Inheritance-composition isomorphism by Mark Seemann
Reuse via inheritance is isomorphic to composition.
This article is part of a series of articles about software design isomorphisms.
Chapter 1 of Design Patterns admonishes:
Favor object composition over class inheritancePeople sometimes struggle with this, because they use inheritance as a means to achieve reuse. That's not necessary, because you can use object composition instead.
In the previous article, you learned that an abstract class can be refactored to a concrete class with injected dependencies.
Did you notice that there was an edge case that I didn't cover?
I didn't cover it because I think it deserves its own article. The case is when you want to reuse a base class' functionality in a derived class. How do you do that with Dependency Injection?
Calling base #
Imagine a virtual method:
public virtual OutVirt1 Virt1(InVirt1 arg)
In C#, a method is virtual when explicitly marked with the virtual keyword, whereas this is the default in Java. When you override a virtual method in a derived class, you can still invoke the parent class' implementation:
public override OutVirt1 Virt1(InVirt1 arg) { // Do stuff with this and arg var baseResult = base.Virt1(arg); // return an OutVirt1 value }
When you override a virtual method, you can use the base keyword to invoke the parent implementation of the method that you're overriding. The enables you to reuse the base implementation, while at the same time adding new functionality.
Virtual method as interface #
If you perform the refactoring to Dependency Injection shown in the previous article, you now have an interface:
public interface IVirt1 { OutVirt1 Virt1(InVirt1 arg); }
as well as a default implementation. In the previous article, I showed an example where a single class implements several 'virtual member interfaces'. In order to make this particular example clearer, however, here you instead see a variation where the default implementation of IVirt1 is in a class that only implements that interface:
public class DefaultVirt1 : IVirt1 { public OutVirt1 Virt1(InVirt1 arg) { // Do stuff with this and arg; return an OutVirt1 value. } }
DefaultVirt1.Virt1 corresponds to the original virtual method on the abstract class. How can you 'override' this default implementation, while still make use of it?
From base to composition #
You have a default implementation, and instead of replacing all of it, you want to somehow enhance it, but still use the 'base' implementation. Instead of inheritance, you can use composition:
public class OverridingVirt1 : IVirt1 { private readonly IVirt1 @base = new DefaultVirt1(); public OutVirt1 Virt1(InVirt1 arg) { // Do stuff with this and arg var baseResult = @base.Virt1(arg); // return an OutVirt1 value } }
In order to drive home the similarity, I named the class field @base. I couldn't use base as a name, because that's a keyword in C#, but you can use the prefix @ in order to use a keyword as a legal C# name. Notice that the body of OverridingVirt1.Virt1 is almost identical to the above, inheritance-based overriding method.
As a variation, you can inject @base via the constructor of OverridingVirt1, in which case you have a Decorator.
Isomorphism #
If you already have an interface with a 'default implementation', and you want to reuse the default implementation, then you can use object composition as shown above. At its core, it's reminiscent of the Decorator design pattern, but instead of receiving the inner object via its constructor, it creates the object itself. You can, however, also use a Decorator in order to achieve the same effect. This will make your code more flexible, but possibly also more error-prone, because you no longer have any guarantee what the 'base' is. This is where the Liskov Substitution Principle becomes important, but that's a digression.
If you're using the previous abstract class isomorphism to refactor to Dependency Injection, you can refactor any use of base to object composition as shown here.
This is a special case of Replace Inheritance with Delegation from Refactoring, which also describes the inverse refactoring Replace Delegation with Inheritance, thereby making these two refactorings an isomorphism.
Summary #
This article focuses on a particular issue that you may run into if you try to avoid the use of abstract classes. Many programmers use inheritance in order to achieve reuse, but this is in no way necessary. Favour composition over inheritance.
Next: Tester-Doer isomorphisms.
