…………………………………………………………Page 443…………………………………………………………… 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 …………………………………………………………Page 444…………………………………………………………… 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： …………………………………………………………Page 445…………………………………………………………… 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 ty