Monday, August 01, 2016

Infrastructure (Accessor) pattern - an interesting design choice for the .NET Core team

While researching the new .NET Core features and functionalities I've stumbled upon this pattern for hiding functionality, but also making it accessible when needed.

There was a long history of Microsoft writing the code as closed as possible: classes and interfaces are internal protected and sealed and all that jazz. If you have ever tried to copy paste Microsoft .NET source code into your project, in order to modify it to your needs, you know what I mean. More times than not I gave up because of the immense chain of dependencies that had to be all copy pasted in order for a small piece of code to work.

Well, .NET Core is now open source and there is a strong current of moving away from such practices. One pattern that drew my attention is the IInfrastructure<T> interface and pattern used in EntityFramework. Basically, instead of exposing rarely used members directly, you hide them within a generic interface that can be retrieved at will.

Yes, it is possible to do the same thing with an explicitly implemented interface, but this is more of a two step way of doing it (and also of uncluttering class signatures). The concrete example is DbContext, which is an explicit implementation of IInfrastructure<IServiceProvider>. IService provider has a GetService<T> method that returns specific implementation of interfaces of base classes. Then, with a nice extension method called GetInfrastructure<T>, one can get the service provider. For example one can retrieve the relational type mapper from a context using:
var serviceProvider=context.GetInfrastructure();
var mapper=serviceProvider.GetService<IRelationalTypeMapper>();

I find it interesting as a general pattern, allowing one to expose innumerable interface signatures without inheriting from them all. A class can enable any number of mechanisms for discovery and execution simply by implementing its own service provider. Moreover, if there is some sort of general Service Locator pattern in place, classes can locally override that mechanism while leaving the rest in place. Clearly there is potential for abuse, but I also see it as a way to clearly represent and separate concerns.