Limit Order Line Quantity

public class ProductAddValidationHandler : ValidationEventHandlerBase<ValidateOrderProductAdd>
{
    private readonly IProductService _productService;

    public ProductAddValidationHandler(IProductService productService)
    {
        _productService = productService;
    }

    public override void Validate(ValidateOrderProductAdd evt)
    {
        var order = evt.Order;
        var productReference = evt.ProductReference;

        var stock = _productService.GetProductStock(productReference);

        var totalQuantities = order?.OrderLines.Where(x => x.ProductReference == productReference).Sum(x => x.Quantity) ?? 0;

        if (stock.HasValue && totalQuantities >= stock.Value)
            evt.Fail($"Only {stock} quantities can be purchased for {productReference}.");
    }
}

public class OrderLineQuantityValidationHandler : ValidationEventHandlerBase<ValidateOrderLineQuantityChange>
{
    private readonly IProductService _productService;

    public OrderLineQuantityValidationHandler(IProductService productService)
    {
        _productService = productService;
    }

    public override void Validate(ValidateOrderLineQuantityChange evt)
    {
        var orderLine = evt.OrderLine;
        var productReference = orderLine.ProductReference;

        var stock = _productService.GetProductStock(productReference);

        if (stock.HasValue && evt.Quantity.To > stock.Value)
            evt.Fail($"Only {stock} quantities can be purchased for {productReference}.");
    }
}

public static class UmbracoCommerceUmbracoBuilderExtensions
{
    public static IUmbracoCommerceBuilder AddEventHandlers(IUmbracoCommerceBuilder builder)
    {
        // Register event handlers
        builder.WithValidationEvent<ValidateOrderProductAdd>()
            .RegisterHandler<ProductAddValidationHandler>();

        builder.WithValidationEvent<ValidateOrderLineQuantityChange>()
            .RegisterHandler<OrderLineQuantityValidationHandler>();

        return builder;
    }
}