db-subclass-to-dto/src/Before/Client.cs

using System.Collections.ObjectModel;

namespace Before;

/// <summary>
/// A client, backed by the database: it derives from <see cref="DbBase"/> and
/// holds its orders as a navigation collection. In the "after" situation this
/// becomes a plain POCO (no <see cref="DbBase"/>) with a <c>ClientDto</c>
/// carrying its data to the WebApp.
/// </summary>
public class Client : DbBase
{
    /// <summary>Display name of the client.</summary>
    public string Name { get; set; } = string.Empty;

    /// <summary>
    /// Navigation collection of this client's orders. Adding to it performs
    /// EF-style navigation fix-up (see <see cref="ClientOrders"/>).
    /// </summary>
    public ClientOrders Orders { get; }

    public Client()
    {
        Orders = new ClientOrders(this);
    }
}

/// <summary>
/// An <see cref="Order"/> collection that fakes EF's navigation fix-up: when an
/// order is added, its <see cref="Order.Client"/> back-reference is set to the
/// owning client, exactly as EF would wire up the two ends of the relation.
/// </summary>
public sealed class ClientOrders : Collection<Order>
{
    private readonly Client _owner;

    public ClientOrders(Client owner)
    {
        _owner = owner;
    }

    // Both Add(...) and Insert(...) funnel through InsertItem, so overriding
    // it covers every way an order can be added to the collection.
    protected override void InsertItem(int index, Order item)
    {
        ArgumentNullException.ThrowIfNull(item);
        item.Client = _owner;   // navigation fix-up: order now knows its client
        base.InsertItem(index, item);
    }
}