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ular problem； but not raise the old problems of spaghetti code making a mess of things。 So if
you need to use a GoTo statement； go ahead； just don’t use it excessively。
One example where it is not easily possible to avoid using a GoTo statement is in the
following pseudo…code。
Do While FirstActionLoop（）
Do While SecondActionLoop（）
If BreakOutOfLoops（） Then
GoTo EXIT_ALL
End If
Loop
Loop
EXIT_ALL：
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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 423
This code has two loops。 If the code is executing the second loop and decides to end pro
cessing； then exiting the loop bees a bit tricky； as you can break execution only one loop at a
time using the Exit Do statement。 Thus； one solution is to use the GoTo statement； as shown。
The GoTo statement is associated with an identifier that represents a label。 The label can be
placed almost anywhere in the declared method。 An exception is that you can’t place a label in
a Select statement。 However； you can place the label before or after the GoTo keyword。
Generics Constraints
generics have been covered in several chapters。 An additional aspect of generics is
constraints； which can be optionally used to optimize programming。 Constraints limit the types
that the parameter can be。
Three types of constraints are Class； New； and a specific type。 As a general rule； a constraint
is added in the form of an As statement； as follows：
Class Example（Of DataType As ｛New｝）
End Class
This example limits DataType to types that have a default constructor。
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：
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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。
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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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