Decorator

Context: Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

public interface IComponent
{
    void Operation();
}

public class ConcreteComponent : IComponent
{
    public void Operation() => Console.WriteLine("Component operation");
}

public abstract class Decorator : IComponent
{
    protected IComponent _component;
    public Decorator(IComponent component) => _component = component;
    public virtual void Operation() => _component.Operation();
}

public class ConcreteDecoratorA : Decorator
{
    public ConcreteDecoratorA(IComponent c) : base(c) { }
    public override void Operation()
    {
        base.Operation();
        Console.WriteLine("Added behavior A");
    }
}

Real-world usage example

Streams in .NET: FileStream can be decorated with BufferedStream, CryptoStream, GZipStream to add buffering, encryption, or compression without changing the original stream.

Example: ASP.NET Core middleware is a decorator pipeline: each middleware decorates the next, adding logging, authentication, caching, etc. System.IO.Stream decorators include GZipStream, CryptoStream.

Decorator

Real-world usage example

Related sections