What is the Dependency Inversion Principle and why is it important?

Question

What is the Dependency Inversion Principle and why is it important?

Answer 1

Check this document out: The Dependency Inversion Principle.

It basically says:

• High level modules should not depend upon low-level modules. Both should depend upon abstractions.
• Abstractions should never depend upon details. Details should depend upon abstractions.

As to why it is important, in short: changes are risky, and by depending on a concept instead of on an implementation, you reduce the need for change at call sites.

Effectively, the DIP reduces coupling between different pieces of code. The idea is that although there are many ways of implementing, say, a logging facility, the way you would use it should be relatively stable in time. If you can extract an interface that represents the concept of logging, this interface should be much more stable in time than its implementation, and call sites should be much less affected by changes you could make while maintaining or extending that logging mechanism.

By also making the implementation depend on an interface, you get the possibility to choose at run-time which implementation is better suited for your particular environment. Depending on the cases, this may be interesting too.

Answer 2

The point of dependency inversion is to make reusable software.

The idea is that instead of two pieces of code relying on each other, they rely on some abstracted interface. Then you can reuse either piece without the other.

The way this is most commonly achieved is through an Inversion of Control container like Spring in Java. In this model, properties of objects are set up through an XML configuration instead of the objects going out and finding their depenency.

Imagine this pseudocode...

public class MyClass
{
  public Service myService = ServiceLocator.service;
}

MyClass directly depends on both the Service class and the ServiceLocator class. It needs both of those if you want to use it in another application. Now imagine this...

public class MyClass
{
  public IService myService;
}

Now, MyClass relies on a single interface, the IService interface. We'd let the IoC container actually set the value of that variable.

So now, MyClass can easily be reused in other projects, without bringing the dependency of those other two classes along with it.

Even better, you don't have to drag the dependencies of MyService, and the dependencies of those dependencies, and the... well, you get the idea.

Answer 3

Inversion of Control Containers and the Dependency Injection pattern by Martin Fowler is a good read too. I found Head First Design Patterns an awesome book for my first foray into learning DI and other patterns.

Answer 4

Inversion of Control is a design pattern where an Objects gets handed its dependency by an outside framework, rather than asking a framework for its dependency.

Pseudocode example using traditional lookup:

class Service {
    Database database;
    init() {
        database = FrameworkSingleton.getService("database");
    }
}

Similar code using IoC:

class Service {
    Database database;
    init(database) {
        this.database = database;
    } 
}

The benefis of IoC are:

• You have no dependency on a central framework, so this can be changed if desired.
• Since objects are created by injection, preferably using interfaces, it's easy to create unit tests that replace dependencies with mock versions.
• Decoupling off code.

Answer 5

Good answers and good examples are already given by others here.

The reason DIP is important is because it ensures the OO-principle "lousely coupled design".

The objects in your software should NOT get into a hierarchy where some objects are the top-level ones, dependent on low-level objects. Changes in low-level objects will then ripple-through to your top-level objects which makes the software very fragile for change.

You want your 'top-level' objects to be very stable and not fragile for change, therefore you need to invert the dependencies.

Answer 6

The books Agile Software Development, Principles, Patterns, and Practices and Agile Principles, Patterns, and Practices in C# are the best resources for fully understanding the original goals and motivations behind the Dependency Inversion Principle. The article "The Dependency Inversion Principle" is also a good resource, but due to the fact that it is a condensed version of a draft which eventually made its way into the previously mentioned books, it leaves out some important discussion on the concept of a package and interface ownership which are key to distinguishing this principle from the more general advise to "program to an interface, not an implementation" found within the book Design Patterns (Gamma, et. al).

To provide a summary, the Dependency Inversion Principle is primarily about reversing the conventional direction of dependencies from "higher level" components to "lower level" components such that "lower level" components are dependent upon the interfaces owned by the "higher level" components. (Note: "higher level" component here refers to the component requiring external dependencies/services, not necessarily its conceptual position within a layered architecture.) In doing so, coupling isn't reduced so much as it is shifted from components that are theoretically less valuable for reuse to components which are theoretically more valuable for reuse.

This is achieved by designing components whose external dependencies are expressed in terms of an interface for which an implementation must be provided by the consumer of the component. In other words, the defined interfaces express what is needed by the component, not how you use the component (e.g. "INeedSomething", not "IDoSomething").

What the Dependency Inversion Principle does not refer to is the simple practice of abstracting dependencies through the use of interfaces (e.g. MyService → [ILogger ⇐ Logger]). While this decouples a component from the specific implementation detail of the dependency, it does not invert the relationship between the consumer and dependency (e.g. [MyService → IMyServiceLogger] ⇐ Logger.

A longer discussion of this principle as it relates to the simple use of interfaces, Dependency Injection, and the Separated Interface pattern can be found here.

Answer 7

To me, the Dependency Inversion Principle, as described in the official article, is really a misguided attempt to increase the reusability of modules that are inherently less reusable, as well as a way to workaround an issue in the C++ language.

The issue in C++ is that header files typically contain declarations of private fields and methods. Therefore, if a high-level C++ module includes the header file for a low-level module, it will depend on actual implementation details of that module. And that, obviously, is not a good thing. But this is not an issue in the more modern languages commonly used today.

High-level modules are inherently less reusable than low-level modules because the former are normally more application/context specific than the latter. For example, a component that implements an UI screen is of the highest-level and also very (completely?) specific to the application. Trying to reuse such a component in a different application is counter-productive, and can only lead to over-engineering.

So, the creation of a separate abstraction at the same level of a component A that depends on a component B (which does not depend on A) can be done only if component A will really be useful for reuse in different applications or contexts. If that's not the case, then applying DIP would be bad design.