Beginning C# 2008-from Novice-to-professional - A2Z Dotnet

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178CHAPTER 7 ■ LEARNING ABOUT COMPONENTS AND OBJECT HIERARCHIES}public virtual ITaxIncome CreateIncome(double amount) {return new TaxIncome(amount, 1.0);}public abstract ITaxAccount CreateTaxAccount();The base class must implement all interface methods, regardless of whether or notthe method has an implementation. The CalculateTaxToPay(), CreateDeduction(), andCreateIncome() methods have implementations. The CreateTaxAccount() method does nothave an implementation and is declared as abstract. The methods with implementations havea virtual keyword associated with them, indicating any class derived from BaseTaxEngine canoverride the functionality if they don’t like the default functionality.In the implementation of CalculateTaxToPay(), the income (account.Income) is addedtogether and deductions (account.Deductions) are subtracted from the income. The resultingtotal is used as a query amount (account.GetTaxRate()) to retrieve the actual tax rate used tocalculate against the payable tax.■Note The implementation of CalculateTaxToPay() is a shared functionality, which implies that therecannot be any code specific to a derived type. All of the calculations and data manipulations are executedagainst an interface, making it possible to generalize operations. Whenever you implement base class methodsor shared pieces of code, you should try to keep the source code derived class agnostic.Overriding for Specialized FunctionalityIn the base class implementation, the data member _calculatedTaxable is declared as protected.As you learned in the previous chapter, this means that _calculatedTaxable can be manipulatedin a derived class. However, if you look at how the data member is used, you will see thatonly CalculateTaxToPay() assigns the data member. The purpose of the data member is toprovide information about the operation CalculateTaxToPay() without giving the exact detailsof the operation.The idea behind _calculatedTaxable and the declaration of CalculateTaxToPay() is toprovide a mechanism where the derived class does not need to calculate things again. Considerthe example of a country where, if your taxable income is above 400 currency units,a surtax of 10 currency units is calculated. You don’t know what your taxable income is untilthe function CalculateTaxToPay() is executed, and that function returns only the total payabletaxes. So how do you know if you should apply the surtax in this situation? One solution is toreverse-calculate the payable taxes, but that would involve quite a few additional steps. Aneasier solution is to write some code in the base class method of CalculateTaxToPay() thatstores the taxable income so the subclass has access to it.The original implementation of CalculateTaxToPay() does not consider a surtax, so thederived class must contain that functionality. Since CalculateTaxToPay() can be overriddenwithout the data member _calculatedTaxable, the derived class would need to implement thefunctionality in the base class to calculate whether or not the surtax applies. Following is anexample derived class implementation of the tax engine for such a situation, stored in a namespacecalled LibTax.Surtax, to distinguish it from the base functionality.
CHAPTER 7 ■ LEARNING ABOUT COMPONENTS AND OBJECT HIERARCHIES 179namespace LibTax.Surtax{internal class TaxEngine : BaseTaxEngine {public override double CalculateTaxToPay(ITaxAccount account) {double taxToPay = base.CalculateTaxToPay(account);if (_calculatedTaxable > 400) {taxToPay += 10;}return taxToPay;}public override ITaxAccount CreateTaxAccount() {throw new Exception("The method or operation is not implemented.");}}}In the implementation of CalculateTaxToPay(), we replace the virtual keyword with override,implying that the functionality of TaxEngine replaces the functionality of BaseTaxEngine.However, if TaxEngine is called, and the TaxEngine.CalculateTaxToPay() implementation isempty, then tax is not calculated. Since our fictional country calculates the basic tax similarlyto most countries, the functionality of BaseTaxEngine.CalculateTaxToPay() can be used. Thus,the first line of TaxEngine.CalculateTaxToPay() is base.CalculateTaxToPay(), meaning thebase class (BaseTaxEngine) method CalculateTaxToPay() is called.Calling the base class results in calculating a basic tax to pay amount. We need to figureout if a surtax applies, and that is where the protected data member _calculatedTaxablecomes into play. Having called BaseTaxEngine.CalculateTaxToPay(), the data member_calculatedTaxable is assigned and contains the amount that is being taxed. Thus, TaxEngine.CalculateTaxToPay() can make a decision if more than 400 currency units have been earned.And if so, then the variable taxToPay is incremented with another 10 currency units. Had_calculatedTaxable not existed, TaxEngine.CalculateTaxToPay() would have needed to callthe base class functionality to get the basic tax rate, and then recalculate the taxable monies tofigure out if the surtax applied.■Note When you override methods, you are saying that you need something special. This does not implythat the base class functionality will be called. It implies that you might call the base class functionality andperform some additional operations. Thus, when designing base class functionality, it is important to trackcalculations or operations using protected data members. The data members avoid having derived classesperform the same operations multiple times, slowing down the application and avoiding potential errors.
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The EXPERT’s VOIce ® in .NETBegi
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Some food for thought when writing
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ContentsAbout the Author . . . . .
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■CONTENTSixOrganizing the Search
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xii■CONTENTSTweaking Serializatio
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xiv■CONTENTSSelecting Data from a
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About the AuthorMany people say tha
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IntroductionThe first computer prog
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2CHAPTER 1 ■ READY, STEADY, GO!Fo
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4CHAPTER 1 ■ READY, STEADY, GO!3.
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20CHAPTER 1 ■ READY, STEADY, GO!}
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26CHAPTER 1 ■ READY, STEADY, GO!
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28CHAPTER 2 ■ LEARNING ABOUT .NET
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CHAPTER 3■ ■ ■Learning About
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CHAPTER 3 ■ LEARNING ABOUT STRING
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78CHAPTER 4 ■ LEARNING ABOUT DATA
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CHAPTER 5 ■ LEARNING ABOUT C# EXC
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CHAPTER 9 ■ LEARNING ABOUT LISTS,
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360CHAPTER 13 ■ LEARNING ABOUT MU
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CHAPTER 15 ■ LEARNING ABOUT LINQ
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CHAPTER 16■ ■ ■Writing Functi
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CHAPTER 16 ■ WRITING FUNCTIONAL C
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456CHAPTER 17 ■ LEARNING ABOUT OT
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472■INDEXincrement, 458-459modulu
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474■INDEXconfiguration files, 336
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476■INDEXdouble value, 325Double.
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478■INDEXIExample interface, 169#
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480■INDEXMonitor.Wait( ) method,
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482■INDEXusing consoles, 258-278b
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484■INDEXSQL (Structured Query La
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486■INDEXTranslateWord( ) method,
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Beginning C# 2008-from Novice-to-professional - A2Z Dotnet

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