Clean Architecture in .NET: A Practical Guide

namespace Shop.Domain.Orders;

public sealed class Order
{
    private readonly List<OrderLine> _lines = [];

    private Order() { } // Used by EF Core when materializing.

    public Guid Id { get; private set; }
    public Guid CustomerId { get; private set; }
    public OrderStatus Status { get; private set; }
    public DateTimeOffset CreatedAt { get; private set; }
    public IReadOnlyCollection<OrderLine> Lines => _lines.AsReadOnly();
    public decimal Total => _lines.Sum(line => line.UnitPrice * line.Quantity);

    public static Order Create(Guid customerId, DateTimeOffset now) => new()
    {
        Id = Guid.CreateVersion7(now),
        CustomerId = customerId,
        Status = OrderStatus.Draft,
        CreatedAt = now
    };

    public void AddLine(string sku, int quantity, decimal unitPrice)
    {
        EnsureDraft();
        var existing = _lines.Find(line => line.Sku == sku);
        var newQuantity = (existing?.Quantity ?? 0) + quantity;
        if (quantity < 1 || newQuantity > 100)
            throw new DomainException("Each SKU must have between 1 and 100 units.");

        if (existing is null)
            _lines.Add(new OrderLine(sku, quantity, unitPrice));
        else
            existing.Increase(quantity);
    }

    public void Submit()
    {
        EnsureDraft();
        if (_lines.Count == 0)
            throw new DomainException("An order needs at least one line.");
        Status = OrderStatus.Submitted;
    }

    private void EnsureDraft()
    {
        if (Status != OrderStatus.Draft)
            throw new DomainException($"Order {Id} is {Status} and can no longer change.");
    }
}

public sealed class OrderLine
{
    private OrderLine() { }

    internal OrderLine(string sku, int quantity, decimal unitPrice) =>
        (Sku, Quantity, UnitPrice) = (sku, quantity, unitPrice);

    public string Sku { get; private set; } = "";
    public int Quantity { get; private set; }
    public decimal UnitPrice { get; private set; }

    internal void Increase(int quantity) => Quantity += quantity;
}

public enum OrderStatus { Draft, Submitted, Shipped, Cancelled }

public sealed class DomainException(string message) : Exception(message);