C# Design Patterns — Real-World Implementations

Clean, practical implementations of the most important design patterns in C#/.NET, using real-world business scenarios — POS systems, invoices, inventory, payments, and more.

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📦 Patterns Included

Creational Patterns

Pattern	File	Real-World Example
Singleton	DatabaseConnection.cs	Thread-safe database connection manager
Factory Method	PaymentProcessorFactory.cs	Payment gateway selector (CreditCard, JazzCash, Bank Transfer)
Builder	InvoiceBuilder.cs	Fluent invoice generator with line items & tax

Behavioral Patterns

Pattern	File	Real-World Example
Observer	OrderSystem.cs	Order notification system (Email, SMS, Analytics)
Strategy	PosCheckout.cs	POS discount strategies (%, BOGO, Loyalty)
Command	InventoryCommands.cs	Inventory management with Undo/Redo

Structural Patterns

Pattern	File	Real-World Example
Adapter	NotificationAdapters.cs	Unified notification service (Twilio, SendGrid, Slack)
Decorator	OrderDecorator.cs	Restaurant order customization (toppings, combos)
Repository	ProductRepository.cs	Generic product CRUD with soft-delete

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🚀 Quick Start

git clone https://github.com/iamahsanmehmood/csharp-design-patterns.git
cd csharp-design-patterns
dotnet run

The Program.cs entry point runs all pattern demos with formatted console output.

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🗂️ Project Structure

csharp-design-patterns/
├── src/
│   ├── Creational/
│   │   ├── Singleton/      → Thread-safe lazy initialization
│   │   ├── Factory/        → Payment processor factory
│   │   └── Builder/        → Fluent invoice builder
│   ├── Behavioral/
│   │   ├── Observer/       → Event-driven order notifications
│   │   ├── Strategy/       → Interchangeable discount algorithms
│   │   └── Command/        → Undo/redo inventory operations
│   └── Structural/
│       ├── Adapter/        → Third-party API integration
│       ├── Decorator/      → Dynamic order customization
│       └── Repository/     → Data access abstraction
├── Program.cs
├── DesignPatterns.csproj
└── README.md

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💡 Why These Examples?

Each pattern uses a real business scenario rather than abstract shapes or animals:

• 🏪 POS Systems — Strategy, Observer, and Decorator patterns mirror actual point-of-sale logic
• 💳 Payment Processing — Factory Method shows real gateway switching
• 📦 Inventory — Command pattern with real undo/redo for stock adjustments
• 📄 Invoicing — Builder pattern for constructing complex invoice documents
• 🔔 Notifications — Adapter and Observer patterns for multi-channel alerting

These examples come from 5+ years of building production software at XechTech.

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🛠️ Tech Stack

• Language: C# 12
• Framework: .NET 8
• IDE: Visual Studio 2022 / VS Code
• No external dependencies — pure .NET

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📚 Learning Resources

• Refactoring.Guru — Design Patterns
• Microsoft — .NET Design Patterns
• Head First Design Patterns

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👤 Author

Ahsan Mehmood — Full-Stack Developer & Technical Founder

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📄 License

This project is licensed under the MIT License. See LICENSE for details.