Association, aggregation and composition — OOP Concepts in C#

12. Association, aggregation and composition

Intermediate★ Must knowLesson 12 of 15

Relationship	Meaning	Lifetime	Example
Association	"uses" / knows about	Independent	`Teacher` ↔ `Student`
Aggregation	"has-a", weak ownership	Part can exist without whole	`Department` has `Employees`
Composition	"owns-a", strong ownership	Part dies with whole	`Order` owns `OrderLines`; `House` owns `Rooms`
Inheritance	"is-a"	—	`Car` is a `Vehicle`

csharp

public class Order               // composition: lines are created and owned by the order
{
    private readonly List<OrderLine> _lines = [];
    public void Add(string sku, int qty) => _lines.Add(new OrderLine(sku, qty));
}

public class Department          // aggregation: employees exist independently
{
    public List<Employee> Members { get; } = [];
}