How many patterns are there?

The original Gang of Four book included 23 patterns divided into 3 categories:

• Creational patterns used to create objects without instantiating them directly. This category includes Abstract Factory, Builder, Factory Method, Prototype, and Singleton.
• Structural patterns that define ways of class and object composition. Adapter, Bridge, Composite, Decorator, Facade, Flyweight, and Proxy are all structural patterns.
• Behavioral patterns that suggest various ways for objects to communicate and/or evolve. Behavioral patterns include Chain of Responsibility, Command, Interpreter, Iterator, Mediator, Memento, Observer, State, Strategy, Template Method, and Visitor.

This is the original list, but it's not set in stone. If you look around, you will find variations where some patterns are omitted and others are introduced.

Selected design patterns

Some software engineers argue that the most common design pattern is The Big Ball of Mud. However, we'll look at a selection of patterns that are arguably more useful. We'll use C# as the language of choice for sample code.