Who cares about Domain Rules?

namespace BusinessRules.Evolution_Three
{
    /// <summary>
    /// Investor Business Rules Domain object that is abstract to form the base 
    /// for all other specific business rules domain objects.
    /// </summary>
    public abstract class IncomeTaxEngine : IIncomeTaxEngine
    {
        #region Factory Method

        /// <summary>
        /// Creates the specific instance of the income tax engine domain object 
        /// dependant on the current culture.
        /// </summary>
        /// <returns>Returns a specific income tax engine domain object.</returns>
        public static IIncomeTaxEngine CreateInstance()
        {
            ICurrentCultureInfo cultureInfo = new CurrentCultureInfo();
            return CreateInstance(cultureInfo);
        }

        /// <summary>
        /// Creates the specific instance of the income tax engine domain object 
        /// dependant on the current culture.
        /// </summary>
        /// <param name="cultureInfo">An instance of the current culture info.</param>
        /// <returns>Returns a specific income tax engine domain object.</returns>
        public static IIncomeTaxEngine CreateInstance(ICurrentCultureInfo cultureInfo)
        {
            switch (cultureInfo.CurrentCultureName)
            {
                case "en-NZ":
                    return new NewZealandIncomeTaxEngine();
                case "en-AU":
                    return new AustralianIncomeTaxEngine();
                case "en-US":
                    return new UsaIncomeTaxEngine();
                default:
                    return new NullIncomeTaxEngine();
            }
        }

        #endregion

        #region Methods

        /// <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 abstract double CalculateTaxLiability(double income);

        /// <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 abstract double CalculateTaxRate(double income);

        #endregion
    }
}