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Designing with the Dependency Inversion Principle

The Dependency Inversion Principle says "the interface belongs to the client." As I've said elsewhere, adopting this principle means a reversing of the way applications used to be built: Design the database, build the objects to maintain the tables, wrap a UI around those objects and then bring the users in for training because they'd never figure the application out on their own.

The Dependency Inversion Principle says: Build the UIs that your users will understand (the interface belongs to them), design the objects that will make those UIs easy to build, build those classes, design the objects that make those classes easy to build and carry on until code-first Entity Framework generates the database you need.

You know this, already: the Dependency Inversion Principle is what drives the essential difference between ADO.NET and Entity Framework. With ADO.NET, it was your responsibility to create a connection (because you need a connection to the database), create a correctly configured command object (because you have to issue commands), call the appropriate execute method (because different SQL commands work differently) and then manage fetching rows and turning them into objects (because ... well, you get the picture). In other words, ADO.NET's API was driven by how the ADO.NET objects worked -- the reverse of the Dependency Inversion Principle.

On the other hand, essentially what Entity Framework says is, "Tell me what objects you want and I'll get them for you." Entity Framework provides the API that the application wants: An object-oriented way of retrieving, adding, updating and deleting data.

There are real cost-savings associated with the Dependency Inversion Principle. Because the principle requires that objects deliver the functionality that the client wants, interfaces tend to be more stable. Following the principle, APIs only change because the client program wants to do something differently (which, when you think about it, is the only reason we should be changing our code). You're welcome to upgrade how your objects work, of course ... but you're not allowed to change the API.

Of course, this level of abstraction isn't free: Entity Framework doesn't have the performance that pure ADO.NET has, even for the scenarios it targets: online, transactional applications. However, it does improve the productivity of developers who, let's face it (and given the current cost of hardware), are the most expensive part of an application -- just ask my client. And, when you do need "bare metal" levels of performance, there's always Dapper.

And, quite frankly, if you wanted the fastest performance, you'd be writing your code in assembler and running it on MS-DOS. Let's not be silly about performance.

Posted by Peter Vogel on 02/27/2019

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