Who cares about Domain Rules?

using System;

namespace BusinessRules.Evolution_Five
{
    /// <summary>
    /// The Tax Band domain object symbolizes the bands found in countries tax regimes. 
    /// </summary>
    /// <remarks>
    /// <para>The tax band domain object is an immutable object and contains three properties:</para>
    /// <list type="number">
    /// <item><description><c>StartAmount</c> is the lower limit in the band.</description></item>
    /// <item><description><c>EndAmount</c> is the upper limit in the band.</description></item>
    /// <item><description><c>TaxRate</c> is the rate applied to the band.</description></item>
    /// </list>
    /// </remarks>
    public class TaxBand
    {
        #region Contants

        private const double ZeroValue = 0.0;

        #endregion

        #region Fields

        private double lowerLimitAmount;
        private double upperLimitAmount;
        private double taxRate;

        #endregion

        #region C'tors

        /// <summary>
        /// Initializes a new instance of the <see cref="TaxBand"/> class.
        /// </summary>
        /// <param name="lowerLimitAmount">The start amount.</param>
        /// <param name="upperLimitAmount">The end amount.</param>
        /// <param name="taxRate">The tax rate.</param>
        public TaxBand(double lowerLimitAmount, double upperLimitAmount, double taxRate)
        {
            this.lowerLimitAmount = lowerLimitAmount;
            this.upperLimitAmount = upperLimitAmount;
            this.taxRate = taxRate;
        }

        #endregion

        #region Methods

        /// <summary>
        /// Calculates the band tax portion.
        /// </summary>
        /// <param name="income">The investors income.</param>
        /// <returns>
        /// Returns a double representing the portion of the tax liability of this tax band.
        /// </returns>
        public double CalculateTaxPortion(double income)
        {
            if (EqualToOrLessThan(income, LowerLimitAmount))
            {
                return ZeroValue;
            }
            else if (FallWithinTaxBand(income))
            {
                return (income - lowerLimitAmount) * taxRate;
            }
            else if (EqualToOrGreaterThan(income, UpperLimitAmount))
            {
                return (upperLimitAmount - lowerLimitAmount) * taxRate;
            }
            else
            {
                return ZeroValue;
            }
        }


        /// <summary>
        /// Determines whether the income fall within the tax band.
        /// </summary>
        /// <param name="income">The income.</param>
        /// <returns>
        /// 	<c>true</c> if the income fall within the tax band; otherwise, <c>false</c>.
        /// </returns>
        private bool FallWithinTaxBand(double income)
        {
            return EqualToOrGreaterThan(income, LowerLimitAmount) && EqualToOrLessThan(income, UpperLimitAmount);
        }

        /// <summary>
        /// Income is equal to or greater than the band value.
        /// </summary>
        /// <param name="income">The income.</param>
        /// <param name="bandValue">The band value.</param>
        /// <returns>
        /// 	<c>true</c> if the income is equal to or greater than the band value; otherwise, <c>false</c>.
        /// </returns>
        private bool EqualToOrGreaterThan(double income, double bandValue)
        {
            return income >= bandValue;
        }

        /// <summary>
        /// Income is equal to or less than the band value.
        /// </summary>
        /// <param name="income">The income.</param>
        /// <param name="bandValue">The band value.</param>
        /// <returns>
        /// 	<c>true</c> if the income is equal to or less than the band value; otherwise, <c>false</c>.
        /// </returns>
        private bool EqualToOrLessThan(double income, double bandValue)
        {
            return income <= bandValue;
        }

        #endregion

        #region Properties

        /// <summary>
        /// Gets the tax rate.
        /// </summary>
        /// <value>The tax rate.</value>
        public double TaxRate
        {
            get { return taxRate; }
        }

        /// <summary>
        /// Gets the lower limit amount.
        /// </summary>
        /// <value>The lower limit amount.</value>
        public double LowerLimitAmount
        {
            get { return lowerLimitAmount; }
        }

        /// <summary>
        /// Gets the upper limit amount.
        /// </summary>
        /// <value>The upper limit amount.</value>
        public double UpperLimitAmount
        {
            get { return upperLimitAmount; }
        }

        #endregion
    }
}