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01-Introductory-Sessions/Session05/Session05 Notes.md

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+# 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
+
+````c#
+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.
+
+```c#
+class Engine { }
+class Car { Engine engine = new Engine(); }
+```
+
+#### Association (HAS-A)
+
+- Loose relationship between objects.
+- Both can exist independently.
+
+```c#
+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.
+
+```c#
+class Address { }
+class Student {
+    public Address address;
+}
+```
+
+#### Dependency
+
+- Temporary usage of one class by another.
+
+```c#
+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](https://chatgpt.com/c/68715298-27f8-8010-a29b-8f6e457f6179#) 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)