Create a decorator
Last modified: 21 July 2022The Decorator design pattern is used to dynamically add additional behavior to an object. In addition, by using interfaces, a decorator can be used to unify types in a manner similar to multiple inheritance.
tip
For more information on the decorator pattern, check out this Wikipedia article.
Let’s take an example – suppose you have two objects, called Bird and Lizard, that you want to put into a Decorator:
class Bird
{
public void Fly()
{
...
}
}
class Lizard
{
public void Walk()
{
...
}
}1. Get object interfaces with the extract interface refactoring
Since a decorator cannot be inheriting from both of these classes, we can invoke the Extract Interface refactoring from the Refactor this menu Ctrl+Shift+R for both of these objects to get their interfaces:
Calling this refactoring pops up a window asking us which members should appear in the interface:
After we do this for both our classes, we end up with the following code:
internal interface IBird
{
void Fly();
}
class Bird : IBird
{
public void Fly()
{
...
}
}
internal interface ILizard
{
void Walk();
}
class Lizard : ILizard
{
public void Walk()
{
...
}
}2. Declare a decorator class
With these interfaces, we can make a decorator. First, we declare a class called Dragon, indicating that it implements both IBird and ILizard:
class Dragon : IBird, ILizard
{
...
}Now for the aggregated members. The easiest thing to do is to declare both of these as fields first, that is:
class Dragon : IBird, ILizard
{
private Bird bird;
private Lizard lizard;
}3. Use quick-fix to initialize unused fields
Now, we can use apply a quick-fix to unused fields and initialize both of them from constructor parameters:
After this, our class will look as follows:
class Dragon : IBird, ILizard
{
private Bird bird;
private Lizard lizard;
public Dragon(Bird bird, Lizard lizard)
{
this.bird = bird;
this.lizard = lizard;
}
}4. Generate delegating members for source objects.
And now for the finishing touch – we want to generate delegating members for both bird and lizard. This is easy — we open the Generate menu Alt+Insert and choose Delegating Members:
ReSharper then asks us which members we need to delegate:
And here is the end result:
class Dragon : IBird, ILizard
{
public void Fly()
{
bird.Fly();
}
public void Walk()
{
lizard.Walk();
}
private Bird bird;
private Lizard lizard;
public Dragon(Bird bird, Lizard lizard)
{
this.bird = bird;
this.lizard = lizard;
}
}That’s it! Our decorator is ready.