Is this called adapter? + adapter vs decorator

Question

I have 2 projects: A and B that should interactwith each other.

1. Project A introduce interface names ISpecialTask and Project B should implement it.

2. Projet B has an entity named TaskWithListOfProperties that cannot implement ISpecialTask because it has different structure of properties (in addition, all the system knows how to work with TaskWithListOfProperties and I don't want to change its structure).

So I decided to create a class named SpecialTaskFromListOfProperties that implements ISpecialTask and uses TaskWithListOfProperties instance in order to use it for the interaction between the projects.

interface ISpecialTask {
    long Id{get;}
    long WorkerId{get;}
    long VehicleId{get;}
    long CustomerId{get;}
}

class TaskWithListOfProperties {
    IDictionary<string, long> Properties {get;
}

class SpecialTaskFromListOfProperties : ISpecialTask  {
    public SpecialTaskFromListOfProperties(TaskWithListOfProperties  ins) {
        ...
        ...
    }
    public long Id { get{ ... } }
    public long WorkerId { get{ ... } }
    public long VehicleId { get{ ... } }
    public long CustomerId { get{ ... } }
}

Is SpecialTaskFromListOfProperties actually the Adapter pattern?
What is the difference between the adapter pattern and decorator pattern?

Answer 1

Depends on what you're actually trying to achieve. Adapter and Decorator are pretty much similar, however when you implement an Adapter you bring no new logic besides conversion. When implementing a Decorator you actually bring in some brand new functionality that never existed before in the object you're decorating.

So, long story short, if interface properties Id, WorkerId etc are naturally coming from TaskWithListOfProperties - then you should consider it as an Adapter. Otherwise it's a Decorator.

Answer 2

From the original GoF book, the intent of the Adapter pattern ^{[Black Wasp] [Wikipedia]} is to...

convert the interface of a class into another interface clients expect. Adapter lets classes work together that couldn't otherwise because of incompatible interfaces.

While the intent of the Decorator pattern ^{[Black Wasp] [Wikipedia]} is to...

attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending funtionality.

Though the patterns are similar, from the definition it's clear that this is the adapter pattern. You've got a square peg (TaskFromListOfProperties) that needs to fit into a round hole (ISpecialTask), so you've adapted it using SpecialTaskFromListOfProperties.

A decorator would augment/extend the existing functionality of TaskFromListOfProperties, i.e. it wouldn't change its existing interface. That's not what SpecialTaskFromListOfProperties is doing.