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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 421
Public ReadOnly Property Real（） As Double
Get
Return _real
End Get
Set（ByVal value As Double）
_real = value
End Set
End Property
End Class
plexType is an immutable type that has two data members representing the real and
imaginary number parts。
The goal is to define the ＋ operator so that the following code can be piled。
Dim a As plexType = New plexType（1。0； 10。0）
Dim b As plexType = New plexType（2。0； 20。0）
Dim c As plexType = a ＋ b
Overloading the ＋ operator means to add a method that has a special notation。 The following
is the modified plexType type with the overloaded operator implemented （bolded）。
Public NotInheritable Class plexType
Private ReadOnly _imaginary As Double
Private ReadOnly _real As Double
Public Sub New（ByVal real As Double； ByVal imaginary As Double）
_real = real
_imaginary = imaginary
End Sub
Public Shared Operator ＋（ByVal a As plexType； ByVal b As plexType） _
As plexType
Return New plexType（（a。Real ＋ b。Real）； （a。Imaginary ＋ b。Imaginary））
End Operator
Public Overrides Function ToString（） As String
Return String。Concat（New Object（） ｛〃（〃； _real； 〃） （〃； _imaginary； 〃）i〃｝）
End Function
Public ReadOnly Property Imaginary（） As Double
Get
Return _imaginary
End Get
End Property
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422 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
Public ReadOnly Property Real（） As Double
Get
Return _real
End Get
End Property
End Class
The declaration of the overloaded operator is a specially defined function； which follows
these rules：
o The method is always declared as Shared in the context of the type。
o The method has a return type； which should be the type that you want to construct。 In
most cases； it is the type of the declaration （except where a Boolean is required）。
o The method identifier starts with the keyword Operator； followed by a space； and then
the operator being overloaded （＋； …； and so on）。
The GoTo Statement
The GoTo statement allows you to jump from one spot in the code to another。 In the past； when
we did not have objects； methods； and other advanced programming constructs； developers
used the GoTo statement because they had no other choice。 Currently； many in the software
industry dislike the GoTo statement。 They think that the GoTo statement is a sign of poor program
ming； and that you never need to use it。 The Channel 9 forum （http：//channel9。msdn。/
ShowPost。aspx？PageIndex=1&PostID=14652） has a good discussion on the use of GoTo statements。
In this discussion； a person who was against the GoTo statement said this：
The only possible exception would be if you are doing some sort of puter graphics
app； where I would tolerate a （for y） （for x） nesting； because the inner loop is likely to be
quite simple and the structure makes more sense as an entirety。
So； he would tolerate the GoTo statement in a specific situation； and that means that one
can’t argue that GoTo is all bad。
What I like about how Visual Basic implemented GoTo is that it is designed to solve a partic
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（）