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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。
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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
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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。
ToString（） method on value； an exception would be generated。 So； what the code has created
is a variable value that references nothing and is nothing。 This works because the example uses
a reference type。 Let’s see what happens when you assign a value to a value type that we
equate to no real value。
Dim value As Integer = 0
If value = 0 Then
Console。WriteLine（〃Yup a Nothing？？？〃）
End If
This time； value is not of the type Object； but is of the type Integer。 And value is assigned
a value of 0 to indicate no real value。 Now many of you will be saying； “Hey； wait a minute—
0 Nothing。” Yet； in the programming languages C＋＋ and C； Nothing and 0 have the same
contextual meaning； so it’s not so outlandish。
Visual Basic has a programming construct called the nullable type。 The nullable type solves the
problem of knowing whether or not a value type has been assigned。 This is needed when inter
acting with relational databases。 For example； there is no way to indicate that an integer value
is not assigned； since declaring a value type means declaring a stack variable； and it will have a
default value。 In other words； there is no chance a value type will have a null reference。
A nullable type in Visual Basic is used to define a value…type variable that might have a null
state。 In the value type example； the integer value was assigned a value of 0； but a value of
Nothing would have been more appropriate。 However； value types cannot be assigned a value
of Nothing； they must be assigned some valid value。
To further demonstrate the restrictions of a value type； the following is a Structure
declaration。
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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 427
Structure ExampleStructure
Public value As Integer
End Structure
Here’s how we might use it：
Dim ex As ExampleStructure = Nothing
The structure ExampleStructure is defined using the Structure keyword； which means that
ExampleStructure is a value type。 The next line； where the variable ex is assigned a value of
Nothing； will result in an action where nothing happens other than resetting the variable ex to
0。 And as was illustrated earlier； 0 is not Nothing。
The inability to assign Nothing is odd； because structures behave similarly to reference
types in many ways。 Instantiating a default value of a structure is not always the right answer；
since the data members might not have a default value。
The nullable type makes it possible for value types （for example； structures） to be nothing。
In Visual Basic； you define a nullable type by using the generics class Nullable or the
nullable type notation； as follows：
Dim ex As Nullable（Of ExampleStructure） = Nothing
or
Dim ex As ExampleStructure？ = Nothing
Using nullable types； it is possible to assign a Nothing value to a value type。 To assign a
value to a nullable type； you need to explicitly assign the variable； as in the following example。
ex = New ExampleStructure（） With ｛。value = 10｝
And then to reference the structure—in particular； the property value—use the following
code。
Console。WriteLine（〃（〃 & ex。Value。value & 〃）〃）
The additional reference of Value is the link between the reference type Nullable and the
value type ExampleStructure。
■Note Nullable types are not intended to be used in conjunction with reference types。 If you attempt to use
a nullable type with a reference type； you will get a