Design Patterns Refresher

The most common design patterns from "Head First Design Patterns" by Eric Freeman and Elisabeth Freeman - the best book ever about design patterns:

Strategy
Encapsulate algorithms. Allows object's behaviour to be changed at runtime by substituting default algorithms. 

Observer 
Notify dependents of state changes. A very common pattern, often part of the language, see events and listeners in C#.

Decorator

Use and extend behaviour of the object being wrapped, while acting as that object to the external world. A decorator object is created with an instance of an object of the same type. When a method is invoked, it calls the embedded object's method, modifies the result, and returns it.

Simple Factory

Encapsulate object creation. Useful when you need to decide at runtime which object to create.

Factory Method

Same purpose as Simple Factory, but more abstract: moves actual object creation to other factory objects. 

Singleton

Indispensable when there is one of something in the system, like one database, or one set of configuration data. Implemented as a sealed class with private constructor and a static method that returns the only instance.

Command

Encapsulate actions. Every action object has "execute" method. Very useful for a processing system that needs to queue commands and then execute them in order. 

Adapter
Provide a converter that allows clients to use the interface they expect with an object that has a different interface.

Facade

Provide a simplified interface. Used to hide complexity of the underlying system from the external world.

Template Method

Provides extension points or hooks by defining steps of an algorithm and allowing or forcing subclasses to provide implementation for some of them. This pattern is often used in frameworks.

Iterator 

Provides a uniform way of accessing sequentially elements in an aggregate object of any type. Typically found in collections.

Composite

Organize objects into tree structures so a group of them can be treated the same way as a single element. Each object in the hierarchy is a component that can have sub-components. User interfaces are usually built as composites.

State
Encapsulate behaviours of an object in state objects. Current state object can request transition to another state.

Proxy

Provide a surrogate object that controls access to an object that is remote, needs to be secured or is expensive to create.