Business rules that explain themselves.

public sealed class OrderMustHaveItemsRule : BaseBusinessRule
{
    private readonly Order _order;

    public OrderMustHaveItemsRule(Order order)
    {
        _order = order;
    }

    public override string Code => "ORDER.NO_ITEMS";

    public override string Message => "An order must contain at least one item.";

    public override bool IsBroken() => _order.Items.Count == 0;
}

IBusinessRule minimumTotalRule = BusinessRuleBuilder.Create("ORDER.BELOW_MINIMUM")
    .WithName("Minimum order total")
    .WithMessage("Order total must be at least 10.00.")
    .WithSeverity(RuleSeverity.Error)
    .WithCategory("Checkout")
    .WithTags("orders", "checkout")
    .When(() => order.Total < 10.00m)
    .Build();

minimumTotalRule.Check(); // throws BusinessRuleValidationException when broken

public sealed class OrderItemsRequiredRule : BaseBusinessRule<Order>
{
    public override string Code => "ORDER.NO_ITEMS";
    public override string Message => "Order must contain at least one item.";

    public override bool IsBroken(Order order) => order.Items.Count == 0;
}

IBusinessRule<Order> rule = new OrderItemsRequiredRule();
bool isBroken = rule.IsBroken(order);

IBusinessRule orderLimit = BusinessRuleBuilder.Create("ORDER.TOO_LARGE")
    .WithMessage("Order exceeds the customer's limit.")
    .WithCategory("Orders")
    .WithParameter("maxOrderAmount", 10_000m)
    .WhenExpression("amount > @maxOrderAmount")
    .Build();