Expression.Sum C# (CSharp) Method

    private static double Sum(string expression, ref int index)
    {
        double result = Product(expression, ref index);

        while(index < expression.Length)
        {
            if(expression[index] == '+')
            {
                ++index;
                result += Product(expression, ref index);
            }
            else if(expression[index] == '-')
            {
                ++index;
                result -= Product(expression, ref index);
            }
            else
            {
                break;
            }
        }

        return result;
    }

        public Constraints5ConstraintElement(
            IaIndexElement aIndexElement,
            IsIndexElement sIndexElement,
            Ip p,
            Idur dur,
            ILENGTH LENGTH,
            IP P,
            ISPx x,
            ISPz z)
        {
            Expression LHS = Expression.Sum(
                p.Value
                .Where(
                    i =>
                    P.IsThereElementAt(
                        sIndexElement,
                        i))
                .Select(
                    i =>
                    (double)dur.GetElementAtAsdecimal(
                        i)
                    *
                    x.Value[i, aIndexElement]));

            Expression RHS =
                z.Value[sIndexElement, aIndexElement]
                *
                (double)LENGTH.Value.Value.Value;

            this.Value = LHS <= RHS;
        }