WarehouseProduct.cs

public class WarehouseProduct
{
    private readonly string _sku;
    private readonly List<IEvent> _uncommittedEvents = new();

    private int _quantityOnHand = 0;

    public WarehouseProduct(string sku)
    {
        _sku = sku;
    }

    public void ShipProduct(int quantity)
    {
        if (quantity > _quantityOnHand)
        {
            throw new InvalidDomainException("Ah... we don't have enough product to ship?");
        }

        AddEvent(new ProductShipped(_sku, quantity, DateTime.UtcNow));
    }

    private void Apply(ProductShipped evnt)
    {
        _quantityOnHand -= evnt.Quantity;
    }

    public void ReceiveProduct(int quantity)
    {
        AddEvent(new ProductReceived(_sku, quantity, DateTime.UtcNow));
    }

    private void Apply(ProductReceived evnt)
    {
        _quantityOnHand += evnt.Quantity;
    }

    public void AdjustInventory(int quantity, string reason)
    {
        if (_quantityOnHand + quantity < 0)
        {
            throw new InvalidDomainException("Cannot adjust to a negative quantity on hand.");
        }

        AddEvent(new InventoryAdjusted(_sku, quantity, reason, DateTime.UtcNow));
    }

    private void Apply(InventoryAdjusted evnt)
    {
        _quantityOnHand += evnt.Quantity;
    }

    public void ApplyEvent(IEvent evnt)
    {
        switch (evnt)
        {
            case ProductShipped shipProduct:
                Apply(shipProduct);
                break;
            case ProductReceived receiveProduct:
                Apply(receiveProduct);
                break;
            case InventoryAdjusted inventoryAdjusted:
                Apply(inventoryAdjusted);
                break;
            default:
                throw new InvalidOperationException("Unsupported Event.");
        }
    }

    private void AddEvent(IEvent evnt)
    {
        ApplyEvent(evnt);
        _uncommittedEvents.Add(evnt);
    }

    public IReadOnlyCollection<IEvent> Events() => _uncommittedEvents.AsReadOnly();
}