🧩 Façade Design Pattern in C# – Simplifying Complex Systems

🔍 What is the Façade Design Pattern?

The Façade Design Pattern is a structural design pattern that provides a simplified interface to a complex subsystem of classes, libraries, or frameworks.
In simple terms, it hides the complexity of multiple interdependent systems behind a single, easy-to-use interface.

Think of a hotel receptionist — you don’t directly talk to housekeeping, room service, or maintenance. The receptionist (Façade) takes your request and communicates with the right departments internally.

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🧠 Intent of the Façade Pattern

• Simplify interaction between client and complex subsystems.

• Reduce dependencies between client and internal components.

• Make the system easier to use and maintain.

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🧩 Structure (UML Conceptually)

Client → Facade → SubsystemA
                 → SubsystemB
                 → SubsystemC

The Client interacts with the Facade, which delegates calls to one or more Subsystems.

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💻 C# Example: Home Theater System

Let’s say you’re building a Home Theater Application that involves multiple components:

• DVD Player

• Projector

• Sound System

• Lights

Instead of the client calling all these subsystems directly, we can use a Facade class to control them with a single method call.

✅ Step 1: Subsystems

public class DVDPlayer
{
    public void On() => Console.WriteLine("DVD Player On");
    public void Play(string movie) => Console.WriteLine($"Playing '{movie}'");
    public void Off() => Console.WriteLine("DVD Player Off");
}

public class Projector
{
    public void On() => Console.WriteLine("Projector On");
    public void SetInput(string source) => Console.WriteLine($"Projector input set to {source}");
    public void Off() => Console.WriteLine("Projector Off");
}

public class SoundSystem
{
    public void On() => Console.WriteLine("Sound System On");
    public void SetVolume(int level) => Console.WriteLine($"Volume set to {level}");
    public void Off() => Console.WriteLine("Sound System Off");
}

public class Lights
{
    public void Dim(int level) => Console.WriteLine($"Lights dimmed to {level}%");
}

✅ Step 2: Façade Class

public class HomeTheaterFacade
{
    private readonly DVDPlayer dvd;
    private readonly Projector projector;
    private readonly SoundSystem sound;
    private readonly Lights lights;

    public HomeTheaterFacade(DVDPlayer dvd, Projector projector, SoundSystem sound, Lights lights)
    {
        this.dvd = dvd;
        this.projector = projector;
        this.sound = sound;
        this.lights = lights;
    }

    public void WatchMovie(string movie)
    {
        Console.WriteLine("Get ready to watch a movie...");
        lights.Dim(10);
        projector.On();
        projector.SetInput("DVD Player");
        sound.On();
        sound.SetVolume(5);
        dvd.On();
        dvd.Play(movie);
    }

    public void EndMovie()
    {
        Console.WriteLine("Shutting down the home theater...");
        dvd.Off();
        sound.Off();
        projector.Off();
        lights.Dim(100);
    }
}

✅ Step 3: Client Code

class Program
{
    static void Main()
    {
        var dvd = new DVDPlayer();
        var projector = new Projector();
        var sound = new SoundSystem();
        var lights = new Lights();

        var homeTheater = new HomeTheaterFacade(dvd, projector, sound, lights);

        homeTheater.WatchMovie("Avengers: Endgame");
        Console.WriteLine("\n--- Movie Finished ---\n");
        homeTheater.EndMovie();
    }
}

🧾 Output:

Get ready to watch a movie...
Lights dimmed to 10%
Projector On
Projector input set to DVD Player
Sound System On
Volume set to 5
DVD Player On
Playing 'Avengers: Endgame'

--- Movie Finished ---

Shutting down the home theater...
DVD Player Off
Sound System Off
Projector Off
Lights dimmed to 100%

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🚀 Real-Time Use Cases of Façade Pattern

Scenario	How Façade Helps
Banking Systems	Simplifies complex operations like fund transfers by combining multiple services (accounts, validation, notification) into one interface.
E-commerce Checkout	Combines inventory, payment, and order services into one checkout process.
Hotel Booking APIs	Wraps flight, hotel, and transport systems behind a single booking interface.
Logging or Notification Systems	Provides one class to log to multiple targets (database, file, email).
Azure / AWS SDK Wrappers	Developers use simplified API wrappers to avoid dealing with multiple low-level SDK services.

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⚖️ Advantages of the Façade Pattern

✅ Simplifies complex systems for clients
✅ Reduces coupling between client and subsystems
✅ Improves code readability and maintenance
✅ Makes the system more modular

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⚠️ Disadvantages

❌ Overuse may hide useful functionality from the client
❌ Can become a "God Object" if it grows too large
❌ Difficult to maintain if subsystems frequently change

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💡 Best Practices

• Use when you have a complex system with multiple dependencies.

• Keep the Facade thin — it should only simplify, not duplicate logic.

• Combine with Singleton pattern for global access if needed.

• Avoid making it responsible for business rules — just coordination.

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🧭 Conclusion

The Façade Design Pattern acts like a front desk for your codebase — it hides unnecessary complexity and makes client interactions smooth and simple.
When used properly, it makes large systems more maintainable, readable, and user-friendly.

🏭 Factory Design Pattern in C# with Real-Time Example

📌 What is the Factory Design Pattern?

The Factory Design Pattern is a creational pattern used in object-oriented programming to abstract the process of object creation. Instead of instantiating classes directly using new, the Factory Pattern provides a method that returns instances of different classes based on input parameters.

This pattern is especially useful when:

• The exact type of object to create is determined at runtime.
• You want to encapsulate object creation logic.
• You need to adhere to the Open/Closed Principle (open for extension, closed for modification).

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🚗 Real-Time Example: Vehicle Factory in C#

Imagine you're building a transportation management system that handles different types of vehicles: Car, Bike, and Truck. Each vehicle has its own behavior, but the client code shouldn't worry about how these objects are created.

✅ Step-by-Step Implementation

1. Define a Common Interface

public interface IVehicle
{
    void Start();
}

2. Create Concrete Implementations

public class Car : IVehicle
{
    public void Start()
    {
        Console.WriteLine("Car started.");
    }
}

public class Bike : IVehicle
{
    public void Start()
    {
        Console.WriteLine("Bike started.");
    }
}

public class Truck : IVehicle
{
    public void Start()
    {
        Console.WriteLine("Truck started.");
    }
}

3. Implement the Factory Class

public static class VehicleFactory
{
    public static IVehicle GetVehicle(string vehicleType)
    {
        switch (vehicleType.ToLower())
        {
            case "car":
                return new Car();
            case "bike":
                return new Bike();
            case "truck":
                return new Truck();
            default:
                throw new ArgumentException("Invalid vehicle type");
        }
    }
}

4. Client Code

class Program
{
    static void Main(string[] args)
    {
        IVehicle vehicle1 = VehicleFactory.GetVehicle("car");
        vehicle1.Start();

        IVehicle vehicle2 = VehicleFactory.GetVehicle("bike");
        vehicle2.Start();
    }
}

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🎯 Benefits of Using Factory Pattern in C#

• Encapsulation: Object creation logic is hidden from the client.
• Scalability: Easily add new vehicle types without changing client code.
• Maintainability: Centralized object creation makes debugging and updates easier.
• Loose Coupling: Promotes interface-based programming.

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📝 Final Thoughts

The Factory Design Pattern is a cornerstone of clean, scalable software architecture. Whether you're building enterprise applications or small utilities, this pattern helps you write flexible and maintainable code.

Write a program to Generate all possible permutations of the string "ABCDEF" using C#.

 Since "ABCDEF" has 6 characters, the number of permutations = 6! = 720.

Here’s a C# program to generate and print them:

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        string input = "ABCDEF";
        var results = new List<string>();

        GetPermutations(input.ToCharArray(), 0, results);

        Console.WriteLine($"Total Permutations: {results.Count}");
        foreach (var str in results)
        {
            Console.WriteLine(str);
        }
    }

    static void GetPermutations(char[] arr, int index, List<string> results)
    {
        if (index == arr.Length - 1)
        {
            results.Add(new string(arr));
        }
        else
        {
            for (int i = index; i < arr.Length; i++)
            {
                Swap(ref arr[index], ref arr[i]);
                GetPermutations(arr, index + 1, results);
                Swap(ref arr[index], ref arr[i]); // backtrack
            }
        }
    }

    static void Swap(ref char a, ref char b)
    {
        if (a == b) return;
        char temp = a;
        a = b;
        b = temp;
    }
}

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🔹 How it works:

1. Uses recursion to generate all permutations.

2. Swaps characters, calls recursively, then backtracks to restore the original order.

3. Stores all unique permutations in a List<string>.

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🔹 Sample Output:

Total Permutations: 720
ABCDEF
ABCDE F
ABCDFE
ABCEDF
...
FEDCBA

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👉 This prints all 720 unique permutations of "ABCDEF".