolver(abstract factory) for the IRepository interface type
var resolver = myContainer.Resolve<Func<IRepository>>();
// ... other code here...
// Register mappings for the IRepository interface to appropriate concrete types
myContainer.RegisterType<IRepository, ARepository>("A");
myContainer.RegisterType<IRepository, BRepository>("B");
Then in the implementation of FooController and BarController receive the func factory by injection and select the right instance.
public class FooController
{
IRepository repository;
public FooController(IService service, Func<IRepository> repositoryFactory)
{
repository = repositoryFactory("A");
}
}
You can read more about this here:
http://msdn.microsoft.com/en-us/library/ff660854%28v=pandp.20%29.aspx
You can register the type with a name and then use that in a Dependency attribute of the target classes:
// Register mappings for the IRepository interface to appropriate concrete types
myContainer.RegisterType<IRepository, ARepository>("A");
myContainer.RegisterType<IRepository, BRepository>("B");
Then in the FooController and BarController, declare which implementation you require with the Dependency attribute:
public class FooController
{
public FooController(IService service, [Dependency("A")] IRepository repository)
{
}
}
public class BarController
{
public BarController(IService service, [Dependency("B")] IRepository repository)
{
}
}
You can use a public const string in ARepository and BRepository rather than "A" and "B" in both RegisterType and Dependency.
One possibility is to register the specific type to be resolved by the UnityContainer when resolving IRepository:
IUnityContainer container = new UnityContainer();
container.RegisterType<IRepository, BRepository>(new ContainerControlledLifetimeManager());
Using this depends on whether you require finer grained control over what types are resolved in certain contexts - if you do, you might consider using a local IUnityContainer instance and using RegisterInstance() instead:
//Assumes container is instantiated and already populated with other instances/type mappings.
IUnityContainer childContainer = container.CreateChildContainer();
container.RegisterInstance<IRepository>(new BRepository(), new ContainerControlledLifetimeManager());
JoefGoldstein got me most of the way, but I got a few errors when attempting that.
I had to register my classes with a named dependency and use the InjectionFactory class to resolve the dependency.
// register interfaces to implementations with a named dependency
myContainer.RegisterType<IRepository, ARepository>("A");
myContainer.RegisterType<IRepository, BRepository>("B");
// register Injection factory to resolve the dependency
container.RegisterType<Func<string, IRepository>>(
new InjectionFactory(c =>
new Func<string, IRepository>(name => c.Resolve<IRepository>(name)))
);
Then in my controller
public class FooController
{
IRepository repository;
public FooController(IService service, Func<string, IRepository> repositoryFactory)
{
repository = repositoryFactory("A");
}
}
I want to inject Container property via SimpleInjector. I didn't find any functionality of SimpleInjector for that.
Then I wanted to register self container to itself, but Container has no interface.
I want this functionality because I don't to transfer Container object via constructor - because why if I can use auto inject of register objects.
My usage idea:
var container = new Container();
container.Options.AutowirePropertiesWithAttribute<InjectableProperty>();
container.Register<ISomething, Something>(Lifestyle.Singleton);
ISomething:
public interface ISomething
{
void SomeMethod();
}
Something class:
public class Something : ISomething
{
public void SomeMethod()
{
var environment = _container.GetInstance<IEnvironment>();
environment.DoSomething();
}
[InjectableProperty] // - maybe it is not possible (I don't know it)
Container Container {get;set;}
}
Do you have any idea to achieve that?
Thank you very much.
Prevent having your application code depend upon the container. The only place in your application that should know about the existence of your DI library is the Composition Root (the place where you register all your dependencies).
Instead of letting each class call back into the container (which is called the Service Locator anti-pattern), prefer using Dependency Injection. With Dependency Injection you inject dependencies instead of asking for them.
So you can rewrite your class to the following:
public class Something : ISomething
{
private readonly IEnvironment environment;
public Something (IEnvironment environment)
{
this.environment = environment;
}
public void SomeMethod()
{
this.environment.DoSomething();
}
}
Also, prevent doing any logic in your constructors besides storing the incoming dependencies. This allows you to compose object graphs with confidence.
In some cases however, it can still be useful to inject the Container into another class. For instance when creating a factory class that is located inside the Composition Root. In that case you can still use constructor injection, like this:
// Defined in an application layer
public interface IMyFactory
{
IMyService CreateService();
}
// Defined inside the Composition Root
public class MyFactory : IMyFactory
{
private readonly Container container;
public MyFactory(Containter container)
{
this.container = container;
}
public IMyService CreateService(ServiceType type)
{
return type == ServiceType.A
? this.container.GetInstance<MyServiceA>()
: this.container.GetInstance<MyServiceB>();
}
}
If Simple Injector detects a Container constructor argument, it will inject itself into the constructor automatically.
I have the IRespository registered twice (with names) in the following code:
// Setup the Client Repository
IOC.Container.RegisterType<ClientEntities>(new InjectionConstructor());
IOC.Container.RegisterType<IRepository, GenericRepository>
("Client", new InjectionConstructor(typeof(ClientEntities)));
// Setup the Customer Repository
IOC.Container.RegisterType<CustomerEntities>(new InjectionConstructor());
IOC.Container.RegisterType<IRepository, GenericRepository>
("Customer", new InjectionConstructor(typeof(CustomerEntities)));
IOC.Container.RegisterType<IClientModel, ClientModel>();
IOC.Container.RegisterType<ICustomerModel, CustomerModel>();
But then when I want to resolve this (to use the IRepository) I have to do a manual resolve like this:
public ClientModel(IUnityContainer container)
{
this.dataAccess = container.Resolve<IRepository>(Client);
.....
}
What I would like to do is to have it resolved in the constructor (just like IUnityContainer). I need some way to say which named type to resolve to.
Something like this: (NOTE: Not real code)
public ClientModel([NamedDependancy("Client")] IRepository dataAccess)
{
this.dataAccess = dataAccess;
.....
}
Is there a way to make my fake code work?
You can configure dependencies with or without names in the API, attributes, or via the config file. You didn't mention XML above, so I'll assume you're using the API.
To tell the container to resolve a named dependency, you'll need to use an InjectionParameter object. For your ClientModel example, do this:
container.RegisterType<IClientModel, ClientModel>(
new InjectionConstructor( // Explicitly specify a constructor
new ResolvedParameter<IRepository>("Client") // Resolve parameter of type IRepository using name "Client"
)
);
This tells the container "When resolving ClientModel, call the constructor that takes a single IRepository parameter. When resolving that parameter, resolve with the name 'Client' in addition to the type."
If you wanted to use attributes, your example almost works, you just need to change the attribute name:
public ClientModel([Dependency("Client")] IRepository dataAccess)
{
this.dataAccess = dataAccess;
.....
}
This is a very late response but the question still shows up in Google.
So anyways, 5 years later...
I have a pretty simple approach. Usually when you need to use "named dependency" it's because you're trying to implement some kind of strategy pattern. In that case, I simply create a level of indirection between Unity and the rest of my code called the StrategyResolver to not be directly depending on Unity.
public class StrategyResolver : IStrategyResolver
{
private IUnityContainer container;
public StrategyResolver(IUnityContainer unityContainer)
{
this.container = unityContainer;
}
public T Resolve<T>(string namedStrategy)
{
return this.container.Resolve<T>(namedStrategy);
}
}
Usage:
public class SomeClass: ISomeInterface
{
private IStrategyResolver strategyResolver;
public SomeClass(IStrategyResolver stratResolver)
{
this.strategyResolver = stratResolver;
}
public void Process(SomeDto dto)
{
IActionHandler actionHanlder = this.strategyResolver.Resolve<IActionHandler>(dto.SomeProperty);
actionHanlder.Handle(dto);
}
}
Registration:
container.RegisterType<IActionHandler, ActionOne>("One");
container.RegisterType<IActionHandler, ActionTwo>("Two");
container.RegisterType<IStrategyResolver, StrategyResolver>();
container.RegisterType<ISomeInterface, SomeClass>();
Now, the nice thing about this is that I will never have to touch the StrategyResolver ever again when adding new strategies in the future.
It's very simple. Very clean and I kept the dependency on Unity to a strict minimum. The only time I would have touch the StrategyResolver is if I decide to change container technology which is very unlikely to happen.
Hope this helps!
Edit: I don't really like the accepted answer because when you use the Dependency attribute in your service's constructor you actually have a hard dependency on Unity. The Dependency attribute is part of the Unity library. At that point you might as well pass an IUnityContainer dependency everywhere.
I prefer having my service classes depend on objects that I completely own instead of having a hard dependency on an external library all over the place. Also using Dependency attribute makes the constructors signatures less clean and simple.
Furthermore, this technique allows to resolve named dependencies at runtime without having to hardcode the named dependencies in the constructor, in the application configuration file or use InjectionParameter which are all methods that require to know what named dependency to use at design time.
Edit (2016-09-19):
For those that might wonder, the container will know to pass itself when you are requesting IUnityContainer as dependency, as shown in the StrategyResolver constructor signature.
Edit (2018-10-20):
Here's another way, simply using a factory:
public class SomeStrategyFactory : ISomeStrategyFactory
{
private IStrategy _stratA;
private IStrategy _stratB;
public SomeFactory(IStrategyA stratA, IStrategyB stratB)
{
_stratA = stratA;
_stratB = stratB;
}
public IStrategy GetStrategy(string namedStrategy){
if (namedStrategy == "A") return _stratA;
if (namedStrategy == "B") return _stratB;
}
}
public interface IStrategy {
void Execute();
}
public interface IStrategyA : IStrategy {}
public interface IStrategyB : IStrategy {}
public class StrategyA : IStrategyA {
public void Execute(){}
}
public class StrategyB : IStrategyB {
public void Execute() {}
}
Usage:
public class SomeClass : ISomeClass
{
public SomeClass(ISomeStrategyFactory strategyFactory){
IStrategy strat = strategyFactory.GetStrategy("HelloStrategy");
strat.Execute();
}
}
Registration:
container.RegisterType<ISomeStrategyFactory, SomeStrategyFactory>();
container.RegisterType<IStrategyA, StrategyA>();
container.RegisterType<IStrategyB, StrategyB>();
container.RegisterType<ISomeClass, SomeClass>();
This 2nd suggestion is the same thing but using the factory design pattern.
Hope this helps!
You should be able to use ParameterOverrides
var repository = IOC.Container.Resolve<IRepository>("Client");
var clientModel = IOC.Container.Resolve<ClientModel>(new ParameterOverrides<ClientModel> { {"dataAccess", repository } } );
edit:
I'm not sure why you're passing around the UnityContainer - personally, we inject our dependencies into the constructor themselves (which is "normal" from what I've seen). But regardless, you can specify a name in your RegisterType and Resolve methods.
IOC.Container.RegisterType<IRepository, GenericRepository>("Client");
IOC.Container.Resolve<IRepository>("Client");
and it will give you the type you registered for that name.
Don't do this - just create a class ClientRepository : GenericRepository { } and utilise the Type system.