Who cares about Domain Rules?

namespace BusinessRules.Evolution_Three
{
    /// <summary>Null Object Pattern</summary>
    /// <remarks>
    /// <list type="bullet">
    /// <item><description>Provides an interface identical to Income Tax Engine Object's so that a null 
    /// object can be substituted for a real object</description></item>
    /// <item><description>Implements its interface to do nothing. What exactly it means to do 
    /// nothing depends on what sort of behaviour the client is expecting</description></item>
    /// <item><description>When there is more than one way to do nothing, more than one Null Object 
    /// class may be required</description></item>
    /// </list>
    /// </remarks>
    internal class NullIncomeTaxEngine : IncomeTaxEngine
    {
        private const double ZERO_DOUBLE_VALUE = 0.0;

        #region Methods

        /// <summary>
        /// Calculates the tax rate.
        /// </summary>
        /// <param name="income">The investor.</param>
        /// <returns>
        /// Returns a double representing the tax liability for the investor.
        /// </returns>
        public override double CalculateTaxRate(double income)
        {
            return ZERO_DOUBLE_VALUE;
        }

        /// <summary>
        /// Calculates the tax liability.
        /// </summary>
        /// <param name="income">The investor.</param>
        /// <returns>
        /// Returns a double representing the tax rate for the investor.
        /// </returns>
        public override double CalculateTaxLiability(double income)
        {
            return ZERO_DOUBLE_VALUE;
        }

        #endregion
    }
}