Subscription & Billing Engine (.NET / C#)

A robust, object-oriented subscription management and billing system designed to simulate real-world SaaS billing architectures (e.g., Stripe, Spotify), featuring automated grace period enforcement and dynamic discount logic.

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Key Features & Business Rules

• Dynamic Pricing Engine: Automated annual discount calculation (e.g., applying a 15% discount for 12-month advance commitments).
• Financial Precision: Eliminates floating-point rounding errors by utilizing the 128-bit decimal data type across all monetary calculations.
• Grace Period Lifecycle:
  • 1–6 days overdue: Subscription access remains active while transitioning to PastDue status for payment retries.
  • 7+ days overdue: Automatic cancellation (Canceled) to prevent revenue leakage.
• Domain-Driven OOP Design: Strict encapsulation, strongly-typed enum state management, and separation of concerns between domain entities and business services.

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Project Structure

├── Models/
│   ├── Plan.cs                 # Subscription tier definition and base pricing
│   ├── Customer.cs             # Customer identity and contact profile
│   ├── SubscriptionStatus.cs   # Enum states (Active, PastDue, Canceled)
│   ├── Subscription.cs         # Lifecycle evaluation and grace period business logic
│   └── Invoice.cs              # Generated billing document model
├── Services/
│   └── BillingService.cs       # Calculation engine and invoice generation
└── Program.cs                  # Interactive execution demo & lifecycle simulation

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Getting Started

Prerequisites

• .NET 8.0 SDK or higher

Installation & Run

1. Clone the repository:

   git clone [https://github.com/YOUR_USERNAME/subscription-billing-engine.git](https://github.com/YOUR_USERNAME/subscription-billing-engine.git)
   cd subscription-billing-engine

2. Run the application:

   dotnet run