I've a WCF Service which is hosted inside MVC Application. Service interacts with several Repository (I've Code First design) to do its job. Currently I create an instance of all Repository classes inside each Service method, I think it is bad and my Service is totally coupled to Repository classes. I want to know how should I implement a nice and clean DI for WCF Service.
Thanks in Advance.
One approach you can take is to inject a repository factory in your service class and then call/get your repository from the factory.
Repository Factory:
public interface IRepositoryFactory
{
IRepositoryOne GetRepositoryOne();
IRepositoryTwo GetRepositoryTwo();
}
public class RepositoryFactory: IRepositoryFactory
{
public DataAccess.RepositoryInterfaces.IRepositoryOne GetRepositoryOne()
{
return new RepositoryOne();
}
public DataAccess.RepositoryInterfaces.IRepositoryTwo GetRepositoryTwo()
{
return new RepositoryTwo();
}
}
Service Class:
public ServiceClass: IService
{
private readonly IRepositoryFactory _repositoryFactory;
public ServiceClass(IRepositoryFactory factory)
{
_repositoryFactory = factory;
}
public IList<YourItems> GetYourItems()
{
var repository = _repositoryFactory.GetRepositoryOne();
return repository.GetItems(....);
}
}
With this approach, you'll need to register and resolve only your repository factory, not all the individual repositories. This is sort of hybrid approach, but I think it's very clean and easy to understand. Of course, you can always not use a factory and resolve your repositories in every call. I can show a sample of that too, if you'd like.
I would recommend using the Dependency Inversion Principle: Have your repositories implement a specific interface, then have your service classes take in an object (or objects) of that interface (or interfaces). Do not have it directly reference the concrete class. Then, all you'd need to do on your service class is call a method that's exposed by the interface to bring up any/all of the information that you want.
Doing so will de-couple the code from each other, since they'd both be relying on abstractions, and you'll still get the wonderful functionality that you're requesting.
Here's how you could go about doing it: Let's say your WCF service class needs RepositoryA, which implements IRepositoryA. What you would do is have a field (usually private) of type IRepositoryA on it. Then create a constructor in the service that takes in an object of type IRepositoryA, and then sets the field variable with that object being passed in. Something like what's found on this site:
For more information on the Dependency Inversion Principle, just read what Uncle Bob has to say.
Related
I am using the Microsoft.Extensions.DependencyInjection 5.0.1 nuget package.
Here's the Dependency Injection service that I created:
public partial class BehaviorService : IBehaviorService
{
public BehaviorService()
{
}
public class MyClass : MyClassBase
{
public void MyMethod()
{
}
}
}
public interface IBehaviorService
{
}
As MyClass inherits from MyClassBase, I was thinking it was best to put that inside of a service—but I am very new to DI, so maybe it should be it's own service? I hope I can get advice here.
What I am not sure how to do is access the method inside the MyClass class that's inside of my BehaviorService, and how can I identify that in my interface?
Note: To access the service, I was thinking to use something like:
var x = Startup.ServiceProvider.GetRequiredService<BehaviorService>();
There are a few conceptual problems here that you’re going to run into. I’m going to approach the problem from the outside in, eventually getting down to your specific question. The short answer to your immediate question, though, is that you either need to register and consume MyClass as its own service, or you need to instantiate and expose it from BehaviorService, and communicate that via your IBehaviorService interface. Before you get there, though, you’ll want to work through the broader conceptual problems discussed below.
Constructor Injection
Your dependencies should be passed into consuming class via the constructor, and wired up by your dependency injection container (i.e., the Microsoft dependency injection library). You should not call out to your dependency injection container from within your consuming class—by e.g. calling GetRequiredService()—as that just establishes a hard-coded dependency on your dependency injection container itself, while also obscuring the dependencies of your application from external callers.
Note: Calling to your dependency injection container from within a consuming class is known as the service locator pattern, and it’s usually regarded as an anti-pattern.
Code to Interfaces
The dependency passed into your consuming class’s constructor should be your IBehaviorService interface, and that’s all your consuming class should ever be aware of. That means your consuming class has no awareness of implementation details that are specific to your concrete BehaviorService class unless they’re reflected in the IBehaviorService interface. This allows you to swap out implementations without maintaining a dependency on any one concrete implementation—which is the primary goal of dependency injection. Currently, your IBehaviorService contains no members, so there’s nothing consuming classes can do to interact with it.
Note: Acknowledging that part of your question is how to expose your nested method via your interface—I’ll circle back to that below.
Accessing a Nested Class
There are three basic ways of accessing your nested MyClass class:
Establish an interface for it, register it with your dependency injection container, and inject it into a consuming class via the injector—i.e., to treat it exactly like an independent service.
Do the same as the above, but inject it into BehaviorService instead of your consuming class. That makes sense if BehaviorService relies on MyClass, but your consuming class doesn’t—which doesn’t sound like the case here.
Have your parent BehaviorService class create and expose an instance of MyClass via a member registered on the IBehaviorService. In this case, that member needs to be added to your IBehaviorService interface.
Of these, the first should be your preference unless BehaviorService depends on MyClass, in which case the second should be your preference. If both your consuming class and BehaviorService depend on MyClass then it should be injected into both. Finally, there are a lot of potential issues with the last option, so I’d avoid it; if you need to expose another class from a dependency, it should either be a well-known class (e.g., in the same project as your interface), or should adhere to an interface or base class which is.
Nested Classes vs. Services
Critically, in any of these cases, there is no real need or benefit to having MyClass nested; the same options are available if it were a separate class. From dependency injection’s perspective, it doesn’t really matter either way. So the question you should be asking yourself is what problem are you trying to solve by nesting it?
(Personally, I very rarely find cases where a nested classes are desirable—and especially public nested classes—but I don’t know your use case.)
Accessing Just a Nested Method
The above assumes you want to access the nested class. If you really only care about the method on the nested class, and only need it it context of your service, then the simpler approach is for your interface expose that method:
public interface IBehaviorService
{
void MyMethod();
}
And then have your BehaviorService proxy a request to an instance of the nested class:
public partial class BehaviorService : IBehaviorService
{
private readonly MyClass myClass;
public BehaviorService()
{
myClass = new MyClass();
}
public void MyMethod() => myClass.MyMethod();
private class MyClass : MyClassBase
{
public void MyMethod()
{
}
}
}
In this case, neither your interface or your consumers need to be aware of the existence of the nested class—and, in fact, you might even be able to mark it as private. Your BehaviorService is effectively acting as a facade to the nested class. This works well if a) MyClass is really specific to your BehaviorService implementation, and/or b) you only need access to a couple of members of MyClass.
I'm new to DI in .NET C# & autofac and having problems to understand how to use DI when I can't fully control the caller side.
There are two scenarios I have problems to understand.
Scenario 1: Caller expects a default constructor (without any parameters)
How to handle this scenario when I still want to inject some Service Interfaces when the class is constructed? I was thinking of constructor chaining, but that would mean I have to know the concrete type and it works around the idea of DI. (at least I think).
public class ServiceWorker
{
IService _service;
public ServiceWorker(IService service)
{
_service = service
}
}
public class Caller
{
// No way to change this.
var serviceWorker = new ServiceWorker();
}
Scneario 2: Caller expects a specific constructor signature (e.g.
Same question here. How can I inject additional dependencies when the caller expects an exact match for the constructor signature?
I think my main issue in understanding the concept is, that I don't see how to do DI only partially when not everything is constructed by DI (caller)
public class ServiceWorker
{
IService _service;
public ServiceWorker(string name, string id, IService service)
{
_service = service
}
}
public class Caller
{
// No way to change this.
var serviceWorker = new ServiceWorker(name, id);
}
I know, this is pretty basic, but I believe I need to understand this first before moving on. Are there alternatives?
As Steven correctly points out in his comment, being restricted to a specific constructor signature is an instance of the Constrained Construction anti-pattern. Sometimes, however, that can be outside your control.
An example is the so-called 'Provider pattern', which isn't a pattern at all. In such an example, you may have to play by the rules of a third-party framework, so there isn't much you can do about it. Such frameworks should be considered unfriendly to Dependency Injection.
Consider scenario 2 in the OP, since scenario 1 is just a special case of scenario 2. If you must supply a class with a certain constructor, you could create a Facade that has the required constructor signature. This enables you to avoid polluting your well-designed classes that use Dependency Injection with the constraints imposed by the third-party framework. It could look like this:
public class ServiceWorker
{
IService _service;
public ServiceWorker(string name, string id, IService service)
{
_service = service
}
}
public class ServiceWorkerFacade
{
ServiceWorker imp;
public ServiceWorkerFacade(string name, string id)
{
imp =
new ServiceWorker(
name,
id,
new FooService(
new BarService(),
new BazService());
}
}
public class Caller
{
// No way to change this.
var serviceWorker = new ServiceWorkerFacade(name, id);
}
FooService implements IService. Just to make things interesting, I've assumed that FooService has dependencies of its own, and that those dependencies are satisfied by BarService and BazService.
As Steven suggests, then, you can (from necessity) consider the Facade's constructor the Composition Root.
If you have any opportunity to influence the design of the framework in question, you could point the developers to my guidance on designing DI-friendly frameworks.
I'm trying to implement IoC in my app. I have this model:
interface IService;
interface IComponent;
class Service : IService
Service()
class Component : IComponent
Component(IService service, object runtimeValue) { }
At some point in my app I need to get a IComponent. My app uses a IoC container (Unity). I can register Service with the container but I can't do the same for Component b/c of its dependency runtimeValue. According to this I have to use a factory and inject that wherever I need to get a IComponent:
interface IComponentFactory
IComponent CreateComponent(object runtimeValue)
class ComponentProvider : IComponentProvider
ComponentProvider(IComponentFactory factory) { }
IComponent CreateAndCacheComponent(object runtimeValue) {
_component = factory.CreateComponent(runtimeValue)
return _component
}
// other methods
I must be able to register the factory with the container, so it must have only static dependencies. At the same time it must be able to provide a service instance of type IService required to create a component.
Here is the factory implementation. The only thing I could think of was to use a Func<> delegate as dependency:
class ComponentFactory : IComponentFactory
ComponentFactory(Func<IService> serviceFactoryDelegate)
IComponent CreateComponent(object runtimeValue) {
return new Component(serviceFactoryDelegate.Invoke(), runtimeValue)
}
... and register the delegate with the container as static factory, so that it calls back the container to resolve the service (I'm using Unity 1.2 on .net 2.0):
Container
.Configure<IStaticFactoryConfiguration>()
.RegisterFactory<Func<IService>>(container => (Func<IService>)container.Resolve<IService>)
Now I can use the container to resolve a ComponentProvider and get a component based on a runtime value:
// this happens inside CompositionRoot
provider = Container.Resovle<IComponentProvider>()
component = provider.CreateAndCacheComponent("the component")
Now I have some questions about this:
I'm not happy that the factory calls new Component(...). Isn't this poor man's DI?
Does the Hollywood principle still stand when using Func<IService> on factory's constructor? I mean, it ultimately calls container.Resolve<>... kind of like SL. The only difference is the code is in the container registration part of the app rather than inside the factory class.
Is there anything (else) wrong with this implementation, as far as DI and IoC are concerned?
It's a big step away from Poor Man's DI, but it would be nice if you didn't have to change this factory method every time a new dependency gets added to the Component's constructor.
This isn't a problem per se. Think of it like you're injecting an anonymous factory class. It can still be mocked for unit testing, and the bindings can be changed, so you're still getting the benefits of DI. But it is an added layer of abstraction which is probably not necessary. You can still avoid it in this case by injecting the IService directly into the factory, rather than a Func.
Typically when using dependency injection, you want to inject services rather than values. The fact that you're finding that you have to have both may indicate that you need to reconsider your class's API. For example, maybe you should be passing the value in to the methods on the class rather than the constructor. It's hard to say what the best approach would be without knowing more details.
No, it isn't. The whole purpose of a factory is to create an instance of a concrete class.
Basically, yes, but as I already asked in my comment, I don't see why this is necessary. You could inject an instance of IService directly
It's a bit more complicated than it needs to be. Why the double redirection IComponentProvider -> IComponentFactory? It looks like IComponentFactory doesn't add any benefit.
Implement ComponentProvider like this:
class ComponentProvider : IComponentProvider
{
ComponentProvider(IService service) { _service = service; }
IComponent CreateAndCacheComponent(object runtimeValue) {
_component = new Component(_service, runtimeValue);
return _component;
}
This would give you the following benefits:
You get rid of the unnecessary interface IComponentFactory along with the corresponding implementation.
No need to register a factory for IService
Generally speaking, how you implement this it depends on what you really need:
"runtimeValue" can be the same throughout the runtime, e.g. a connection string that is read from the settings. In that case, there would be no need for a factory / provider, you could simply new up the instance and register it with the container. Everyone who needs an IComponent requests one in the constructor instead of the provider.
You would only implement a factory and pass that as a dependency around if the "runtimeValue" really changes between calls to CreateAndCacheComponent.
To question 1: there is nothing wrong with calling new in the factory. You have isolated instantiation to one place in your application; you just made that one place the factory instead of the container.
If you ever needed to mock or change implementations, you would just mock or change the factory implementation, rather than the Component alone.
I am looking to see how Ioc/Di can simplify wiring the following classes I use often.
Consider a library that has an abstract notion of an Entity and an interface for a data access object:
public abstract class EntityWithTypedId<TId> : IEntityWithTypedId<TId>{...}
public interface IDao<T, TId> where T : IEntityWithTypedId<TId>
For the dao, I have one implementation for NHibernate as well as a fake dao I find useful for testing:
// has an NHib implementation
public class Dao<T, TId> : IDao<T, TId> where T : EntityWithTypedId<TId> {...}
public class DaoFakeBase<T> : IDao<T, int>, IDisposable where T : IEntityWithTypedId<int> {...}
I currently do the following to define an Entity and Dao type for a given project:
/// <summary>
/// <see cref="IEntityWithTypedId{IdT}"/> with an int for an id
/// </summary>
[Serializable]
public abstract class Entity : EntityWithTypedId<int>
{
}
public class Dao<T> : Dao<T, int> where T : Entity
{
protected Dao(ISessionFactory sessionFactory) : base(sessionFactory) { }
}
Can I use a DI tool to define the Entity instead? Can someone show me a code sample of how to do it if so?
Can you also lay out how I might tell my test assembly to use DaoFakes and production to use NHib.Dao
I have been looking at Windsor, mostly because NHibernate contrib projects use it, but am also interested in MEF, AutoFac and Ninject, in that order. I realize that MEF is not an IoC container in the sense that Windsor is. From what I can see with Windsor I would use Installer classes, maybe an EntityInstaller and a DaoInstaller, although I might be missing a FActory type of object here too.
Cheers,
Berryl
UPDATE # KeithS
Are you saying to change something like:
class MyViewModel(IDao<MyClass, int> dao) {...}
becomes something like
class MyViewModel(Func<IDao<MyClass, int>, obj> getDaoFunc) {
_dao = getDaoFunc(this);
}
In your example...
class MyViewModel(IDao<MyClass, int> dao) {...}
...IDao would get resolved at runtime based on a previous registration within your container. The syntax for a Prism/Unity implementation is below...
IUnityContainer.RegisterType<IDao..., DaoFakeBase...>();
The RegisterType takes place within IModule.Initialize() of a given module as defined in the UnityBootstrapper class.
protected override IModuleCatalog GetModuleCatalog()
{
ModuleCatalog catalog = new ModuleCatalog();
catalog.AddModule(typeof(project.Init));
return catalog;
}
You can also register a given type based on a lifetime manager; to behave as a Singleton...
IUnityContainer.RegisterType<IShellController, ShellController>(new ContainerControlledLifetimeManager());
...where the IShellController resolved instance will remain the same returned instance throughout the lifetime of the IUnityContainer.
UPDATE:
Using your code the registration would look like this...
public interface IDao<T, TId> where T : IEntityWithTypedId<TId>
{ }
public class Dao<T, TId> : IDao<T, TId> where T : EntityWithTypedId<TId>
{ }
public class TId
{ }
public abstract class EntityWithTypedId<TId> : IEntityWithTypedId<TId>
{ }
public interface IEntityWithTypedId<TId>
{ }
IUnityContainer.RegisterType<IEntityWithTypedId<TId>, EntityWithTypedId<TId>>();
IUnityContainer.RegisterType<IDao<IEntityWithTypedId<TId>, TId>, Dao<IEntityWithTypedId<TId>, TId>>();
IDao<IEntityWithTypedId<TId>, TId> dao = IUnityContainer.Resolve<IDao<IEntityWithTypedId<TId>, TId>>();
I would not use IoC to register the relationship between DAOs and their types (which is basically what you'd be doing). This will lead to you using the IoC container as a "service locator", a known anti-pattern where you pass the IoC container into objects that will use it to get the DAO they need.
I think the best way to simplify this from a consumption perspective would be to define a strategy pattern, using a factory class or method:
public Dao<T, TId> GetDaoFor<T, TId>(T objectInstance) where T:EntityWithTypedId<TId>
{
//Here, you could use a Dictionary, Linq with some reflection, etc.
}
This one method can be injected as a delegate into classes dependent upon DAOs. The difference is that classes that need a DAO are dependent on a method that can give it to them, which can be provided by the IoC container; they are NOT dependent on the container itself (which is the primary source of evil inherent in the "service locator" pattern). This reduces the number of things you'd have to change if you re-wrote how you got these DAOs.
EDIT: A bit off-topic, but I opened the door:
The service location pattern is generally to be avoided, because it results in code that relies on the service locator. For instance, the following is common in code where the IoC has been exposed at child levels:
private IDependency _dependency;
public IDependency MyDependency
{
get {
_dependency = _dependency ?? IoC.Resolve<IDependency>();
return _dependency;
}
}
While this seems like a nice pattern (dependencies are lazily initialized, consuming code doesn't need to know about dependencies of the child, and you always* get a reference), this code will ALWAYS require the IoC singleton to exist. You can change the IoC framework behind it, you can remove the third-party tool altogether and roll your own, but this class will always require something on which to statically call Resolve<IDependency>().
You also don't ALWAYS get a reference; you get a reference only if you properly registered IDependency with IoC. This produces two more weaknesses; 1) you don't know what the class will need without opening it up, and 2) if/when the call fails, it will fail deep in the bowels of the dependent class' inner workings. If you develop a new class, and plug it into IoC, it may pass integration, and even work in production for a while, until you start getting weird "object reference set to null" errors in really weird places in code, which are, trust me, a nightmare to debug.
Lastly, unit-testing service-locator-pattern code is more difficult, for the simple reason that you must mock the service locator as well as the dependency provided by the service locator. You can leave the production service locator in use, and simply register mocked classes as the dependencies, but that's not a unit test; the test relies on, and thus to some extent tests, that the integration of the class and its service locator works as expected. That's an integration test.
By contrast, dependency injection patterns free you from any dependency on how dependencies are resolved. The only requirement (in constructor-injection) is that they be around when the class is created. This has several advantages:
If not using an IoC framework, you have to know what the class will need to instantiate it.
If using an IoC framework, you get a runtime error when attempting to instantiate the dependent class, not sometime later when the object actually gets resolved.
When testing a dependent class, you can more easily mock the dependency, because the dependency does not have to be fed in via the service locator.
You can in most IoC frameworks still lazily initialize dependencies by providing a factory method instead of the actual dependency to the constructor. The above pattern then calls that delegate, which could come from anywhere, instead of a static named method which is satisfied by one and only one construct in the entire codebase.
We are building an ASP.NET project, and encapsulating all of our business logic in service classes. Some is in the domain objects, but generally those are rather anemic (due to the ORM we are using, that won't change). To better enable unit testing, we define interfaces for each service and utilize D.I.. E.g. here are a couple of the interfaces:
IEmployeeService
IDepartmentService
IOrderService
...
All of the methods in these services are basically groups of tasks, and the classes contain no private member variables (other than references to the dependent services). Before we worried about Unit Testing, we'd just declare all these classes as static and have them call each other directly. Now we'll set up the class like this if the service depends on other services:
public EmployeeService : IEmployeeService
{
private readonly IOrderService _orderSvc;
private readonly IDepartmentService _deptSvc;
private readonly IEmployeeRepository _empRep;
public EmployeeService(IOrderService orderSvc
, IDepartmentService deptSvc
, IEmployeeRepository empRep)
{
_orderSvc = orderSvc;
_deptSvc = deptSvc;
_empRep = empRep;
}
//methods down here
}
This really isn't usually a problem, but I wonder why not set up a factory class that we pass around instead?
i.e.
public ServiceFactory
{
virtual IEmployeeService GetEmployeeService();
virtual IDepartmentService GetDepartmentService();
virtual IOrderService GetOrderService();
}
Then instead of calling:
_orderSvc.CalcOrderTotal(orderId)
we'd call
_svcFactory.GetOrderService.CalcOrderTotal(orderid)
What's the downfall of this method? It's still testable, it still allows us to use D.I. (and handle external dependencies like database contexts and e-mail senders via D.I. within and outside the factory), and it eliminates a lot of D.I. setup and consolidates dependencies more.
Thanks for your thoughts!
One argument against this is that it doesn't make your dependencies clear. It shows that you depend on "some of the stuff in the service factory" but not which services. For refactoring purposes it can be helpful to know exactly what depends on what.
Dependency injection should make this kind of thing easy, if you're using an appropriate framework - it should just be a matter of creating the right constructor, defining what implements which interface, and letting it sort everything out.
Such a factory is essentially a Service Locator, and I consider it an anti-pattern because it obscures your dependencies and make it very easy to violate the Single Responsibility Principle (SRP).
One of the many excellent benefits we derive from Constructor Injection is that it makes violations of the SRP so glaringly obvious.
If most of your classes depend on this three interfaces you could pass an object around that wraps them together, BUT: if most of the classes just depend on one or two of them then it's not a good idea since those classes will have access to objects they don't need and they have no business with and some programmers will always call the code they are not supposed to call just because it's available.
Btw, it's not a factory unless you always create a new object in the Get[...]Service() methods and doing that just for passing a few methods around is bad. I'd just call it ServiceWrapper and turn them into the properties EmployeeService, DepartmentService and OrderService.