is not the responsibility of the IWorkbook interface； but of the IWorkbook interface implementation。 ■Note The IWorkbook interface does not provide a Clear（） method to reset the workbook and delete all of the referenced worksheets。 It would seem logical to have a Clear（） method； but in a garbage…collected environment； that’s pletely unnecessary。 If you don’t want to use a workbook anymore； just don’t refer ence it； and the garbage collector will take care of the rest。 Think of it as having the option of serving dinner to your guests on real plates or paper plates。 Real plates might seem better； but they break and you need to wash them。 Paper plates are used once and thrown away。 Of course； with paper plates you have recycling issues that you don’t have in ； because the memory is recycled for you。 The property Identifier identifies the workbook represented by the current IWorkbook object。 The identifier might be a path or file name and is pletely dependent on the imple mentation of IWorkbook。 …………………………………………………………Page 321…………………………………………………………… 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 299 The default property； Item； is the primary way of getting and retrieving worksheets； where each worksheet is referenced using a string identifier。 The identifier does not need to be a string— it could have been a custom type； enumeration； or interface that is implemented。 Using a string keeps things simple； but there are maintenance issues。 Let’s say all workbooks have a configuration worksheet。 So for most of the code； the string identifier 〃configuration〃 is used。 However； a new programmer decides to use 〃Configuration〃 （with a capital C）。 This slight change will cause problems because 〃configuration〃 is meant to have a lowercase c。 Here’s the example： Dim workbook As IWorkbook Dim worksheet1 As IWorksheetBase = workbook（〃configuration〃） Dim worksheet2 As IWorksheetBase = workbook（〃Configuration〃） This approach uses a buffer that is typed in by hand and is considered hard…coded。 The preferred alternative is to hard…code a structure that is then referenced throughout the source code； like this： Public Module WorksheetIdentifiers Public Const Configuration As String = 〃configuration〃 End Module Dim workbook As IWorkbook Dim worksheet1 As IWorksheetBase = workbook（WorksheetIdentifiers。Configuration） Dim worksheet2 As IWorksheetBase = workbook（WorksheetIdentifiers。Configuration） The module WorksheetIdentifiers still contains a hard…coded string buffer； but this buffer has been centralized to a single location。 The workbook default property references the identifier within the class WorksheetIdentifiers。 Thus； if you change the class WorksheetIdentifiers； you also change the identifiers used by the default property。 That way； the chances of having a typo break an application are reduced。 Let’s get back to the IWorkbook interface and； in particular； the default property。 The default property is of the type IWorksheetBase； which is a bit tedious because the IWorksheetBase inter face is a rudimentary interface and most likely not the interface that you will use； because we have IWorksheet（Of BaseType）； and therein lies the problem。 To get an IWorksheet instance； you would need a cast； as follows： Dim workbook As IWorkbook Dim worksheet As IWorksheet（Of String） = _ TryCast（workbook（WorksheetIdentifiers。Configuration）； IWorksheet（Of String）） The bolded code is the cast you need each and every time you want to reference an IWorksheet instance。 The cast is not a big deal； but it is tedious。 I personally would prefer being able to call a property； method； or default property that returns the type I want。 Therein lies the problem； because the way I want to use a default property； property； or method is as a mixed type； and you can’t define a default property with mixed types。 You can define a default property that is a fixed type。 To understand this problem； look at the following declaration of a workbook； which does pile。 …………………………………………………………Page 322…………………………………………………………… 300 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 Imports System。Reflection Imports Devspace。Trader。mon _ Public Interface IWorkbook（Of BaseType） Inherits IDebug Default Property Item（ByVal identifier As String） As IWorksheet（Of BaseType） ReadOnly Property Identifier（） As String End Interface In this declaration of IWorkbook； a generics type is used； but then the problem is that the default property can return only IWorksheet instances of a single type； such as Double or String。 Remember that there are multiple worksheet types （as illustrated in Figure 11…2）。 What we want to do is use method…level generics declarations； like this： _ Public Interface IMixedType Default Property Item（Of BaseType）（ByVal identifier As String） _ As IWorksheet（Of BaseType） End Interface The problem with the method…level declaration is that it does not pile。 There are two ways to declare a generics parameter。 The first is what you have seen the most often； and that is at the type level： Class MyType（Of GenericType） End Class Declaring at the type level means that whenever you use the type and specify a type for the generics parameter； MyTyp