Library Management System — Low Level Design

Problem Statement

Build a console-driven library system that supports:

1. Adding a book (title, author, quantity)
2. Displaying available books
3. Borrowing a book
4. Returning a book
5. Quitting cleanly

Design

• ILibraryService / LibraryService — owns the Library entity and exposes AddBook, DisplayBooks, BorrowBook, ReturnBook.
• Book — title, author, quantity, and borrowed count; Available is derived (quantity - borrowed), not stored.
• Command pattern — each menu action (AddBookCommandRunner, BorrowBookCommandRunner, DisplayBookCommandRunner, ReturnBookCommandRunner, QuitCommandRunner) is its own class implementing a common CommandRunner abstraction, so Program.cs never branches on user choice directly.
• CommandRunnerFactory — maps the user's menu input to the right command runner, keeping Program.cs's loop a thin dispatch layer.
• IConsoleWrapper — abstracts console I/O behind an interface so command runners and the service are unit-testable without a real console.
• InvalidCommandException — surfaces bad menu input as a typed error instead of crashing the loop.
• Unit tests (Library_Management_Test) — one test class per command runner, plus the service itself.

Tech

C# / .NET 6

How to Run

• Rebuild the solution
• Navigate to /Library_Management/bin/debug/net6.0
• Run Library_Management.exe
• Menu: Add Book → Display Available Books → Borrow Book → Return Book → Quit