End Sub End Class Having one class subclass the other is fine； and it works。 The class BaseGenerated could be stored in one file； and Example could be stored in another file。 But that approach can be a bit problematic； as it implies a specific architecture where one class subclasses the other。 Another approach would be to use the Partial keyword in the context of a class； as follows： Class Example Const BaseValue As Integer = 10 End Class Partial Class Example Public Sub AddNumbers（ByVal value1 As Integer； _ ByVal value2 As Integer； _ ByRef response As Integer） response = BaseValue ＋ value1 ＋ value2 End Sub End Class When you use the Partial keyword； you are defining a partial class。 The plete class is the sum of all the separate partial class definitions for the class （which could； and typically would； reside in separate files）。 Thus； one part of the class could be autogenerated； and the other piece could be hand…coded。 …………………………………………………………Page 451…………………………………………………………… CH AP T E R 1 6 ■ L E A R N I N G A B OU T O TH E R V IS U AL B A SI C T E C HN IQ U E S 429 A requirement of partial classes is that they must be wholly defined in an assembly。 The partial class is of relevance only to the Visual Basic piler； and not to the language。 Another usage of the Partial keyword is to have methods that are defined in one place but implemented in another。 Think of partial methods as defining an abstract class method and implementing it within the same class。 The following is an example of using a partial method。 Class Example Partial Private Sub AddNumbers（ByVal value1 As Integer； _ ByVal value2 As Integer； _ ByRef response As Integer） End Sub Public Sub Method（） Dim response As Integer = 0 AddNumbers（1； 2； response） Console。WriteLine（〃Added numbers （〃 & response & 〃）〃） End Sub End Class Partial Class Example Private Sub AddNumbers（ByVal value1 As Integer； _ ByVal value2 As Integer； _ ByRef response As Integer） response = value1 ＋ value2 End Sub End Class Partial methods are defined in the context of a partial class。 In one part of your class； you would prefix the identifier Partial to the method and declare it。 In the other part of the class； you would implement the method。 Partial methods； like partial classes； have some restrictions： o They must be declared as a Sub。 o They must be declared to be Private。 o The methods can be shared and use extension methods。 Overall； partial methods and classes should not be used as a general programming practice； because they are intended to be employed in the context of autogenerated code （such as Form1。vb in Windows forms）。 When used in that context； partial methods and classes are an excellent solution。 Partial methods are also useful substitutes for conditional pilation constants； because if you don’t implement a partial method； any mention of it is removed from the piled class。 This means； for example； that you can wrap the implementation with a single #If； rather than littering many instances of #If in your code。 Here’s an example： …………………………………………………………Page 452…………………………………………………………… 430 CH AP T E R 1 6 ■ L E A R N I N G A B OU T O TH E R V IS U AL B A SI C TE C H N IQ U E S Partial Private Sub Display（ByVal message As String） End Sub #If DEBUG Then Private Sub Display（ByVal message As String） Console。WriteLine（message） End Sub #End If Sub RunAll（） RunFirst（） Display（〃Done RunFirst（）〃） RunSecond（） Display（〃Done RunSecond（）〃） RunThird（） Display（〃Done RunThird（）〃） End Sub The Important Stuff to Remember In this chapter； you learned about some Visual Basic techniques that are useful in specific situ ations。 Here are the points you should keep in mind： o You will need to use the arithmetic operators。 Some are obvious； others are not。 Take the time to get used to coding with them。 o Overloading operators makes sense when you need to define custom types and you want them to support basic operators such as add； subtract； and so on。 When imple menting the operators； be careful to do a logically correct implementation。 o To be able to use methods or properties on generics parameters； they need to be constrained。 Constraints are an effective way of indicating basic functionality possessed by generics types。 Even though you could usually achieve the same results using interfaces； that approach is not as elegant。 o Visual Basic has the GoTo keyword； even though some consider the use of GoTo as poor programming。 Visual Basic has ensured that you can’t easily shoot yourself in the foot。 o The nullable type is used to indicate whether or not a value type has been assigned。 You will use this type when interacting wi