What is a Factory in DDD and when do you use it?

A Factory encapsulates the complex logic of creating domain objects, especially when creation involves multiple steps or validation.

When to use:

• Complex object creation logic
• Multiple ways to create the same object
• Creation involves external dependencies
• Need to enforce creation invariants

public class OrderFactory
{
    public Order CreateOrder(Customer customer, List<OrderItemRequest> items)
    {
        ValidateCustomer(customer);
        ValidateItems(items);
        
        var order = new Order(customer.Id);
        
        foreach (var item in items)
        {
            var product = _productRepository.GetById(item.ProductId);
            order.AddItem(product, item.Quantity);
        }
        
        return order;
    }
    
    private void ValidateCustomer(Customer customer)
    {
        if (!customer.IsActive)
            throw new InvalidOperationException("Inactive customer");
    }
}