…………………………………………………………Page 327…………………………………………………………… 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 305 Public Interface IWorksheetSerialize Sub AssignCellState（Of ValueType）（ByVal row As Integer； _ ByVal col As Integer； ByVal value As ValueType） Sub AssignCellState（ByVal row As Integer； ByVal col As Integer； _ ByVal value As Object） End Interface The problem is that the function passes in as a parameter of one type and is assigned to CellState； which is another undefined generics type。 The implementation of the AssignCellState（） method is as follows： Public Sub AssignCellState（ByVal row As Integer； ByVal col As Integer； _ ByVal value As Object） Implements IWorksheetSerialize。AssignCellState CellState（row； col） = DirectCast（value； BaseType） End Sub The code looks so simple and innocent； but it is actually masking many potential problems。 Here； the value of the parameter value is of type Object。 Then to convert the value to type BaseType—because that is what CellState is defined to be—you use a cast。 For the most part； this code will work； if value is the correct type。 Consider the following code； which would generate an exception。 Dim worksheet As Worksheet（Of Double） = New Worksheet（Of Double）（） worksheet。Dimension（10； 10）Dim buffer As String = 〃hello； world〃 worksheet。AssignCellState（1； 2； buffer） The variable worksheet is declared to be of type Worksheet（Of Double）。 When the method AssignCellState（） is called； the cell 1； 2 is assigned to be type String。 Calling AssignCellState（） is not a problem； but in the implementation； the assignment will fail。 You can’t willy…nilly assign a String to a Double。 Of course； this begs the question； “Why have a function of type Object？” Sometimes you can’t get around it； and you need to write a general object method。 The proper way to call the method would be as follows： Dim worksheet As Worksheet（Of Double） = New Worksheet（Of Double）（） worksheet。Dimension（10； 10） Dim buffer As String = 〃hello； world〃 worksheet。AssignCellState（1； 2； Double。Parse（buffer）） The bolded code shows that the string buffer is parsed by the Double。Parse（） method； converting a string buffer into a Double value。 Of course； the conversion will fail； since the string buffer represents a string； but that is another problem。 Another way of solving the problem is to avoid the object altogether and declare the method as being a generics method。 The advantage of the generics method is that you could execute in a type…safe manner without explicitly forcing the user to implement parsing routines。 Consider the following modified generics method declaration of AssignCellState（）。 …………………………………………………………Page 328…………………………………………………………… 306 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 Public Sub AssignCellState（Of ValueType）（ByVal row As Integer； _ ByVal col As Integer； _ ByVal value As ValueType） _ Implements IWorksheetSerialize。AssignCellState If GetType（BaseType）。IsAssignableFrom（GetType（ValueType）） Then CellState（row； col） = DirectCast（DirectCast（value； Object）； BaseType） ElseIf TypeOf （value） Is String And _ GetType（Double）。IsAssignableFrom（GetType（BaseType）） Then Dim obj As Object = DirectCast（value； Object） Dim dValue As Double = Double。Parse（CStr（obj）） Dim objDValue As Object = CType（dValue； Object） CellState（row； col） = DirectCast（objDValue； BaseType） Else Throw New InvalidCastException（〃Could not perform conversion〃） End If End Sub The cell state to be assigned is a generics parameter and defined to be the type ValueType。 You can only guess what ValueType is； it is determined when the method AssignCellState（Of ）（） is called。 For example； suppose this method call is made： Dim buffer As String = 〃hello； world〃 worksheet。AssignCellState（1； 2； buffer） The type for ValueType will be String； even though you have not explicitly specified it。 One of the things that is possible with generics methods is that types can be deduced implic itly。 The following would be an explicit usage of AssignCellState（Of ）（）。 Dim buffer As String = 〃hello； world〃 worksheet。AssignCellState（Of String）（1； 2； buffer） Knowing that ValueType is String； AssignCellState（Of ）（） will then first check if ValueType can be assigned to BaseType： If GetType（BaseType）。IsAssignableFrom（GetType（ValueType）） Then This code is rather clever； because it uses what is known as reflection to determine if one type can be assigned to another type。 Essentially； it asks if it is OK via a cast to assign value to CellState。 You could try to do this without the If statement； but then you risk an unnecessary exception。 If it is permissible to assign； then the assignment is done via a two…step casting： CellState（row； col） = DirectCast（DirectCast（value； Object）； BaseType） Here； you first convert the type to an Object； and then convert the type to BaseType； which happens to be the type that the spreadsheet is declared as。 It is absolutely imperative that the cast to the object is added； otherwise； the Visual Basic piler will plain that the cast is not possible。 But suppose tha