Using the Type Constraint Type constraints allow you to associate a minimal type with the generics parameter。 For example； suppose this interface is defined： Interface IExample Sub Method（） End Interface Adding an IExample constraint to the generics parameter allows you to define a class as follows： Class ExampleMgr（Of DataType As ｛ IExample｝） Private _inst As DataType Public Sub New（ByVal inst As DataType） _inst = inst End Sub Public Sub DoSomething（） _inst。Method（） End Sub End Class In the example； the constraint of IExample allows a developer to call Method（）。 If the constraint were not there； the reference to Method would generate a piler error； because Method（） is not a method of DataType。 But is this ability to reference a method an advantage？ After all； you could write the ExampleMgr code without using generics； like this： …………………………………………………………Page 446…………………………………………………………… 424 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 Class ExampleMgr Dim _inst As IExample Public Sub New（ByVal inst As IExample） Me。_inst = inst End Sub Public Sub DoSomething（） _inst。Method（） End Sub End Class The generics code and the interface…based code do the exact same thing； and using generics offers no advantage in this example。 But that is not always the case。 Consider the following modified example of ExampleMgr。 Class ExampleMgr（Of DataType As ｛ IExample ｝） Dim _inst As DataType Public Sub New（ByVal inst As IExample） Me。_inst = CType（inst； DataType） End Sub Public Sub DoSomething（） _inst。Method（） End Sub Public ReadOnly Property Inst（） As DataType Get Return _inst End Get End Property End Class With generics； you can write a property that contains a reference to the base type。 Had you used the pure interface…based code； the Inst property would need to be of type IExample； and to get the same effect as the generics example； a cast would be involved。 A cast is not the end of the world； but it is inconvenient and not type…safe； since you don’t know if a cast will work until you execute the code。 You can add multiple interfaces and classes as constraints； although you are limited。 The limitations are identical to the limitations when subclassing a class or interface： o You can subclass only a single class。 o You can subclass as many interfaces as desired； but they must be declared after the class。 …………………………………………………………Page 447…………………………………………………………… 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 425 Using the New Constraint The purpose of the New constraint is to limit the data type to one that has a default constructor； as in this example： Class Example（Of DataType As ｛New｝） Private _value As DataType Public Sub New（） _value = New DataType（） End Sub End Class Without the New constraint； the bolded code cannot be piled。 The constructor that is defined with the New constraint does not take parameters； and it would seem that this is a bother。 After all； you might want to instantiate the type with some state。 What you can do is constrain DataType； and then instantiate the type using object initial izers。 For example； the following is an interface that has a single property。 Interface IBase Property Value（） As Integer End Interface bining the IBase interface and the New keyword as constraints； you could write the following code。 Class Example（Of DataType As ｛IBase； New｝） Private _value As DataType Public Sub New（） _value = New DataType（） With ｛。Value = 10｝ End Sub End Class In the modified example； the bolded code instantiates DataType； and then using the constraints； the object initializer that defines _value is possible。 Using the Class Constraint The main purpose of the Class constraint is to indicate whether the generics type is a value type or a reference type。 Here is an example of a type that wants to manage only reference types： Class AssumeReferenceType（Of DataType As ｛Class｝） Private _value As DataType Public Sub New（ByVal value As DataType） End Sub End Class …………………………………………………………Page 448…………………………………………………………… 426 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 If you were to declare the AssumeReferenceType using an Integer； as shown in the following example； the code would not pile。 Dim cls As AssumeReferenceType（Of Integer） = _ New AssumeReferenceType（Of Integer）（Nothing） Nullable Types In ； Nothing means no value； and we all understand what it represents； right？ For example； the following code shows how to use Nothing。 Dim value As Object = Nothing If value Is Nothing Then Console。WriteLine（〃Yup a Nothing〃） End If The example is obvious and does nothing useful。 If you attempted to call the Object。 ToSt