In object-oriented programming (OOP), relationships between classes describe how different classes interact or are connected to each other.
Think of classes like people in a family or team — they can be related in different ways (parent-child, friend, colleague, etc.).

A) Association

Definition: A general connection between two classes where one knows about the other.

Example: A Customer places an Order.

Notes: Can be one-to-one, one-to-many, or many-to-many.

public class Customer
{
    public string Name { get; set; }
    public Order Order { get; set; } // Association
}

public class Order
{
    public int OrderId { get; set; }
}

B) Aggregation (Has-A)

Definition: A special type of association where one class contains another, but the contained object can exist independently.

Example: A Team has Players, but a Player can exist without a Team.

public class Team
{
    public List<Player> Players { get; set; } // Aggregation
}

public class Player
{
    public string Name { get; set; }
}

C) Composition (Strong Has-A)

Definition: A stronger form of aggregation where the contained object cannot exist without the container.

Example: A House has Rooms — if the House is destroyed, the Rooms are gone too.

public class House
{
    public List<Room> Rooms { get; set; } = new List<Room>(); // Composition
}

public class Room
{
    public string Name { get; set; }
}

D) Inheritance (Is-A)

Definition: A relationship where one class is a specialized version of another.

Example: Car is a Vehicle.

public class Vehicle
{
    public int Speed { get; set; }
}

public class Car : Vehicle
{
    public string Brand { get; set; }
}

E) Dependency (Uses-A)

Definition: A temporary relationship where one class uses another class to perform a task.

Example: A ReportService uses a PdfGenerator.

public class ReportService
{
    public void GenerateReport(PdfGenerator generator) // Dependency
    {
        generator.Create();
    }
}

public class PdfGenerator
{
    public void Create() { }
}

Summary Table

Relationship	Example (English)	Lifespan Link?	Strong/Weak?
Association	Customer ↔ Order	No	Weak
Aggregation	Team → Player	No	Weak
Composition	House → Room	Yes	Strong
Inheritance	Car → Vehicle	N/A	Strong
Dependency	Service → Logger	No	Weak