which in turn returns the lambda expression that represents the new value stored in the cell： Module CellFactories Public Function DoAdd（ByVal cell1 As Func（Of Object）； _ ByVal cell2 As Func（Of Object）） As Func（Of Object） Return Function（） CType（cell1（）； Double） ＋ CType（cell2（）； Double） End Function Public Function DoMultiply（ByVal cell1 As Func（Of Object）； _ ByVal cell2 As Func（Of Object）） As Func（Of Object） Return Function（） CType（cell1（）； Double） * CType（cell2（）； Double） End Function Public Function FixedValue（ByVal value As Object） As Func（Of Object） Return Function（） value End Function End Module The lambda expressions can be used to add two cells together； multiply two cells together； or store a fixed value。 With the lambda expressions and the spreadsheet； you have two pieces of source code that； when bined； have the ability to solve plicated problems。 The key idea that you need to take away is that the Spreadsheet class and the lambda expressions defined in CellFactories do not know about each other。 The lambda expressions could be used in a context other than a spreadsheet。 The only requirement is that the function types and signatures match。 The sample spreadsheet that would be used to add and multiply some cells together would be as follows： Dim spreadsheet As Spreadsheet = New Spreadsheet（） spreadsheet。Cells（1； 0） = CellFactories。FixedValue（10。0） spreadsheet。Cells（0； 1） = CellFactories。FixedValue（10。0） …………………………………………………………Page 313…………………………………………………………… 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 291 spreadsheet。Cells（1； 2） = _ CellFactories。DoAdd（spreadsheet。Cells（1； 0）； spreadsheet。Cells（0； 1）） spreadsheet。Cells（2； 2） = CellFactories。DoMultiply（spreadsheet。Cells（1； 2）； _ CellFactories。FixedValue（2。0）） spreadsheet。Execute（） Console。WriteLine（〃Contents of （1； 2）： 〃 & spreadsheet。State（1； 2）。ToString（）） Console。WriteLine（〃Contents of （2； 2）： 〃 & spreadsheet。State（2； 2）。ToString（）） The sample code illustrates a rudimentary example of a spreadsheet and how lambda expressions can be used。 In this chapter； the focus will be on how to create a spreadsheet implementation that is effective； mostly object…oriented； and maintainable。 Architecting a Server…Side Spreadsheet To architect a server spreadsheet； the following requirements must be met： o Performance： Wherever possible； the design should not sacrifice performance。 o Usability： The server…side spreadsheet must be easy to program from a Visual Basic perspective。 If the server…side spreadsheet is too plex or difficult to understand； it will not be used properly； potentially incurring errors。 o Maintainability： The server…side spreadsheet implementation should be somewhat maintainable。 Otherwise； bugs could creep into the code； impeding the spreadsheet’s effective use。 Not listed is the requirement for extensibility。 A spreadsheet by itself is not extensible because it implements a certain paradigm； which is a two…dimensional document of numbers and calculations。 And if you are wondering where the spreadsheet code es from； it is a subset of the actual code that I use in my own security trading system。 ■Note To code the spreadsheet to do more than it is originally designed for might be an interesting goal； but not one that is worth pursuing。 Sometimes it is best to solve a problem； and leave paradigm thinking for another time。 I have seen developers think about paradigms； not finish their code； and then have the code made obsolete by a paradigm that they did not consider。 Three projects are defined for this example： o Devspace。Trader。mon： A class library that is a distilled form of my trading library。 I decided to include such a library to give you a taste of how a production class library looks and feels。 o ServerSideSpreadsheet： A class library that represents the implementation of a server side spreadsheet。 o TestServerSideSpreadsheet： A console application that tests the ServerSideSpreadsheet project。 …………………………………………………………Page 314…………………………………………………………… 292 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 Designing the Architecture The original implementation of the spreadsheet code provides a great starting point。 Using lambda expressions to calculate the state of a cell makes it easy to create a worksheet of numbers。 What is not so great is the fact that the class SpreadSheet is a single worksheet。 Most spreadsheets applications （like Microsoft Excel） offer the ability to create multiple worksheets。 The server…side spreadsheet that we will create will consist of two concepts： workbook and worksheet； as illustrated in Figure 11…1。 Figure 11…1。 Spreadsheet design based on workbook and worksheet types The workbook is a type that acts like a collection class of the worksheet type。 The work sheet type is an individual spreadsheet of fixed dimension that is responsible for storing the state； and the cell calculations reference the individual lambda expressions。 The workbook and worksheet could be defined as interfaces or as classes。 Which would be better？ Let?