Who cares about Domain Rules?

namespace BusinessRules.Evolution_Three
{
    /// <summary>
    /// Investor Domain object.
    /// </summary>
    public class Investor : IInvestor
    {
        #region Fields

        private double income;
        private IIncomeTaxEngine taxEngine = IncomeTaxEngine.CreateInstance();

        #endregion

        #region C'tors

        /// <summary>
        /// Initializes a new instance of the <see cref="Investor"/> class.
        /// </summary>
        /// <param name="income">The income.</param>
        public Investor(double income)
        {
            this.income = income;
        }

        #endregion

        #region Properties

        /// <summary>
        /// Gets or sets the income.
        /// </summary>
        /// <value>The income.</value>
        public double Income
        {
            get { return income; }
            set { income = value; }
        }

        /// <summary>
        /// Gets the tax liability.
        /// </summary>
        /// <value>The tax liability.</value>
        public double TaxLiability
        {
            get { return taxEngine.CalculateTaxLiability(Income); }
        }

        /// <summary>
        /// Gets the tax rate.
        /// </summary>
        /// <value>The tax rate.</value>
        public double TaxRate
        {
            get { return taxEngine.CalculateTaxRate(Income); }
        }

        /// <summary>
        /// Gets or sets the business rule.
        /// </summary>
        /// <remarks>The Get is only valid within the Investor Domain object.</remarks>
        public IIncomeTaxEngine TaxEngine
        {
            set { taxEngine = value; }
        }

        #endregion
    }
}