ASP.NET/IntroductorySessions/Session05/05_Notes.md

Session 5: Interfaces and Object-Oriented Relationships

📝 Overview

In this session, we explored:

• The concept and syntax of Interfaces in C#
• Implementation of interfaces in real scenarios
• Comparison of Abstract Classes vs Interfaces
• Key OOP relationships: Inheritance, Composition, Aggregation, Association, and Dependency
• Applying these relationships in a Library Management System

📚 Topics Covered

Interfaces in C#

Interfaces define contracts that classes must fulfill. They enable loose coupling and multiple inheritance in C#.

• Syntax: interface IMyInterface { void Method(); }
• Implementation: class MyClass : IMyInterface
• All members are public and abstract by default
• Cannot contain fields
• Supports multiple inheritance

Interface Example

interface IDisplayer { void Display(); }

class Test : IDisplayer {
    public void Display() => Console.WriteLine("Hello from interface!");
}

### Abstract Class vs Interface

|Feature|Abstract Class|Interface|
|---|---|---|
|Implementation Allowed?|Yes|No|
|Multiple Inheritance|No|Yes|
|Constructors|Yes|No|
|Fields|Yes|No|
|Use Case|Partial abstraction|Full abstraction|

### OOP Relationships

#### Inheritance (IS-A)

- A class inherits members from another class.

- Promotes reusability.


```c#
class Person { }
class Student : Person { }

Composition (PART-OF)

• Objects are composed of other objects.
• Strong lifecycle dependency.

class Engine { }
class Car { Engine engine = new Engine(); }

Association (HAS-A)

• Loose relationship between objects.
• Both can exist independently.

class Teacher { }
class School {
    public void AddTeacher(Teacher t) { }
}

Aggregation (WEAK-PART-OF)

• Special association where one object "owns" another but the owned object can exist independently.

class Address { }
class Student {
    public Address address;
}

Dependency

• Temporary usage of one class by another.

class Printer { void Print(string msg) => Console.WriteLine(msg); }
class Report { void Generate(Printer p) => p.Print("Report"); }

Applied Example: Library Management System

A real-world example combining all relationships:

• Person → Member/Librarian (Inheritance)
• Library → Bookshelf → Book (Composition)
• Member ↔ LibraryCard (Association)
• Member → Address (Aggregation)
• Report.Generate(Printer) (Dependency)

✔ See full code snippet here or review it during the live session.

🧪 Practice Tasks

• Implement an IVehicle interface in two different classes.
• Use composition to build a Computer with CPU, RAM, and HardDrive.
• Demonstrate aggregation with Employee and Address.
• Create an association between Doctor and Patient.
• Write a class Order that depends on PaymentProcessor.

🙏 Acknowledgments

Sources:

• StackOverflow
• GeeksforGeeks
• C# Corner
• Medium
• ChatGPT Assistance (2025 sessions)