What causes "Discount is not valid" in dotnet/eShop?

Validate discount >= 0 at the call site before invoking SetNewDiscount; reject negative values earlier.

dotnet/eShop · error · OrderingDomainException

Discount is not valid

Error message

Discount is not valid

What it means

Thrown by OrderItem.SetNewDiscount when discount is less than zero. A negative discount is meaningless (it would raise the price), so the method rejects it as an invariant violation. It is an OrderingDomainException. Note this guard checks negativity only — a discount larger than the line total is not caught here.

Source

Thrown at src/Ordering.Domain/AggregatesModel/OrderAggregate/OrderItem.cs:48

        if ((unitPrice * units) < discount)
        {
            throw new OrderingDomainException("The total of order item is lower than applied discount");
        }

        ProductId = productId;

        ProductName = productName;
        UnitPrice = unitPrice;
        Discount = discount;
        Units = units;
        PictureUrl = pictureUrl;
    }
    
    public void SetNewDiscount(decimal discount)
    {
        if (discount < 0)
        {
            throw new OrderingDomainException("Discount is not valid");
        }

        Discount = discount;
    }

    public void AddUnits(int units)
    {
        if (units < 0)
        {
            throw new OrderingDomainException("Invalid units");
        }

        Units += units;
    }
}

View on GitHub (pinned to 9b4f9434f4)

Solutions

Validate discount >= 0 at the call site before invoking SetNewDiscount; reject negative values earlier.
Fix the upstream discount computation so it never yields a negative figure.
If a percentage model is used, compute absolute = subtotal * pct/100 which is inherently non-negative for non-negative inputs.
Add a guard in the command/handler: if (request.Discount < 0) return BadRequest(...).

Example fix

// before
item.SetNewDiscount(computedDiscount);

// after
if (computedDiscount < 0) throw new ArgumentException("Discount cannot be negative");
item.SetNewDiscount(computedDiscount);

Defensive patterns

Strategy: validation

Validate before calling

if (discount < 0) throw new ArgumentOutOfRangeException(nameof(discount));
item.SetNewDiscount(discount);

Type guard

static bool IsValidDiscount(decimal discount) => discount >= 0;

Try / catch

try {
    item.SetNewDiscount(discount);
} catch (OrderingDomainException ex) when (ex.Message == "Discount is not valid") {
    // negative discount upstream — fix the promo calculation, do not retry as-is
}

Prevention

Compute discounts from non-negative inputs (e.g. percentage model).
Guard discount >= 0 at the handler boundary.
Add unit tests covering discount == 0 and discount == subtotal.

When it happens

Trigger: Calling existingOrderForProduct.SetNewDiscount(discount) (reached via Order.AddOrderItem when a higher discount is supplied for an existing line, or directly) with a negative discount value. Usually a sign-inversion or arithmetic bug computing the discount.

Common situations: A promo calculator returning a negative amount; subtraction producing a negative remainder; client sending a signed discount field; tests passing -1 as a sentinel.

Related errors

AI-assisted analysis of dotnet/eShop@9b4f9434f4 (2026-08-13). Data as JSON: /api/errors/80b445c338973774. Report an issue: GitHub.