taxes calculation

Taxes.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    class Program
    {
        private static TaxRate[] _entries = new TaxRate[]
        {
            new TaxRate(5070, 0.10m),
            new TaxRate(8660, 0.14m),
            new TaxRate(14070, 0.23m),
            new TaxRate(21240, 0.30m),
            new TaxRate(40230, 0.33m),
            new TaxRate(null, 0.45m),
        };

        public static void Main()
        {
            var calculator = new TaxCalculator(_entries);

            var taxes = new decimal[] { 5000, 5800, 9000, 15000, 50000 }.Select(sum => calculator.GetTaxes(sum));

            foreach (var sum in taxes)
            {
                Console.WriteLine(sum);
            }
        }
    }

    public class TaxCalculator
    {
        private TaxRate[] _policy;

        public TaxCalculator(TaxRate[] policy)
        {
            //TODO: Ensure that policy entries are ordered and the last one has open boundary value.
            _policy = policy;
        }

        public decimal GetTaxes(decimal sum)
        {
            if (sum < 0)
                throw new ArgumentException("Incorrect sum");

            decimal result = 0;
            decimal previousBound = 0;

            foreach (var entry in _policy)
            {
                bool stop;
                decimal part = CalculatePart(sum, previousBound, entry, out stop);
                
                result += part;
                if (stop) break;

                //Will not affect correctness, not relevant for the last step.
                previousBound = entry.UpperBound ?? 0;
            }

            return result;
        }

        private decimal CalculatePart(decimal sum, decimal previousBound, TaxRate entry, out bool stop)
        {
            stop = false;
            decimal boundary = entry.UpperBound.HasValue ? entry.UpperBound.Value : decimal.MaxValue;
            decimal result;
            if (sum < boundary)
            {
                result = (sum - previousBound) * entry.Rate;
                stop = true;
            }
            else
            {
                result = (boundary - previousBound) * entry.Rate;
            }

            return result;
        }
    }

    public struct TaxRate
    {
        public decimal? UpperBound;
        public decimal Rate;

        public TaxRate(decimal? boundary, decimal rate)
        {
            UpperBound = boundary;
            Rate = rate;
        }
    }
}