Feedback on ASP.NET vNext Dependency Injection by Mark Seemann

ASP.NET vNext includes a Dependency Injection API. This post offers feedback on the currently available code.

As part of Microsoft's new openness, the ASP.NET team have made the next version of ASP.NET available on GitHub. Obviously, it's not yet done, but I suppose that the reasons for this move is to get early feedback, as well as perhaps take contributions. This is an extremely positive move for the ASP.NET team, and I'm very grateful that they have done this, because it enables me to provide early feedback, and offer my help.

It looks like one of the proposed new features of the next version of ASP.NET is a library or API simply titled Dependency Injection. In this post, I will provide feedback to the team on that particular sub-project, in the form of an open blog post. The contents of this blog post is also cross-posted to the official ASP.NET vNext forum.

Dependency Injection support #

The details on the motivation for the Dependency Injection library are sparse, but I assume that the purpose is provide 'Dependency Injection support' to ASP.NET. If so, that motivation is laudable, because Dependency Injection (DI) is the proper way to write loosely coupled code when using Object-Oriented Design.

Some parts of the ASP.NET family already have DI support; personally, I'm most familiar with ASP.NET MVC and ASP.NET Web API. Other parts have proven rather hostile towards DI - most notably ASP.NET Web Forms. The problem with Web Forms is that Constructor Injection is impossible, because the Web Forms framework doesn't provide a hook for creating new Page objects.

My interpretation #

As far as I can tell, the current ASP.NET Dependency Injection code defines an interface for creating objects:

public interface ITypeActivator
{
    object CreateInstance(
        IServiceProvider services,
        Type instanceType,
        params object[] parameters);
}

In addition to this central interface, there are other interfaces that enable you to configure a 'service collection', and then there are Adapters for

• Autofac
• Ninject
• StructureMap
• Unity
• Caste Windsor

As far as I can tell, there's no web code in the ASP.NET Dependency Injection code base. In other words, this is a poster example of a Conforming Container.

My recommendations #

It's an excellent idea to add 'Dependency Injection support' to ASP.NET, for the few places where it's not already present. However, as I've previously explained, a Conforming Container isn't the right solution. The right solution is to put the necessary extensibility points into the framework:

• ASP.NET MVC already has a good extensibility point in the IControllerFactory interface. I recommend keeping this interface, and other interfaces in MVC that play a similar role.
• ASP.NET Web API already has a good extensibility point in the IHttpControllerActivator interface. I recommend keeping this interface, and other interfaces in Web API that play a similar role.
• ASP.NET Web Forms have no extensibility point that enables you to create custom Page objects. I recommend adding an IPageFactory interface, as described in my article about DI-Friendly frameworks. Other object types related to Web Forms, such as Object Data Sources, suffer from the same shortcoming, and should have similar factory interfaces.
• There may be other parts of ASP.NET with which I'm not particularly familiar (SignalR?), but they should all follow the same pattern of defining Abstract Factories for user classes, in the cases where these don't already exist.

In addition to adding these required extensibility points, I recommend completely abandoning the project of defining a Conforming Container. The extensibility points should be added where they're used - the MVC Factories as part of MVC, the Web Form Factories as part of Web Forms, etc. This will have the added benefit of making the ASP.NET Dependency Injection project redundant. Less code is better than more code.

"perfection is attained not when there is nothing more to add, but when there is nothing more to remove." - Antoine de Saint Exupéry

These are my general recommendations to the team, but if desired, I'd also like to offer my assistance with any particular issues the team might encounter.

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