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.
Related
I'm trying to get an understanding of which concrete types are providing the implementations of interfaces in an IOC (dependency injection) container. My implementation works fine when there are no delegates involved. However, I'm having trouble when a delegate method is passed as the type factory, as I can't get Mono.Cecil to give me the concrete type or a method reference to the factory back. I'm specifically in this case trying to build a component that can work with the IServiceCollection container for .Net ASP.Net REST APIs. I've created a 'minimised' set of code below to make it easy to explain the problem.
Consider the following C# code:
interface IServiceProvider {}
interface IServiceCollection {}
class ServicesCollection : IServiceCollection {}
interface IMongoDBContext {}
class MongoDBContext : IMongoDBContext
{
public MongoDBContext(string configName) {}
}
static class Extensions
{
public static IServiceCollection AddSingleton<TService>(this IServiceCollection services, Func<IServiceProvider, TService> implementationFactory) where TService : class
{
return null;
}
}
class Foo
{
void Bar()
{
IServiceCollection services = new ServicesCollection();
services.AddSingleton<IMongoDBContext>(s => new MongoDBContext("mongodbConfig"));
}
}
When successfully locating the 'services.AddSingleton' as a MethodReference, I'm unable to see any reference to the MongoDBContext class, or its constructor. When printing all the instructions .ToString() I also cannot seem to see anything in the IL - I do see the numbered parameter as !!0, but that doesn't help if I can't resolve it to a type or to the factory method.
Does anyone have any ideas on how to solve this?
Most likely your code is looking in the wrong place.
C# compiler will try to cache the conversion of lambda expression -> delegate.
if you look in sharplab.io you'll see that the compiler is emitting an inner class '<>c' inside your Foo class and in that class it emits the method '<Bar>b__0_0' that will be passed as the delegate (see opcode ldftn).
I don't think there's an easy, non fragile way to find that method.
That said, one option would be to:
Find the AddSingleton() method call
From there start going back to the previous instructions trying to identify which one is pushing the value consumed in 1 (the safest way to do that would be to consider how each instruction you are visiting changes the stack). In the code I've linked, it would be IL_0021 (a dup) of Bar() method.
From there, do something similar to 2, but now looking for the instruction that pushes the method reference (a ldftn) used by the ctor of Func<T, R>; in the code linked, it would be IL_0016.
Now you can inspect the body (in the code linked, Foo/'<>c'::'<Bar>b__0_0')
Note that this implementation has some holes though; for instance, if you call AddSingleton() with a variable/parameter/field as I've done (services.AddSingleton(_func);) you'll need to chase the initialization of that to find the referenced method.
Interestingly, at some point Cecil project did support flow analysis (https://github.com/mono/cecil-old/tree/master/flowanalysis).
If you have access to the source code, I think it would be easier to use Roslyn to analyze it (instead of analyzing the assembly).
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.
For those of you that are familiar with the building of the Symfony container, do you know what is the differences (if any) between
Tagged service Collector using a Compiler pass
Tagged service Collector using the supported shortcut
Service Locator especially, one that collects services by tags
Specifically, I am wondering about whether these methods differ on making these collected services available sooner or later in the container build process. Also I am wondering about the ‘laziness’ of any of them.
It can certainly be confusing when trying to understand the differences. Keep in mind that the latter two approaches are fairly new. The documentation has not quite caught up. You might actually consider making a new project and doing some experimenting.
Approach 1 is basically an "old school" style. You have:
class MyCollector {
private $handlers = [];
public function addHandler(MyHandler $hamdler) {
$handlers[] = $handler;
# compiler pass
$myCollectorDefinition->addMethodCall('addHandler', [new Reference($handlerServiceId)]);
So basically the container will instantiate MyCollector then explicitly call addHandler for each handler service. In doing so, the handler services will be instantiated unless you do some proxy stuff. So no lazy creation.
The second approach provides a somewhat similar capability but uses an iterable object instead of a plain php array:
class MyCollection {
public function __construct(iterable $handlers)
# services.yaml
App\MyCollection:
arguments:
- !tagged_iterator my.handler
One nice thing about this approach is that the iterable actually ends up connecting to the container via closures and will only instantiate individual handlers when they are actually accessed. So lazy handler creation. Also, there are some variations on how you can specify the key.
I might point out that typically you auto-tag your individual handlers with:
# services.yaml
services:
_instanceof:
App\MyHandlerInterface:
tags: ['my.handler']
So no compiler pass needed.
The third approach is basically the same as the second except that handler services can be accessed individually by an index. This is useful when you need one out of all the possible services. And of course the service selected is only created when you ask for it.
class MyCollection {
public function __construct(ServiceLocator $locator) {
$this->locator = $locator;
}
public function doSomething($handlerKey) {
/** #var MyHandlerInterface $handler */
$handler = $serviceLocator->get($handlerKey);
# services.yaml
App\MyCollection:
arguments: [!tagged_locator { tag: 'app.handler', index_by: 'key' }]
I should point out that in all these cases, the code does not actually know the class of your handler service. Hence the var comment to keep the IDE happy.
There is another approach which I like in which you make your own ServiceLocator and then specify the type of object being located. No need for a var comment. Something like:
class MyHandlerLocator extends ServiceLocator
{
public function get($id) : MyHandlerInterface
{
return parent::get($id);
}
}
The only way I have been able to get this approach to work is a compiler pass. I won't post the code here as it is somewhat outside the scope of the question. But in exchange for a few lines of pass code you get a nice clean custom locator which can also pick up handlers from other bundles.
For example, let's say I have an interface 'IFeed' and two concrete types ('Feed1' and 'Feed2') that implement this interface. Now let's say I have a 'FeedManager' type that takes multiple parameters that will get resolved dynamically, two of which are of type 'IFeed' and I'd like both concrete type to be injected via constructor injection, not via manual resolve (I only use resolve once at the composition root). I have a feeling that I should be using a factory but I wanted to see what the proper way of doing this might be. Many thanks in advance.
If you want ALL implementations of IFeed, you can use array syntax in your constructor and then nothing special is needed at type registration time.
container.RegisterType<IFeedManager, FeedManager>();
container.RegisterType<IFeed, FeedA>("FeedA"); // The name doesn't matter
container.RegisterType<IFeed, FeedB>("FeedB"); // The name doesn't matter
Then the manager constructor...
public FeedManager(IFeed[] feeds) {...}
or if you want to add a little flare for calling the constructor directly...
public FeedManager(params IFeed[] feeds) {...}
Assuming you want to determine the actual concrete instances at runtime, you need to use named type registrations and then tell unity which one you want. So, use a factory method to construct the types required and pass those in as parameter overrides. Unity will use the overrides and resolve any remaining dependencies.
// register the types using named registrations
container.RegisterType<IFeedManager,FeedManager>()
container.RegisterType<IFeed, Feed1>("Feed1")
container.RegisterType<IFeed, Feed2>("Feed2")
Assuming your feed manager has the following named constructor parameters
class FeedManager : IFeedManager
{
public FeedManager (IFeed Feed1, IFeed Feed2, string someOtherDependency)
{
}
}
and create your feed manager:
static IFeedManager CreateFeedManager()
{
ParameterOverride feed1 = new ParameterOverride("Feed1"
,_container.Resolve<IFeed>("feed1"));
ParameterOverride feed2 = new DependencyOverride("Feed2"
,_container.Resolve<IFeed>("feed2"));
IFeedManager = _container.Resolve<IFeedManager>(feed1,feed2)
return IFeedManager;
}
Obviously this is overly simplified, but you you insert your own logic to determine which instance is to be resolved and then injected for each of the IFeed instances required by the FeedManager.
With Unity you would do this like so:
container.RegisterType<IFeed, Feed1>("Feed1");
container.RegisterType<IFeed, Feed2>("Feed2");
container.RegisterType<FeedManager>(new InjectionConstructor(new ResolvedParameter<IFeed>("Feed1"),
new ResolvedParameter<IFeed>("Feed2")));
This has now configured Unity so that when it needs to resolve a FeedManager, it will resolve Feed1 for the first parameter and Feed2 for the second parameter.
This is a simple questions. I have researched this questions in my notebooks and books and the internet but cant find an answer
Why would we override the default constructor by adding parameters to it?
You would create a constructor for a class to manipulate its member variables according to whatever other conditions as soon as it's created. I get the impression you don't actually know what a constructor is.
Many languages (like C++/C#/Java) automatically create default no-arguments constructor when none defined in the class explicitly.
When you create a constructor in a class with or without arguments usually compiler stop creating default auto-generated constructor (depending on language specification). This is done on assumption if you have some non default initialization than automatically generated one is likely to not create object in a state you would expect.
Since having constructor with arguments is natural way to create objects it is essentially lead to "removing" default auto-generated constructor which probably can be called "overriding default constructor".