I was reading a book about Learning ASP.NET Core API when I run to a part saying:
We create a private read-only field _repository that will be assigned
the injected MockCommandAPIRepo object in our constructor and used
throughout the rest of our code.
Here is some text I thought you'd better have:
Then there are some explanations related to the picture above:
Add the new using statement to reference ICommandAPIRepo.
We create a private read-only field _repository that will be assigned the injected MockCommandAPIRepo object in our constructor
and used throughout the rest of our code.
The Class constructor will be called when we want to make use of our Controller.
At the point when the constructor is called, the DI system will spring into action and inject the required dependency when we ask for
an instance of ICommandAPIRepo. This is Constructor Dependency
Injection.
We assign the injected dependency (in this case MockCommandAPIRepo) to our private field (see point 1).
And that’s pretty much it! We can then use _repository to make use of our
concrete implementation class, in this case MockCommandAPIRepo. As
I’ve stated earlier, we’ll reuse this pattern multiple times through
the rest of the tutorial; you’ll also see it everywhere in code in
other projects – take note.
Now, According to the highlighted part above in number 2, I got a little confused!
I've heard of "to be assigned by some value" before, but here, it is saying that:
that will be assigned the injected MockCommandAPIRepo object in our constructor
and as you see, there is no "by" added before the injected MockCommandAPIRepo object....
So, I have a question now. What does it mean by the highlighted part above in number 2?
Does it mean the same when we add "by" in the sentence? or not?
This is about dependency injection in Asp.Net Core. After we register service to the IOC Container, How to use it in our controller? We can inject them in controller via Constructor Injection. Once we register a service, the IoC container automatically performs constructor injection if a service type is included as a parameter in a constructor. In your question, An IoC container will automatically pass an instance of ICommandAPIRepo(MockCommandAPIRepo) to the constructor of CommandsController. Now we can use MockCommandAPIRepo in the constructor. But it can only be used in constructor, How can we use it in other method in CommandsController? Here we use:
private readonly ICommandAPIRepo _repository;
to create a global variable in CommandsController, Then in constructor, We use:
_repository = repository
assign the value of repository to _repository. Now _repository and repository has the same value, Because _repository is a global variable, So We can use _repository in other method in CommandsController. The whole process of dependency injection is done.
Related
I am building an WPF application using the MVVM Light Toolkit and specifically SimpleIoc.
I have a parent viewmodel that dynamically creates child viewmodels. When doing this I am using "standard" dependency injection to pass a IConfigService as a parameter to the constructor. I want the IConfigService to be a unique instance for each child viewmodel. So I try this:
IConfigService service = SimpleIoc.Default.GetInstance<IConfigService>(key);
ChildViewModel vm = new ChildViewModel(service);
Where key is a unique identifier for each child viewmodel. According to the documentation of MVVM Light and SimpleIoc this GetInstance method:
...provides a way to get an instance of a given type corresponding to a given key. If no instance had been instantiated with this key before, a new instance will be created.
There is also a remark that the class must have been registered before, else it returns null. In my case it has been, in ViewModelLocator:
var configService = new ConfigService();
SimpleIoc.Default.Register<IConfigService>(() => configService);
However, the GetInstance call returns the same instance every time.
What am I doing wrong here?
You registered an already instantiated object.
SimpleIoc does not create its own instances with this overload. It always returns configService. Either you need to perform the instantiation within the lambda, because you are using a factory overload, or you can do this more easily by just passing the ConfigService type. SimpleIoc will take care of the instantiation itself.
I have a middleware library I intend on using in multiple projects. The middleware itself looks something like:
public SdkMiddleware(RequestDelegate next, ILogger<SdkMiddleware> logger, ISdk sdk)
{
this.next = next;
this.logger = logger;
this.sdk = agentSdk;
this.sdk.Init();
...
}
Thanks to DI, I can simply inject my logger:
// Would rather this class be internal...
public class Sdk: ISdk
{
private ILogger<Sdk> logger;
public Sdk(ILogger<Sdk> logger)
{
this.logger = logger;
}
public void Init() {
this.logger.Info(...) // Do some logging
}
The downside to this is my class needs to be registered in every ASP.Net project's Startup.cs:
public void ConfigureServices(IServiceCollection services)
{
services.AddTransient<ISdk, Sdk>();
Is the best/only route? Every time I want to inject a logger into a class, I need to register that class for DI in my composition root?
There is nothing wrong in having the consumer of your library compose the dependencies for this library in the composition root. That's how dependency injection works. You could of course provide some default implementations and a custom extension method that would register those default implementations into the DI and then let the consumer simply call your extension method.
There are a few things that I feel need clarification here:
Dependency injection/inversion of control
To understand what is the benefit of dependency injection(DI) it is better to look at the principle of inversion of control(IoT) that DI implements.
In your case you want SdkMiddleware class to contain a reference to ILogger implementation. The simplest way to do this is for SdkMiddleware class to create an instance of a class that implements ILogger interface. The downside of such approach is that SdkMiddleware needs to know which class implements ILogger interface and how to instantiate it. In other words, SdkMiddleware has control over the creation of ILogger object. The inversion of control happens when the responsibility of knowing which class implements ILogger and how to create an instance of it is taken away from SdkMiddleware to some other class (dependency container in DI) and the instance if it is given to SdkMiddleware to use (through injection in DI). In this case the control over the creation of ILogger object is outside of SdkMiddleware. Since this changes the direction of control, it is called "Inversion of control".
The benefit of this approach is when you will need to provide another implementation of ILogger or change the way you create an instance of that class, you don't need to change SdkMiddleware at all.
Bootstrapping
So now that we clarified why we are using DI, lets take a look at what do we need to actually use it.
The object that creates all instances, controls which object is injected into which and gives away ready to use objects is usually called DI container or IoT container. In asp.net core "IServiceCollection services" is used as such a container. But it still needs to know how to create objects and which implementation to inject for which interface. This is where bootstrapping comes in.
In a bootstrapper class or method you need to specify how objects are built from classes and how classes relate to interfaces. As an example of the former, imagine that you need to pass a connection string for a database from your configuration to a class that creates a db connection. As for the latter, that is exactly what your line "services.AddTransient()" does.
The answer
I am sorry it took so long to get to the actual answer for your question but I wanted to provide some overview first.
Do you need to specify a relation between a class and an interface to inject logger into your class? No. Your class may not even have an interface to begin with and DI container will inject all the dependencies in it by default if you ask for an object of a class instead of an instance of an interface. You can also use or define some convention over configuration solution so that binding of classes and interfaces will happen automatically.
The bottom line is that registration of a class and the actual injection are not connected. But the code you provided is the default way to do this.
At a high level, how do these dep. injection frameworks work?
I can understand if you always instantiate an object via a custom factory like:
IUser user = DepInjector.Get<User>();
I'm guessing what happens is, wherever you defined the mappings, it will look at the type you want and try and find a match, if found, it will via reflection instantiate the type.
Are there dep. inj. frameworks that would work like:
IUser user = new User();
If so, how would it get the correct user, where is it hooking into the CLR to do this? In case of an asp.net website, is it any different?
If you want to know how Ninject works then the obvious place to start would be reading How Injection Works on their official wiki. It does use reflection but it now also uses dynamic methods:
"By default, the StandardKernel will
create dynamic methods (via
System.Reflection.Emit.DynamicMethod)
that can be used to inject values into
the different injection targets. These
dynamic methods are then triggered via
delegate calls."
As for you second example, I don't believe there are any DI frameworks that would do what you ask. However, constructor injection tends to be most common way of implementing IoC, so that when a class is constructed it knows what type to bind to via some configuration binding. So in your example IUser would be mapped to concrete User in config bindings so that any consuming class that has an IUser parameter as part of its constructor would get the correct User type passed in.
AFAIK there's no way to "hook into" object instantiation with the CLR. The only way to use DI in the second case would be to employ an assembly rewriter (i.e. a postprocessor similar to PostSharp) to replace the call to new with a call to the DI factory method (i.e. GetUser) in the compiled code.
Is it possible to make unity try all defined constructors starting with the one with most arguments down to the least specific one (the default constructor)?
Edit
What I mean:
foreach (var constructor in concrete.GetConstructorsOrderByParameterCount())
{
if(CanFulfilDependencies(constructor))
{
UseConstructor(constructor);
break;
}
}
I don't want Unity to only try the constructor with most parameters. I want it to continue trying until it finds a suitable constructor. If Unity doesn't provide this behavior by default, is it possible to create an extension or something to be able to do this?
Edit 2
I got a class with two constructors:
public class MyConcrete : ISomeInterface
{
public MyConcrete (IDepend1 dep, IDepend2 dep2)
{}
public MyConcrete(IDepend1 dep)
{}
}
The class exists in a library which is used by multiple projects. In this project I want to use second constructor. But Unity stops since it can't fulfill the dependencies by the first constructor. And I do not want to change the class since the first constructor is used by DI in other projects.
Hence the need for Unity to try resolving all constructors.
Unity will choose the constructor with the most parameters unless you explicitly tag a constructor with the [InjectionConstructor] attribute which would then define the constructor for Unity to use.
When you state a suitable constructor; that is somewhat contingent on the environment. If for instance you always want to guarantee that a certain constructor is used when making use of Unity use the attribute mentioned previously, otherwise explicitly call the constructor you want to use.
What would be the point of Unity "trying" all constructors? It's purpose is to provide an instance of a type in a decoupled manner. Why would it iterate through the constructors if any constructor will create an instance of the type?
EDIT:
You could allow the constructor with the most params to be used within the project that does not have a reference to that type within its container by making use of a child container. This will not force the use of the constructor with a single param but it will allow the constructor with 2 params to work across the projects now.
You could also switch to using the single constructor across the board and force the other interface in via another form of DI (Property Injection), not Constructor Injection...therefore the base is applicable across the projects which would make more sense.
I am creating a custom route by subclassing RouteBase. I have a dependency in there that I'd like to wire up with IoC. The method GetRouteData just takes HttpContext, but I want to add in my unit of work as well....somehow.
I am using StructureMap, but info on how you would do this with any IoC framework would be helpful.
Well, here is our solution. Many little details may be omitted but overall idea is here. This answer may be a kind of offtop to original question but it describes the general solution to the problem.
I'll try to explain the part that is responsible for plain custom HTML-pages that are created by users at runtime and therefore can't have their own Controller/Action. So the routes should be either somehow built at runtime or be "catch-all" with custom IRouteConstraint.
First of all, lets state some facts and requirements.
We have some data and some metadata about our pages stored in DB;
We don't want to generate a (hypothetically) whole million of routes for all of existing pages beforehand (i.e. on Application startup) because something can change during application and we don't want to tackle with pushing the changes to global RouteCollection;
So we do it this way:
1. PageController
Yes, special controller that is responsible for all our content pages. And there is the only action that is Display(int id) (actually we have a special ViewModel as param but I used an int id for simplicity.
The page with all its data is resolved by ID inside that Display() method. The method itself returns either ViewResult (strongly typed after PageViewModel) or NotFoundResult in case when page is not found.
2. Custom IRouteConstraint
We have to somewhere define if the URL user actually requested refers to one of our custom pages. For this we have a special IsPageConstraint that implements IRouteConstraint interface. In the Match() method of our constraint we just call our PageRepository to check whether there is a page that match our requested URL. We have our PageRepository injected by StructureMap. If we find the page then we add that "id" parameter (with the value) to the RouteData dictionary and it is automatically bound to PageController.Display(int id) by DefaultModelBinder.
But we need a RouteData parameter to check. Where we get that? Here comes...
3. Route mapping with "catch-all" parameter
Important note: this route is defined in the very end of route mappings list because it is very general, not specific. We check all our explicitly defined routes first and then check for a Page (that is easily changeable if needed).
We simply map our route like this:
routes.MapRoute("ContentPages",
"{*pagePath}",
new { controller = "Page", action = "Display" }
new { pagePath = new DependencyRouteConstraint<IsPageConstraint>() });
Stop! What is that DependencyRouteConstraint thing appeared in mapping? Well, thats what does the trick.
4. DependencyRouteConstraint<TConstraint> class
This is just another generic implementation of IRouteConstraint which takes the "real" IRouteConstraint (IsPageConstraint) and resolves it (the given TConstraint) only when Match() method called. It uses dependency injection so our IsPageConstraint instance has all actual dependencies injected!
Our DependencyRouteConstraint then just calls the dependentConstraint.Match() providing all the parameters thus just delegating actual "matching" to the "real" IRouteConstraint.
Note: this class actually has the dependency on ServiceLocator.
Summary
That way we have:
Our Route clear and clean;
The only class that has a dependency on Service Locator is DependencyRouteConstraint;
Any custom IRouteConstraint uses dependency injection whenever needed;
???
PROFIT!
Hope this helps.
So, the problem is:
Route must be defined beforehand, during Application startup
Route's responsibility is to map the incoming URL pattern to the right Controller/Action to perform some task on request. And visa versa - to generate links using that mapping data. Period. Everything else is "Single Responsibility Principle" violation which actually led to your problem.
But UoW dependencies (like NHibernate ISession, or EF ObjectContext) must be resolved at runtime.
And that is why I don't see the children of RouteBase class as a good place for some DB work dependency. It makes everything closely coupled and non-scalable. It is actually impossible to perform Dependency Injection.
From now (I guess there is some kind of already working system) you actually have just one more or less viable option that is:
To use Service Locator pattern: resolve your UoW instance right inside the GetRouteData method (use CommonServiceLocator backed by StructureMap IContainer). That is simple but not really nice thing because this way you get the dependency on static Service Locator itself in your Route.
With CSL you have to just call inside GetRouteData:
var uow = ServiceLocator.Current.GetService<IUnitOfWork>();
or with just StructureMap (without CSL facade):
var uow = ObjectFactory.GetInstance<IUnitOfWork>();
and you're done. Quick and dirty. And the keyword is "dirty" actually :)
Sure, there is much more flexible solution but it needs a few architectural changes. If you provide more details on exactly what data you get in your routes I can try to explain how we solved our Pages routing problem (using DI and custom IRouteConstraint).