Design Patterns

• Creational Patterns
  • Singleton
  • Factory Method
  • Abstract Factory
  • Builder 🔜
  • Prototype 🔜
• Structural Patterns
  • Adapter 🔜
  • Bridge 🔜
  • Decorator 🔜
  • Composite 🔜
  • Facade 🔜
  • Proxy 🔜
  • Flyweight 🔜
• Behavioral Patterns
  • Template 🔜
  • Strategy 🔜
  • Command 🔜
  • Momento 🔜
  • Mediator 🔜
  • Chain of Responsibility 🔜
  • Observer 🔜
  • State 🔜
  • Iterator 🔜
  • Visitor 🔜
  • Interpreter 🔜
  • Repository 🔜
  • Unit of Work 🔜

Creational Patterns

These patterns deal with object creation.

• Abstract the object instantiation process.
• Help us make your system independent of how its object is created, composed, and represented.

Singleton

Singleton lets you access your object from anywhere in your application. It guarantees that only one instance of this class will be available a time.

Holding the class instance in a global variable doesn’t prevent clients from creating other instances of the class. We need to make the class responsible for ensuring only one instance of itself exists.

Use Cases for the Singleton Pattern

• Should be used when a class must have a single instance available, and it must be accessible to clients from a well-known access point.

• When the sole instance should be extensible by subclassing. and clients should be able to use an extended instance without modifying their code.

Examples

• Managing a database connection pool.

• Caching frequently accessed data (Single instance to evict the cache easily).

• Managing application configuration settings.

Violated the single responsibility principle as the object creating and the lifetime of the object is maintained by the class.

// Program.cs

using Singleton;

Console.Title = "Singleton";

// call the property getter twice
var instance1 = Logger.Instance;
var instance2 = Logger.Instance;

if (instance1 == instance2)
{
    Console.WriteLine("Instances are the same.");
}

instance1.Log($"Message from {nameof(instance1)}");
// or
instance2.Log($"Message from {nameof(instance2)}");

Console.ReadLine();

// Implementation.cs
// without handling the multithreading

namespace Singleton
{
    // Singleton
    public class Logger
    {
        private static Logger? _instance = null;
        // Instance
        public static Logger Instance
        {
            get
            {
                if (_instance == null)
                {
                    _instance = new Logger();
                }
                return _instance;
            }
        }

        protected Logger() { }

        // SingletonOperation
        public void Log(string message)
        {
            Console.WriteLine($"Message to log: {message}");
        }
    }
}

// Implementation.cs
// with handling the multithreading by lazy loading

namespace Singleton
{
    // Singleton
    public class Logger
    {
        // Lazy<T> for lazy initialization
        private static readonly Lazy<Logger> _lazyLogger = new(() => new Logger());

        // Instance
        public static Logger Instance
        {
            get
            {
                return _lazyLogger.Value;
            }
        }

        protected Logger() { }

        // SingletonOperation
        public void Log(string message)
        {
            Console.WriteLine($"Message to log: {message}");
        }
    }
}

Factory Method

Abstract Factory

Structural Patterns

Adapter

Decorator

Facade

Behavioral Patterns

Command

Observer