using the Calculate（） method。 Had the GetCellState（） method been used； the average might not have been calculated； and thus the difference calculation would have been corrupted。 But having each and every cell calculate the average whenever a small change is made is a waste of resources； since the change might not affect a cell that is recalculated。 To avoid this； built into the spreadsheet is a version…control mechanism that calculates a cell to the latest version。 Then； if another calculation is called with the same version number； the value is retrieved from the cell state。 The following is the code from Worksheet（Of BaseType） that manages the version number。 Public Function Calculate（ByVal row As Integer； ByVal col As Integer） As BaseType _ Implements IWorksheet（Of BaseType）。Calculate If （CurrVersion 》 CalculationVersion（row； col）） Then CellState（row； col） = Cells（row； col）（Me； row； col） CalculationVersion（row； col） = CurrVersion End If Return CellState（row； col） End Function …………………………………………………………Page 333…………………………………………………………… CH AP T E R 1 1 ■ L E A R N IN G AB O U T 。 N E T G E N E R I CS 311 Public Sub Calculate（） Implements IWorksheet（Of BaseType）。Calculate CurrVersion ＋= 1 Dim row As Integer For row = 0 To Cells。GetLength（0） 1 Dim col As Integer For col = 0 To Cells。GetLength（1） 1 If Cells（row； col） IsNot Nothing Then Calculate（row； col） End If Next col Next row End Sub Calling the method Calculate（） without parameters indicates a desire to recalculate the entire spreadsheet。 In the implementation of Calculate（）； the variable CurrVersion； which represents the version number of the current calculation； is incremented。 Then each cell is iter ated； and if it exists； the individual cell form of Calculate（） （Calculate（） with the parameters row and col） is called。 In the individual cell form of Calculate（）； a check is made to see if the calcu lation version number of the cell is the latest of the spreadsheet； if not； the cell’s lambda expression is called。 After the new CellState has been assigned； the cell…state version number is incre mented； and the cell state is returned。 ■Note The calculation of the spreadsheet is nothing earth…shattering； and you might be tempted to argue it is irrelevant to the scope of the book。 In fact； the calculation and its side effects are of major relevance。 With lambda expressions； you have a form of asynchronous processing； very much like a spreadsheet。 The cell calculations of the spreadsheet do not know when they will be called； and they cannot make assumptions about the state。 Thus； when a lambda expression is created； the state at the time of the lambda expression may not be the same as when the lambda expression is executed。 If you are not acutely aware of this potential pitfall； you could have some major bugs in your code。 The Important Stuff to Remember In this chapter； you learned how to use generics and also expanded your knowledge of lambda expressions。 The main items to remember are as follows： o generics code can use generics； or it can be code that provides types based on generics。 o Performance and type…safe characteristics are primary reasons for using generics。 o You can use generics at the type level or at the method level。 Using generics at the type level implies concretizing the type when the type is instantiated。 Using generics at the method level implies concretizing the type when the method is called。 o Lambda expressions may be shared state or individual state； depending on how they are declared and manipulated。 …………………………………………………………Page 334…………………………………………………………… 312 CH AP T E R 1 1 ■ L E A R N I N G A B OU T 。 N E T G E N E R I CS o For plex types； you should always implement ToString（） as a way of figuring out the state of an instance。 o Lambda expressions act asynchronously。 When you use them； remember to not make assumptions of a particular state。 Some Things for You to Do The following are some exercises that allow you to apply what you’ve learned so far。 1。 The Worksheet（Of BaseType） class always requires you to dimension the fixed…cell array ahead of time。 Change this code so that the number of rows and columns can change dynamically。 Remember that the focus is on performance； and the fastest approach is a fixed…dimension array。 2。 The average calculation knows how many items there are by the row number。 Rewrite the average calculation code so that the user of the average code does not need to deal with the plexities of adding more elements； moving the cell calculation； and so on。 3。 The methods Calculate（） and GetCellState（） seem to do the same thing。 So； is there a need to have separate methods？ Answer the question； and then make any necessary changes to the source code based on your answer。 …………………………………………………………Page 335…………………………………………………………… C H A P T E R 1 2 ■ ■ ■ Learning About Application Configuration and Dynamic Loading All of the examples in the preceding chapters have demonstrated how to use an application with a specific ponent。 You knew which type to instantiate； which interface to use； and which pro