fewer than a dozen variations。 The better approach is to add an interface specific to the Canadian tax system。 Think of it as follows。 When you are instantiating the tax engine； you will need to know which tax system to use。 The factory protects you from needing to know which type to instantiate； but there is nothing wrong with giving some extra information that could be used by a factory。 Thus； the solution is to define a new interface called ICanadaTaxEngine。 The purpose of ICanadaTaxEngine is to add factory methods that are used to instantiate types with parameters specific to the implementation。 Following is the definition of the ICanadaTaxEngine。 Public Enum Province Alberta BritishColumbia Manitoba NewBrunswick NewfoundlandLabrador NovaScotia Nunavut Ontario PrinceEdwardIsland Quebec Saskatchewan Yukon End Enum Public Interface ICanadaTaxEngine Function CreateTaxAccount（ByVal province As Province； _ ByVal year As Integer） As ITaxAccount Function CreateCapitalGain（ByVal amount As Double） As ITaxIne End Interface …………………………………………………………Page 209…………………………………………………………… CH AP T E R 7 ■ L E AR N IN G AB O U T CO M P O N E N TS AN D C L AS S H I E R AR C HI E S 187 The definition of ICanadaTaxEngine contains two additional methods： o CreateTaxAccount（） is used to instantiate a tax account specific to a province and year。 o CreateCapitalGain（） is used to instantiate an ine using the Canadian capital gains calculation。 Thus； the implementation of TaxEngine bees the following。 Friend Class TaxEngine Inherits BaseTaxEngine Implements ICanadaTaxEngine Public Overrides Function CreateTaxAccount（） As ITaxAccount Return New TaxAccount（Province。Ontario； 2008） End Function Public Overloads Function CreateTaxAccount（ByVal province As Province； _ ByVal year As Integer） _ As ITaxAccount _ Implements ICanadaTaxEngine。CreateTaxAccount Return New TaxAccount（province； year） End Function Public Function CreateCapitalGain（ByVal amount As Double） As ITaxIne _ Implements ICanadaTaxEngine。CreateCapitalGain Return New TaxIne（amount； 0。50） End Function End Class In the modified implementation of TaxEngine； the class still derives from BaseTaxEngine； fulfilling the requirement of being a general tax engine。 However； for the additional require ments of the Canadian tax system； we implement the ICanadaTaxEngine interface。 Defining a specific interface that implies a certain implementation is fine； because the specific interface is not bound to a certain implementation。 The better way to understand this implementation technique is to consider the specific interface as a characteristic that an implementation is capable of supporting。 This goes back to the shape example； where a square can support both the ISquare and IRectangle interfaces。 Using the Tax Engine The last step is to use the tax engine to calculate some taxes。 The following is an example of calculating the taxes for Ontario in 2008。 Dim engine As ITaxEngine = EngineCreator。CreateCanadianTaxEngine（） Dim canadaEngine As ICanadaTaxEngine = TryCast（engine； ICanadaTaxEngine） Dim account As ITaxAccount = canadaEngine。CreateTaxAccount（Province。Ontario； 2008） Dim ine As ITaxIne = engine。CreateIne（100） Dim capitalGain As ITaxIne = canadaEngine。CreateCapitalGain（100） …………………………………………………………Page 210…………………………………………………………… 188 CH AP T E R 7 ■ L E A R N IN G AB OU T CO M P O N E N TS AN D C L AS S H I E R AR C H IE S account。AddIne（ine） account。AddIne（capitalGain） Dim deduction As ITaxDeduction = engine。CreateDeduction（20） account。AddDeduction（deduction） Dim taxToPay As Double = engine。CalculateTaxToPay（account） Console。WriteLine（〃Tax to pay （〃 & taxToPay & 〃）〃） Notice the definition of the variables engine and canadaEngine。 This is fine； because what we are doing with the interfaces is choosing a characteristic that can be dynamically queried。 Learning More About Inheritance and Type Casting This chapter introduced interfaces and ponents； and delved deeper into inheritance。 Here； I’ll provide more details about inheritance and type casting。 More Inheritance Details In this section； I am going to clearly lay out how inheritance works in Visual Basic。 I will present examples of usage for six scenarios。 Each example is followed by a list explaining the key points to understand。 My goal is to illustrate all of the possible scenarios so that you have a reference to how inheritance works。 ■Note All of the examples demonstrate using methods； but it is possible to apply the same inheritance techniques using properties。 Scenario 1： Overloading Base Class Functionality Class Base Public Sub Method（） Console。WriteLine（〃Base。Method〃） End Sub End Class Class Derived Inherits Base Public Overloads Sub Method（） Conso