Can you explain what the Domain layer is in application architecture?

The Domain layer in application architecture is responsible for business logic and domain rules. It is the core of the application where the main entities, their properties, behaviors, and business rules that define how these entities interact are described.

Typically, this layer contains:

• Domain models (entities)
• Services implementing business logic
• Repository interfaces for data access (without specific implementation)

The Domain layer is independent of infrastructure, databases, or UI, which allows for easy modification of external components without affecting business logic.

Example in C#:

public class Order
{
    public int Id { get; private set; }
    public List<OrderItem> Items { get; private set; }

    public void AddItem(Product product, int quantity)
    {
        // business rules for adding a product
        Items.Add(new OrderItem(product, quantity));
    }
}