# Aggregate Root

The single entry object for a group of related objects, guarding their rules.

## What is **Aggregate Root**?

An **aggregate root** is the one object in a group of related objects that outside code must go through. To add a line to an order, you call `order.AddLine(...)`, never edit the lines directly. That keeps the group's rules intact.

```csharp
public class Order
{
    private readonly List<OrderLine> lines = new();

    public void AddLine(string product, int quantity)
    {
        if (lines.Count >= 50) throw new InvalidOperationException("Order is full");
        lines.Add(new OrderLine(product, quantity));
    }
}
```

Level: Master · [More OOP terms at this level](https://www.ask-codey.com/glossary/subjects/oop/master)

Updated: September 27, 2026

Source: https://www.ask-codey.com/glossary/aggregate-root