DesignPatterns.Decorator

Decorator pattern

The decorator pattern allows to modify or change behavior of object with wrapping it by another one. The wrapper object itself is transparent for user. This is gained by having same parent class with wrapped object or implementation of the same interface.

Decorator is often used in IO libraries of various programming languages like C#, Go, Java and others, because it well suited for designing stream processing and data pipelines.

E.g, C#'s System.IO.Compression.GZipStream and System.IO.FileStream both have System.IO.Stream as an ancestor. Also, GZipStream has parameter of type Stream in all its constructors, so we can't create GZipStream without having another Stream object to wrap (decorate). And so, we can extract contents of gzip archive by creating new FileStream and passing it to GZipStream constructor, and, finally, read from newly created object using Stream.Read method.

In this sample, decorator is used for logging payment operations.

Sample content:

• IPaymentMethod - interface implemented by payment methods and logger
• Checkout - class, which uses IPaymentMethod implementations to handle payments
• CardPaymentMethod and CashPaymentMethod - payment method classes (IPaymentMethod implementations)
• PaymentMethodLogger - the actual decorator class, which implements IPaymentMethod and wraps any other IPaymentMethod implementation

See usage example in Program.cs.