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generics parameter； MyType bees fixed to a certain type。 So say you declared MyType as
follows：
Dim cls As MyType = New MyType（Of Integer） （）
Now any references to GenericType within MyType will bee Integer。 This form of
generics solves many problems； as illustrated in Chapter 9。
But in the case of the IWorkbook； we don’t want a fixed type。 We want the ability to have a
collection type contain mixed types of IWorksheet。 The way to achieve that is to use generics
methods； like this：
Function Method（Of GenericType）（） As MyType（Of GenericType）
End Function
Now the generics parameter is associated with the method； rather than the type。 And
that means MyType can mix types。 So we could have different IWorksheet types in a single work
book。 And wouldn’t it be great if there were a default property with mixed types？ But you can’t
have generics default properties and properties that are not declared at the type level。
Thus； the use of a generics parameter with a default property or property will not work。 In
my opinion； that is a real design flaw in Visual Basic； because it means we need to write code
like this：
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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 301
Imports Devspace。Trader。mon
Imports System。Reflection
_
Public Interface IWorkbook
Inherits IDebug
Function GetSheet（Of BaseType）（ByVal identifier As String） _
As IWorksheet（Of BaseType）
ReadOnly Property Identifier（） As String
Default Property Item（ByVal identifier As String） As IWorksheetBase
End Interface
In this modified declaration； the method GetSheet（） acts like the Get part of the default
property； but notice where the generics parameter BaseType is declared。 The declaration
is after the method identifier and before the first bracket。 In the case of IWorkbook； we use the
method…level generics parameter to allow the caller to determine the type of the work
sheet instances。 The implementation of IWorkbook has to do nothing other than perform the
appropriate cast。 Method…level generics parameters are great when you are dealing with
mixed types； as in the case of IWorkbook。
The code to retrieve a worksheet that previously needed a cast is rewritten as follows：
Dim workbook As IWorkbook
Dim worksheet As IWorksheet（Of String） = _
workbook。GetSheet（Of String）（WorksheetIdentifiers。Configuration）
The cast has not disappeared pletely。 It is done for us in the implementation of the
GetSheet（Of BaseType）（） method； as demonstrated by the following code。
Friend Class Workbook
Implements IWorkbook； IDebug
Private _worksheets As IDictionary（Of String； IWorksheetBase） = _
New Dictionary（Of String； IWorksheetBase）（）
。 。 。
Public Function GetSheet（Of BaseType）（ByVal identifier As String） _
As IWorksheet（Of BaseType） Implements IWorkbook。GetSheet
SyncLock _worksheets
Dim retval As IWorksheet（Of BaseType） = Nothing
If _worksheets。ContainsKey（identifier） Then
retval = TryCast（_worksheets。Item（identifier）； _
IWorksheet（Of BaseType））
Else
retval = New Worksheet（Of BaseType）（identifier）
_worksheets。Add（identifier； retval）
End If
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302 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
Return retval
End SyncLock
End FunctionEnd Class
The bolded code shows that there is still a cast； but the type cast is in the method； and it
uses the generics parameter declared at the method level。
■Note The SyncLock keyword ensures that the code in this method is not executed by more than one
thread at a time。 That way； the calling code always gets back the correct sheet。
Implementing the Server Spreadsheet
Now let’s look at how the workbook and worksheet are implemented。 I will explain only the
important pieces； but all of the code is available for download from the Source/Downloads
area of the Apress web site （http：//apress。）。 The class Worksheet（Of BaseType） imple
ments the interface IWorksheet； but does not specify on which type the worksheet should be
based。 In this application； we also have a class called TraderBaseClass。 In most applications；
there is some functionality that most classes will need。 That mon functionality is what I
call a domain…specific base class。 In the case of TraderBaseClass； that is the implementation of
the IDebug interface。
Using Lambda Expressions in the Spreadsheet
The data members of Worksheet（Of BaseType） are very similar to the previously defined spread
sheet class； except the declarations are lambda…ready。 Lambda…ready means that you use the
Func（Of ） type whenever you want to declare a variable that references a lambda expression。
The following three data members are used to store the state of the cell； cell calculations that
will calculate the state of a cell； and cell calculations that calculate the state of cells for an entire
column。
Private Cells As Func（Of IWorksheet（Of BaseType）； Integer； Integer； BaseType）（；）
Private CellState As BaseType（；）
Private ColCells As