Design-Patterns

The Command Pattern is a behavioral design pattern that turns a request into a stand-alone object containing all the information about the request. This conversion allows you to parameterize methods with different requests, queue requests, and support undoable operations.

In simpler terms, the Command Pattern encapsulates a request as an object, thereby allowing users to decouple the sender (invoker) from the object that performs the action (receiver).

Key Concepts

1. Command: An object that encapsulates a request or action.

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The Adapter pattern is a structural design pattern that allows two incompatible interfaces to work together by converting the interface of one class into an interface expected by the clients. In other words, it acts as a bridge between two incompatible interfaces, enabling them to communicate with each other.

Adapters in real world?

For a phone that does not support a 3.5 mm audio jack, you can use a USB-C to 3.5 mm audio adapter. The phone has only a USB-C port, so the adapter converts the USB-C connection into a 3.5 mm audio jack, allowing you to use traditional wired headphones with your phone. The adapter bridges the gap between the old headphone interface and the new phone design.

The Facade pattern is a structural design pattern that provides a simplified interface to a complex system of classes or interfaces.

Let’s understand it with example. Imagine a home automation system.

Let’s implement it programmatically.

public class FacadePatternTestDrive
{
    public static void Main(string[] args)
    {
        ILights lights = new Lights();
        IAirConditioner airConditioner = new AirConditioner();
        ISecuritySystem securitySystem = new SecuritySystem();

        // When leaving home
        lights.TurnOff();
        airConditioner.TurnOff();
        securitySystem.Arm();
        Console.WriteLine("You have left home.");

        // When entering home
        lights.TurnOn();
        airConditioner.TurnOn();
        securitySystem.Disarm();
        Console.WriteLine("Welcome home!");
    }

}

Note: AirConditioner, SecuritySystem and Lights class is not present in the above example for the sake of simplicity. We will write those classes when we implement the facade pattern.

The Decorator pattern is a structural pattern that enables you to wrap an object with additional behaviors or responsibilities without changing its external interface. It’s like adding layers of decorations to a gift — you can add or remove decorations without altering the gift itself.

Example:

Let’s say, we have a book whose base price is 500 INR. Some buyers need a hard cover book which costs some extra money, some need a signed copy which also add some additional cost, some needs the both hard cover and a signed copy which adds more cost. May be in future we want to remove the option of hard cover book or may be we want to add some extra features to it. How do we make our system more flexible, that is the big question.

The Observer pattern is a behavioral design pattern that defines a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

In a simple words, it is like a publisher and subscriber pattern. When publisher publishes something, all it’s subscriber gets notified.

One Example could be, when we subscribe someone’s mailing list we get notified whenever the publisher publishes something.

The Abstract Factory pattern is a creational design pattern that provides an interface for creating families of related or dependent objects without specifying their concrete classes. It allows a client to create objects from a variety of related classes without needing to know the specifics of their implementations.

GUI Factory Example

Let’s say, we need to create GUI components for different themes in our application. Each theme includes a consistent set of components such as buttons and checkboxes. We want to ensure that the creation of these components is consistent and interchangeable.

The Factory Method Pattern is a creational design pattern that provides an interface for creating objects in a superclass but allows subclasses to alter the type of objects that will be created.

This pattern is useful when a class cannot anticipate the class of objects it needs to create or when subclasses want to specify the objects to be created.

A Notification System

Imagine we need to develop a notification system that can send different types of notifications such as Email, SMS, and Push Notifications. Each type of notification has a distinct implementation but shares a common interface. Instead of creating each notification type directly in our main logic, we can use the Factory Method Pattern to encapsulate the instantiation process, making our code more modular and extensible.

The Dependency Inversion Principle states that:

1. High-level modules should not depend on low-level modules. Both should depend on abstractions (e.g., interfaces).
2. Abstractions should not depend on details. Details (concrete implementations) should depend on abstractions.

Source: Wikipedia

Other SOLID Principles

• Single Responsibility Principle
• Open Closed Principle
• Liskov Substitution Problem
• Interface Segregation Principle

Example without DIP

public class UserRepository
{
    public string GetUserById(int id)
    {
        // Code to fetch user from the database
        return "User";
    }
}

public class UserService
{
    private readonly UserRepository _userRepository;

    public UserService()
    {
        _userRepository = new UserRepository();
    }
    public string GetUser(int id)
    {
        return _userRepository.GetUserById(id);
    }
}

One problem with this code is , theUserService is tightly coupled with the UserRepository. Another problem is, if you want to use different implementation of UserReposoitory, you can not easily do it. For instance you want to swap your UserRepsoitory to UserRepositoryWithDifferentDatabase.

📢 Clients should not be forced to depend on interfaces they do not use.

In simpler terms, this principle suggests that an interface should not include methods that implementing classes don’t need. It Encourages us to create smaller interfaces over the big fat interface.

Other SOLID principles

• Single Responsibility Principle
• Open Closed Principle
• Liskov Substitution Problem
• Dependency Inversion Principle

Violating example

Imagine you have an interface for workers in a company:

The Liskov Substitution Principle states that a derived type should be completely replaceable for its base type.

In simpler terms, a child class should do at least what the parent class can do. The child class can have additional behaviors, but it must have all the behaviors that the parent class has.

For instance, an Animal has behaviors (eat, sleep, makeSound). A Dog, which is a subtype of Animal, must have all the behaviors (eat, sleep, makeSound).