If I am working on a class library how do I make use of Ninject here? i.e., from the internal class library point of view and also from the client code?
For example:
should the class library have its own IOC set up, or should it always assume the client code will supply?
if no (ie it's up to the client to have the IOC in place) then where is the mapping data stored here'. Is this mapping of the class library's functionality to be placed in the client?
If the client doesn't have an IOC what happens? Should they specify an IOC?
If the client does have an IOC does your IOC need to interact with theirs?
I don't see a problem with 2 (or more) IOC's working independently in the same app. But if the IOC's are creating the same objects then they should be put together.
Related
Following the guidelines I read in:
https://www.devtrends.co.uk/blog/how-not-to-do-dependency-injection-the-static-or-singleton-container
I want to try and avoid using a service locator.
But on the other hand, I don't register all the types in the startup.cs file. I don't think this is right that all these internal types are referenced in the main startup.cs
I currently have a factory class that has a collection of builder classes.
Each builder class is in charge of creating a specific object.
I don't want to create all these builder classes in advance as I might not need to use them and creating them is a bit heavy.
I saw an example of how to achieve this in the link above. However the startup.cs class needs to know all these builders. I don't think this is appropriate, I'd rather have the factory class be the only one that is exposed to them. I was trying to understand if there is some kind of func/action method that I can inject from the startup.cs file into my factory class. This func/action will be in charge of creating/registering the builders and then I can activate this func/action within the class factory. I'd like this func/action to receive the interface/class/maybe name of the builder but using generics isn't working. I searched a lot and didn't find any solution so I assume this is not possible.
Seems I have 2 options:
1. Use service locator. This way only the factory class will know the builders. However if in the future, if I want to change the DI I need to "touch" the factory class (I'm contaminating the factory class). Wanted all the DI code to be located only in the startup.cs class.
2. Register the builders in the startup.cs but now the startup.cs is aware of the builders. This kinda couples the code, not really single role of responsibility
It would have been great to inject the factory class a func/action from the startup.cs that would do the registration but the factory class itself activates it.
Is this possible?
I want to try and avoid using a service locator
Great, because the Service Locator is an anti-patttern.
don't register all the types in the startup.cs file.
You should do your registrations in one single 'area' of your application: the start-up path. This area is commonly referred to as the Composition Root (the place where object graphs are composed).
I don't think this is right that all these internal types are referenced in the main startup.cs
No matter how you design it, the startup assembly is the most volatile part of the system and it always depends on all other assemblies in the application. Either directly or transitively (through another referenced assembly). The whole idea of Dependency Injection is to minimize the coupling between components and the way to do this is to centralize coupling by moving it to the Composition Root. By making types internal however, you are decentralizing object composition and that limits your flexability. For instance, it becomes harder to apply decorators or interceptors for those registered types and control them globally. Read this question and its two top voted answers for more information.
I don't register all the types
The concern of having a Composition Root that is too big is not a valid one. One could easily split out the Composition Root into multiple smaller functions or classes that all reside in the startup assembly. On top of that, if you read this, you'll understand that registering all types explicitly (a.k.a. "Explicit Register") is typically pointless. In that case you're probably better off in using DI without a Container (a.k.a. Pure DI). Composition Roots where all types are registered explicitly are not very maintainable. One of the areas a DI Container becomes powerful is through its batch-registration facilities. They use reflection to load and register a complete set of types in a few lines of code. The addition of new types won't cause your Composition Root to change giving you the highest amount of maintainability.
I don't want to create all these builder classes in advance as I might not need to use them and creating them is a bit heavy
Creation of instances should never be heavy. Your injection constructors should be simple and composing object graphs should be reliable. This makes building even the biggest object graphs extremely fast. Factories should be reduced to an absolute minimum.
TLDR;
Register or compose your object graphs solely in the Composition Root.
Refrain from using the Service Locator anti-pattern; Whole applications can (and should) be built purely with Constructor Injection.
Make injection constructors simple and prevent them from doing anything else than storing their incoming dependencies.
Refrain from using factories to compose services, they are not needed in most cases.
I am trying to incorporate Microsoft.Extensions.DependencyInjection into an existing ASP.NET 4.6.1 app.
I know that while .NET Core has it built-in, for 4.6.1 you need to create some initial classes as outlined in http://scottdorman.github.io/2016/03/17/integrating-asp.net-core-dependency-injection-in-mvc-4/. (The article seems to be out of date since the sample code does not show implementation of BeginScope() and Dispose() for IDependencyResolver. If anybody has more updated examples that would be appreciated.)
"Services" accessed by controllers where you are creating instances via constructors is fairly simple but my problem is when I need an instance of "something" that is from a property or method of an existing object.
For example, I have a inherited DbContext that needs an instance of System.Security.Principal.IIdentity that comes from the logged in user.
Another example is an instance of ApplicationUser. ApplicationUser inherits IdentityUser and the current user can be found by calling FindById() method of AppUserManager.
While AppUserManager can easily be instantiated using DI, how can I use DI the inject the output of the FindById() method? I cannot seem to find any documentation or sample code about this for Microsoft based framework. Other frameworks like Unity seem to support Property-based injection.
In essence, can DI be used with all existing classes or do you specifically need to code out the classes to support DI from the beginning? (i.e. only expect parameters to be passed in via constructors and make sure those instances themselves are created via constructors).
The very simple and short answer to your question "Does Microsoft DependencyInjection support non-constructor injection?" is, no.
Out-the-box Microsoft DI does not currently support property injection like other frameworks such as Ninject. If you need that feature, I suggest using those frameworks instead (I cant imagine MS has any plans to add property injection any time soon).
Your other option is to consider how you can refactor your code to use constructor injection instead which is the preferred method
We've started considering using BreezeSharp as we have a WebAPI ODATA Service that we'd like to re-use with a ASP.NET site (no javascript involved, just pure C#).
Unfortunately, we just noticed that, according to the documentation, all of our model entities should now inherit from Breeze.Sharp.BaseEntity. That's a no go for us as this would mean having a dependency on Breeze in our business model. We'd rather keep this dependency on the WebAPI service only.
Is there anyway we could avoid this ? Having proxy classes on the client-side for instance when they don't inherit from BaseEntity ?
Any thoughts on this ?
The Breeze.Sharp.BaseEntity requirement is purely on the client side, and the reason for it is to provide all of the persistence, navigation, key-fixup, change tracking and notification and other services that make the breeze client so easy to use.
There is an IEntity interface that Breeze.Sharp.BaseEntity implements and you are free to implement it instead of using the Breeze.Sharp.BaseEntity, however, this is a very nontrivial task. We are considering offering some guidance on this at a later date if our community generally finds it desirable.
We are also planning on releasing an AOP implementation of IEntity that can be injected directly on top of POCO model objects, but this is likely to require PostSharp and may also have issues running on some client platforms (Xamarin for Android/IOS). No timeframe for this until we get a sense of the demand.
The current implementation on the other hand is very respectful of your model objects, there is only a single 'EntityAspect' property added to your model along with several events.
We have tried the pure POCO approach in the past, on numerous other platforms and application libs and have found that the disadvantages outweigh the minimal cost of a base class, especially when considering that we wanted this library to run in any .NET client including Xamarin/Mono.
If I understand correctly, your only concern is that you don't want to refer to breeze# libraries in your server model. Apparently you have no issue with close coupling of your client and server entity classes in the sense that they have identical properties and perhaps shared methods as well. I'm not being judgmental; I'm merely trying to confirm your architectural decisions.
Have you considered partial classes?
You define the partial class w/o breeze in your server-side business model project and link to that class source in your client model project ... where you keep the companion partial class with the client-specific functionality. That client partial class file specifies the breeze# base class.
While you are at it, you can segregate server-only logic in partial class files that reside in your server project but not in your client project.
Such source file linking has become even easier with VS now that Microsoft is promoting it in their vision of "Universal apps".
So, I've decided to use the Microsoft Extensibility Framework(MEF) with my new ASP.NET MVC web project. The project is a very typical employee management system, with 3 traditional layers :- a presentation layer (with views and controllers), a business layer (with business objects) and ofcourse, a data access layer. After some research, I read a lot, that MEF is supposed to help us implement the plug-in architecture.And this is where, I seem to get stuck. I'm not able to relate with this pluggable part. I'm pretty sure that, since MEF is the part of the core .NET framework, it is not limited to any specific kind of application, and is supposed to be useful in general. I just need to see my application structure in a new light, and that's where I need some helpful insights.
As I'm still trying to get started with MEF, my main question is; what would be the main extensible(pluggable) points in my application? What objects should one be typically composing using the MEF's compose method, and what would be the advantages of doing so using MEF instead of the traditional way?
MEF will not help you solve extensibility issues in the case you have described in the question, because it doesn't seem to have any. What MEF can bring to the table for this particular case is Dependency Injection. If you develop your application in a way that the presentation layer depends on business abstraction and the business layer depends on a data access abstraction rather than on concrete implementation, you will get a highly decoupled application that is easy to test, maintain and change without affecting unnecessary components. Here is one way to do this:
Lets say you have data access class that performs CRUD operations on employees:
[Export(typeof(IEmployeeRepository))]
public class EmployeeRepository : IEmployeeRepository
{
//Some logic here
}
As you can see the EmployeeRepository class implements IEmployeeRepository interface which adds the level of abstraction that is needed to develop a decoupled application. Now lets say that in the business layer you have a class that needs to call some method from the EmployeeRepository class. To be able to call a method of EmployeeRepository, you would need an instance of it. Without using MEF (or any other IoC framework, this would be a way to do it:
public class EmployeeManager
{
private EmployeeRepository _employeeRepository;
public EmployeeManager
{
_employeeRepository = new EmployeeRepository();
}
}
By using the above code sample, a hard dependency between the EmployeeManager and EmployeeRepository classes is created. This hard dependency is difficult to isolate when writing unit tests and causes any change of the EmployeeRepository class to directly affect the EmployeeManager class. By refactoring the code sample a little bit and putting MEF into the game, you'll get this:
[Export(typeof(IEmployeeManager))]
public class EmployeeManager : IEmployeeManager
{
private IEmployeeRepository _employeeRepository;
[ImportingConstructor]
public EmployeeManager(IEmployeeRepository employeeRepository)
{
_employeeRepository = employeeRepository;
}
}
As you can see the EmployeeManager class now doesn't depend on the EmployeeRepository class, but it depends on IEmployeeRepository interface, in other words it depends on abstraction. And it doesn't create the instance of EmployeeRepository class by itself. That job is left to MEF. By now it should be clear that export and ImportingConstructor attributes are part of MEF and are used by it to discover parts and resolve dependencies at runtime. With the last code sample the classes are decoupled, easy to test and maintain and you can change the internal logic in EmployeeRepository class without making EmployeeManager class aware of it. Of course the contract between them, IEmployeeRepository have to be maintained.
The above said, can be used to decouple the presentation layer from the business layer also. Also the above said, can be implemented by using any other IoC framework, like Ninject, Autofac, StructureMap etc. The difference between these IoC frameworks and MEF is that if you use them, you'll need to configure at application start which instance gets created when some interface is encountered:
//something like this, in the case of Ninject
this.Bind<IEmployeeRepository>().To<EmployeeRepository>();
On the other hand, MEF has the ability to discover parts at runtime. On application start, you'll just need to inform it where to look for parts: Directory, Assembly, Type etc. The auto-wire capabilities of MEF (the ability to discover parts at runtime), make it more than a regular IoC framework. This ability makes MEF great for developing pluggable and extensible applications because you'll be able to add plugins while the application is running. MEF is able load them and let the application to use them.
I have an MVC application with a typical architecture...
ASP.NET MVC Controller -> Person Service -> Person Repository -> Entity Framework DB Context
I am using Castle Windsor and I can see the benefit of using this along with a ControllerFactory to create controller with the right dependencies. Using this approach the Controller gets a Service injected, which in turn knows how to construct the right Repository, which in turn knows the correct DbContext to use.
The windsor config is something like this...
dicontainer = new WindsorContainer();
dicontainer.Register(Component.For<IPersonService>().ImplementedBy<PersonService>());
dicontainer.Register(
Component.For<IPersonRepository>().UsingFactoryMethod(
() => new PersonRepository(new HrContext("connectionString"))));
It this the right way to do it? I don't like the UsingFactoryMethod, but can't think of another way.
Also, what if the Repository needed a dependency (say ILogger) that was not needed by the service layer? Does this mean I have to pass the ILogger into the service layer and not use it. This seems like a poor design. I'd appreciate some pointers here. I have read loads of articles, but not found a concrete example to verify whether I am doing this right. Thanks.
I try to avoid using factory methods (as you mentioned you felt this smelled funny). To avoid this, you could create a database session object that creates a new DbContext. Then your repositories just need to get an instance of IDbSession and use its dbContext property. Then, you can also easily control the scope of the IDbSession object (don't use singleton because it's not thread safe).
I wanted to make that point so that I could make this more important point... Make your constructors take in only objects that are registered in the DI container (no options or configurations in constructors). Options and configurations should be read/writen in classes whose sole purposes it is to read/write those values. If all classes follow this model, then your DI registration becomes easy and classes can just add whatever dependencies they need in their constructors.
If you are trying to use a third party library that has options in constructors, wrap that class in your own class that has an easy to consume constructor and uses a configuration class to read the values needed to pass to the third party library. This design also introduces a layer of abstraction between your code and the third party library which can then be used to more easily swap (or stub) third party libraries when necessary.