🎉 Hooray! After 3 years of work, I've finally released the ebook on design patterns! Check it out »
Decorator

Decorator in C#

Decorator is a Conceptual pattern that allows adding new behaviors to objects dynamically by placing them inside special wrapper objects.

Using decorators you can wrap objects countless number of times since both target objects and decorators follow the same interface. The resulting object will get a stacking behavior of all wrappers.

Learn more about Decorator

Usage of the pattern in C#

Complexity:

Popularity:

Usage examples: The Decorator is pretty standard in C# code, especially in code related to streams.

Identification: Decorator can be recognized by creation methods or constructor that accept objects of the same class or interface as a current class.

Conceptual Example

This example illustrates the structure of the Decorator design pattern. It focuses on answering these questions:

  • What classes does it consist of?
  • What roles do these classes play?
  • In what way the elements of the pattern are related?

Program.cs: Conceptual Example

using System;

namespace RefactoringGuru.DesignPatterns.Composite.Conceptual
{
    // The base Component interface defines operations that can be altered by
    // decorators.
    public abstract class Component
    {
        public abstract string Operation();
    }

    // Concrete Components provide default implementations of the operations.
    // There might be several variations of these classes.
    class ConcreteComponent : Component
    {
        public override string Operation()
        {
            return "ConcreteComponent";
        }
    }

    // The base Decorator class follows the same interface as the other
    // components. The primary purpose of this class is to define the wrapping
    // interface for all concrete decorators. The default implementation of the
    // wrapping code might include a field for storing a wrapped component and
    // the means to initialize it.
    abstract class Decorator : Component
    {
        protected Component _component;

        public Decorator(Component component)
        {
            this._component = component;
        }

        public void SetComponent(Component component)
        {
            this._component = component;
        }

        // The Decorator delegates all work to the wrapped component.
        public override string Operation()
        {
            if (this._component != null)
            {
                return this._component.Operation();
            }
            else
            {
                return string.Empty;
            }
        }
    }

    // Concrete Decorators call the wrapped object and alter its result in some
    // way.
    class ConcreteDecoratorA : Decorator
    {
        public ConcreteDecoratorA(Component comp) : base(comp)
        {
        }

        // Decorators may call parent implementation of the operation, instead
        // of calling the wrapped object directly. This approach simplifies
        // extension of decorator classes.
        public override string Operation()
        {
            return $"ConcreteDecoratorA({base.Operation()})";
        }
    }

    // Decorators can execute their behavior either before or after the call to
    // a wrapped object.
    class ConcreteDecoratorB : Decorator
    {
        public ConcreteDecoratorB(Component comp) : base(comp)
        {
        }

        public override string Operation()
        {
            return $"ConcreteDecoratorB({base.Operation()})";
        }
    }
    
    public class Client
    {
        // The client code works with all objects using the Component interface.
        // This way it can stay independent of the concrete classes of
        // components it works with.
        public void ClientCode(Component component)
        {
            Console.WriteLine("RESULT: " + component.Operation());
        }
    }
    
    class Program
    {
        static void Main(string[] args)
        {
            Client client = new Client();

            var simple = new ConcreteComponent();
            Console.WriteLine("Client: I get a simple component:");
            client.ClientCode(simple);
            Console.WriteLine();

            // ...as well as decorated ones.
            //
            // Note how decorators can wrap not only simple components but the
            // other decorators as well.
            ConcreteDecoratorA decorator1 = new ConcreteDecoratorA(simple);
            ConcreteDecoratorB decorator2 = new ConcreteDecoratorB(decorator1);
            Console.WriteLine("Client: Now I've got a decorated component:");
            client.ClientCode(decorator2);
        }
    }
}

Output.txt: Execution result

Client: I get a simple component:
RESULT: ConcreteComponent

Client: Now I've got a decorated component:
RESULT: ConcreteDecoratorB(ConcreteDecoratorA(ConcreteComponent))

Decorator in Other Languages

Design Patterns: Decorator in Java Design Patterns: Decorator in PHP Design Patterns: Decorator in Python Design Patterns: Decorator in Ruby Design Patterns: Decorator in Swift Design Patterns: Decorator in TypeScript