Write a method that calculates an invoice total from line items and a percentage discount.

• Null lines return 0.
• Quantity and unit price cannot be negative.
• Discount percent must be between 0 and 100.
• Use decimal for money-like values.

I would validate discount first, then walk the lines and validate each quantity and price. The subtotal is the sum of quantity times unit price. The discount multiplier is 1 - discountPercent / 100m. Finally I round the result to two decimals using an explicit rounding mode. I would call out that real billing systems often need more detailed tax, currency, and rounding rules.

public sealed record InvoiceLine(int Quantity, decimal UnitPrice);

public static decimal CalculateTotal(IEnumerable<InvoiceLine>? lines, decimal discountPercent)
{
    if (discountPercent < 0m || discountPercent > 100m)
    {
        throw new ArgumentOutOfRangeException(nameof(discountPercent));
    }

    if (lines is null)
    {
        return 0m;
    }

    decimal subtotal = 0m;
    foreach (var line in lines)
    {
        if (line.Quantity < 0)
        {
            throw new ArgumentException("Quantity cannot be negative.");
        }

        if (line.UnitPrice < 0m)
        {
            throw new ArgumentException("Unit price cannot be negative.");
        }

        subtotal += line.Quantity * line.UnitPrice;
    }

    var multiplier = 1m - discountPercent / 100m;
    var total = subtotal * multiplier;
    return Math.Round(total, 2, MidpointRounding.AwayFromZero);
}

The method is O(n) over invoice lines and O(1) extra space.

• Null or empty lines.
• Zero quantity.
• 100 percent discount.
• Invalid negative quantity or price.
• Rounding values such as 10.005.

• Use discount boundaries 0 and 100.
• Use decimal values that force rounding.
• Use invalid values to confirm validation is not skipped.

• Using double for money-like totals.
• Leaving rounding mode implicit.
• Applying discount per line when the rule says invoice total.
• Ignoring invalid negative values.