Controller Constructor:
IRestaurantData db;
public HomeController(IRestaurantData db)
{
this.db = db;
}
// Container code
public class ContainerConfig
{
internal static void RegisterContainer(HttpConfiguration httpConfiguration)
{
var builder = new ContainerBuilder();
builder.RegisterControllers(typeof(MvcApplication).Assembly);
builder.RegisterApiControllers(typeof(MvcApplication).Assembly);
builder.RegisterType<InMemoryRestaurantData>()
.As<IRestaurantData>()
.SingleInstance();
var container = builder.Build();
// MVC CONTROLLER VERSION
DependencyResolver.SetResolver(new AutofacDependencyResolver(container));
// WEBAPI CONTROLLER VERSION
httpConfiguration.DependencyResolver = new AutofacWebApiDependencyResolver(container);
}
}
So I'm trying to wrap my head around how Inversion of Control containers work. The course I'm in is using Autofac to create these containers. From what I'm currently understanding creating this container is making it so that when I instantiate my HomeController with IRestaurantData the container is pointing the interface AT InMemoryResataurantData everytime it is used. I can understand that much. What Im also confused by is I dont understand WHERE in the program my Controllers are are actually being instantiated? Does anyone know?
That is being done by autofac. whenever you request a class from a container, it will instantiate the type for you using the default constructor. If you have a constructor with parameters, it will look in the container for other registered types and will instantiate a new object or fetch an already used object.
Your instantiated types can be transient (short lived) or not.
So in short; when the object is requested.
I was going through this article about dependency injection http://www.asp.net/web-api/overview/advanced/dependency-injection
It shows Unity Container from Microsoft.
There are a few things that are not making sense to me
for example, following line
public ProductsController(IProductRepository repository)
The above is the constructor of Controller. I need to know who passes the repository to the constructor? ANd is that made possible by registering IProductRepository interface with Unity?
public static void Register(HttpConfiguration config)
{
var container = new UnityContainer();
container.RegisterType<IProductRepository, ProductRepository>(new HierarchicalLifetimeManager());
config.DependencyResolver = new UnityResolver(container);
// Other Web API configuration not shown.
}
Is the above code is all needed to make MVC pass the object to the constructor of a controller?
You answered your own question:
is that made possible by registering IProductRepository interface with Unity?
Yes.
When you request to resolve a type using Unity, the container searches for public constructors. If the constructor needs some implementation (IProductRepository in your case), the container searches within its registrations for an implementation for all the needed parameters. If found, it resolves that. This is a recursive process.
So yes. You need to register an implementation of IProductRepository using the container in order to resolve an instance of the Controller using that container.
I'm experimenting with IoC in my Web App and would like to do things according to best practices. Recently I discovered an IoC framework called DryIoc which is supposed to be small and fast.
I've read through the examples but none seem to point out where I should put the container itself.
Should it reside in the controller? Or in Global.asax? Someplace else maybe? Or perhaps as a static variable in a class?
I'd appreciate if someone would be able to guide me in the right direction, preferrably with some sample code, as I've stalled and don't got a clue on how to continue.
var container = new Container(); // Should obviously NOT be a local variable
container.Register<ISalesAgentRepository, SalesAgentRepository>(Reuse.Singleton);
Usually I do the following:
1 - Create a bootstrapper class
public static class Bootstrapper {
public static Container _container;
public void Bootstrap() {
var container = new Container;
// TODO: Register all types
_container = container;
}
public static T GetInstance<T>() {
return _container.Resolve<T>();
}
}
2 - Call the bootstrap method in the global.asax, in the Application_Start method:
protected void Application_Start() {
Bootstrapper.Bootstrap();
}
And never use the container anywhere directly, you have to hook it somewhere in the MVC lifecycle, and usually the DI package you use can do this for you.
Also note that I've added a GetInstance<T> method to the bootstrapper-class. This method is what makes it possible to use the container directly by requesting instances of types. I've added this method so you know it is possible, but always use constructor-injection if possible.
Actually, you may not need to store container on your side. Here is the DryIoc WebApi Owin sample.
The DryIoc.WebApi extension will store and Dispose the container when it is appropriate in IDependencyResolver implementation.
I'm building an ASP .Net MVC 2 application and I want to follow the ideas from Mark Seemann's book "Dependency Injection in .Net" so I registered my custom Controller Factory in the Global.asax file and I'm configuring the container within the Controller Factory like so:
public IController CreateController(RequestContext context, Type controllerType)
{
var container = new Container();
object controller;
if(controllerType == typeof(MyControllerOne)
{
container.Configure(r => r.
For<IService>().
Use<ServiceOne>());
}
else if(controllerType == typeof(MyControllerTwo)
{
container.Configure(r => r.
For<IService>().
Use<ServiceTwo>());
}
......
return container.GetInstance(controllerType) as IController;
}
Now this code works (though it is possible I may have a mistake somewhere since I'm writing this by memory), the dependencies are being resolved and the correct controller is being instantiated with the correct dependency every time, but it seems that for every request the container is being configured to resolve the dependencies that will be needed at that moment. So my questions are:
Isn't that redundant?
Shouldn't the configuration of the container be done in the Global.asax as well so it is only done once? If so, how could this be done?
By configuring the container the way I'm doing it, how will object lifetime be affected? I mean, eventually there will be repositories that should have a singleton lifetime, some others should be created once by HTTP request, and so for. What could be the implications?
Any comments, ideas and/or suggestions will be much appreciated.
By the way, the IoC container I'm using is StructureMap though I think that for this particular question it might not be too relevant.
It is, indeed, redundant to conditionally register each controller upon request. With StructureMap, the Controller Factory should look like this:
public class StructureMapControllerFactory : DefaultControllerFactory
{
private readonly IContainer container;
public StructureMapControllerFactory(IContainer container)
{
if (container == null)
{
throw new ArgumentNullException("container");
}
this.container = container;
}
protected override IController GetControllerInstance(
RequestContext requestContext, Type controllerType)
{
return (IController)this.container.GetInstance(controllerType);
}
}
All services should be unconditionally registered in a single container instance. As an example, you can register all Controllers with StructureMap like this:
this.Scan(x =>
{
x.AssemblyContainingType<HomeController>();
x.AddAllTypesOf<IController>();
x.Include(t => typeof(IController).IsAssignableFrom(t));
});
This is basically just following the Register Resolve Release pattern.
we want to use Unity for IOC.
All i've seen is the implementation that there is one global static service (let's call it the the IOCService) which holds a reference to the Unity container, which registers all interface/class combinations and every class asks that object: give me an implementation for Ithis or IThat.
Frequently i see a response that this pattern is not good because it leads to a dependency from ALL classes to the IOCService (not to the Unity container because it is only known inside the IOCService).
But what i don't see often, is: what is the alternative way?
Michel
EDIT: found out that the global static service is called the service locator, added that to the title.
The alternative is to have a single instance of your container at the highest application level only, then use that container to resolve every object instance you need to create in that layer.
For example, the main method of most executables just looks like this (minus exception handling):
private static void main(string[] args) {
Container container = new Container();
// Configure the container - by hand or via file
IProgramLogic logic = container.Resolve<IProgramLogic>();
logic.Run();
}
Your program (represented here by the IProgramLogic instance) doesn't have to know anything about your container, because container.Resolve will create all its dependencies - and its dependencies' dependencies, on down to leaf classes with no dependencies of their own.
ASP.NET is a harder case, because web forms doesn't support constructor injection. I typically use Model-View-Presenter in my web forms applications, so my Page classes really only have one dependency each - on their presenter. I don't unit test them (everything interesting and testable is in my presenters, which I do test), and I don't ever substitute presenters. So I don't fight the framework - I just expose a container property on my HttpApplication class (in global.asax.cs) and use it directly from my Page files:
protected void Page_Load(object sender, EventArgs args) {
ICustomerPresenter presenter = Global.Container.Resolve<ICustomerPresenter>();
presenter.Load();
}
That's service locator of course - though the Page classes are the only thing coupled to the locator: your presenter and all of its dependencies are still fully decoupled from your IoC container implementation.
If you have a lot of dependencies in your Page files (that is, if you do not use Model-View-Presenter), or if it's important to you to decouple your Page classes from your Global application class, you should try to find a framework that integrates into the web forms request pipeline and use property injection (as suggested by Nicholas in the comments below) - or write your own IHttpModule and perform the property injection yourself.
+1 for knowing that Service Locator is a Bad Thing.
Problem is - Unity is not very sophisticated so I don't know how easy/hard is it to do IoC the right way with it.
I wrote few blogposts recently that you might find useful.
How I use IoC Containers
Pulling from the container
Instead of using the container explicitly, use it implicitly by leveraging constructor / property injection instead. Create a core class (or set of core classes) that depend on all the major pieces of your application.
Most containers will let you put ISomething[] in your constructor and it will inject all instances of ISomething into your class.
This way, when you bootstrap your application:
Instantiate your container
Register all your goodies
Resolve the core classes (this will pull in all the other dependencies you need)
Run the "main" part of the application
Now, depending on the type of application you are writing, there are different strategies for avoiding marking the IoC container as "static".
For ASP.NET web applications, you'll probably end up storing the container in the Application State. For ASP.NET MVC applications, you need to change out the Controller Factory.
For desktop applications, things get more complicated. Caliburn uses an interesting solution to this problem using the IResult construct (this is for WPF applications but could be adapted for Windows Forms as well.
In theory, to not have to worry about having a static IoC instance, you need to follow the Fight Club Rule - i.e. not to talk about the fight club - i.e. not to mention the IoC container.
This means that your components should largely be unaware about the IoC container. It should only be used at the topmost level when registering components. If a class needs to resolve something, it should really be injected as a dependency.
The trivial case is easy enough. If PaymentService depends on IAccount, the latter should be injected by IoC:
interface IAccount {
Deposit(int amount);
}
interface CreditCardAccount : IAccount {
void Deposit(int amount) {/*implementation*/}
int CheckBalance() {/*implementation*/}
}
class PaymentService {
IAccount account;
public PaymentService (IAccount account) {
this.account = account;
}
public void ProcessPayment() {
account.Deposit(5);
}
}
//Registration looks something like this
container.RegisterType<IAccount, CreditCardAccount>();
container.RegisterType<PaymentService>();
The not so trivial case is where you want to inject multiple registrations. This especialy applies when you are doing any sort of Converntion Over Configuration and creating an object from a name.
For our payment example, say you want to enumerate through all accounts and check their balances:
class PaymentService {
IEnumerable<IAccount> accounts;
public PaymentService (IEnumerable<IAccount> accounts) {
this.accounts = accounts;
}
public void ProcessPayment() {
foreach(var account in accounts) {
account.Chackbalance();
}
}
}
Unity has the ability to register multiple interface to class mappings (they have to have different names thought). It does not, however, automatically inject those into classes that take collections of those registered interfaces. So, the above example will throw a resolution failed exception at runtime.
If you don't care that those objects live forever, you can register PaymentService in a more static fashion:
container.RegisterType<PaymentService>(new InjectionConstructor(container.ResolveAll<IAccount>()));
The above code will register PaymentService and will use a collection of IAccount instances that is resolved at registration time.
Alternatively, you can pass an instance of the container itself as a dependency and let PaymentService perform resolution of accounts. This is not quite following the Fight Club Rule, but is slightly less smelly than static Service Locator.
class PaymentService {
IEnumerable<IAccount> accounts;
public PaymentService (IUnityContainer container) {
this.accounts = container.ResolveAll<IAccount>();
}
public void ProcessPayment() {
foreach(var account in accounts) {
account.Chackbalance();
}
}
}
//Registration is pretty clean in this case
container.RegisterType<IAccount, CreditCardAccount>();
container.RegisterType<PaymentService>();
container.RegisterInstance<IUnityContainer>(container);
If your concern is having a dependency on Unity throughout your application, you can combine the service locator with a facade to hide the IOC implementation. In this way, you do not create a dependency on Unity in your application, only on having something that can resolve types for you.
For example:
public interface IContainer
{
void Register<TAbstraction,TImplementation>();
void RegisterThis<T>(T instance);
T Get<T>();
}
public static class Container
{
static readonly IContainer container;
public static InitializeWith(IContainer containerImplementation)
{
container = containerImplementation;
}
public static void Register<TAbstraction, TImplementation>()
{
container.Register<TAbstraction, TImplementation>();
}
public static void RegisterThis<T>(T instance)
{
container.RegisterThis<T>(instance);
}
public static T Get<T>()
{
return container.Get<T>();
}
}
Now all you need is an IContainer implementation for your IOC container of choice.
public class UnityContainerImplementation : IContainer
{
IUnityContainer container;
public UnityContainerImplementation(IUnityContainer container)
{
this.container = container;
}
public void Register<TAbstraction, TImplementation>()
{
container.Register<TAbstraction, TImplementation>();
}
public void RegisterThis<T>(T instance)
{
container.RegisterInstance<T>(instance);
}
public T Get<T>()
{
return container.Resolve<T>();
}
}
Now you have a service locator that is a facade for IOC services, and can configure your service locator to use Unity or any other IOC container. The rest of the application has no dependency on the IOC implementation.
To configure your service locator:
IUnityContainer unityContainer = new UnityContainer();
UnityContainerImplementation containerImpl = new UnityContainerImplementation(unityContainer);
Container.InitializeWith(containerImpl);
For testing, you can create a stub of IContainer that returns whatever you want, and initialize Container with that.