The Visual Basic .NET Programming Language

Sometimes it is desirable to do some initialization of a type when it is created. Constructors are methods that are run when an instance of a type is created, allowing for user -defined initialization. Constructors look exactly like regular methods (discussed in the next chapter) except that they have the special name New . For example:

Class Customer Public Name As String Public Address As String Public PhoneNumber As String Public Sub New(ByVal Name As String, ByVal Address As String) MyBase.New() Me.Name = Name Me.Address = Address End Sub End Class Module Test Sub Main() Dim Bob As Customer Bob = New Customer("Bob", "134 Main Street") End Sub End Module

In this example, the Customer class defines a constructor that takes two parameters: a customer name and a customer address. It then initializes the Name and Address fields with these values. As shown in the example, arguments can be supplied in a New expression and are passed off to the constructor parameters. Like methods, constructors can be overloaded. If a class or structure doesn't define a constructor, the language will assume that the type has a constructor with no parameters that does nothing.

Unlike methods, constructors are not inherited ”each class must declare its own constructor. To ensure that base classes have a chance to run their constructors, a constructor in a derived class automatically calls its base class constructor first. If there is no parameterless base constructor, the constructor must begin with a call to another constructor in the same class ( Me.New(...) ) or to another a constructor in the base class ( MyBase.New(...) ). This ensures that each base class, all the way up to Object , has a chance to do proper initialization. For example:

Class Person Public Name As String Public Sub New(ByVal Name As String) MyBase.New() Me.Name = Name End Sub End Class Class Customer Inherits Person Public Address As String Public PhoneNumber As String Public Sub New(ByVal Name As String, ByVal Address As String) MyBase.New(Name) Me.Name = Name Me.Address = Address End Sub Public Sub New(ByVal Name As String, ByVal Address As String, _ ByVal PhoneNumber As String) Me.New(Name, Address) Me.PhoneNumber = PhoneNumber End Sub End Class

In this example, Customer provides two constructors: one that specifies a phone number and one that doesn't. The one that provides a phone number simply calls the one that doesn't to initialize the Name and Address fields before setting the PhoneNumber field.

Structure Constructors

Structures can have constructors too, even though structures never have to be allocated using New . Instead of creating a new instance on the heap, a structure constructor creates a new instance of the structure on the stack. This structure can then be assigned to a value type variable or passed as an argument to another function. For example:

Structure Point Public x, y As Integer Public Sub New(ByVal x As Integer, ByVal y As Integer) Me.x = x Me.y = y End Sub End Structure Module Test Sub PrintPoint(ByVal p As Point) Console.WriteLine(p.x & "," & p.y) End Sub Sub Main() Dim p1 As Point = New Point(10, 20) PrintPoint(New Point(20, 30)) End Sub End Module

In this example, the Point constructor simplifies the process of creating a new Point value. The first statement in Main assigns the Point value (10, 20) to the variable p1 , while the second statement passes the Point value (20, 30) to the PrintPoint subroutine.

Unlike classes, structures cannot define a parameterless constructor. In the .NET Framework, a structure's parameterless constructor represents its default value. The following example initializes a Point variable with the default value of the Point type, (0,0) .

Module Test Sub Main() Dim p1 As Point p1 = New Point() End Sub End Module

Shared Constructors

Constructors are primarily used to initialize instances of types, but they can also be used to initialize shared information in a type. A shared constructor is declared the same way as a regular constructor except that it uses the Shared modifier, can have no parameters, and cannot be declared with an access level. The last two restrictions are due to the fact that a program never explicitly calls a shared constructor ”the .NET Framework runtime will automatically call the shared constructor when necessary. For example:

Class Customer Public Prefix As String Public Name As String Shared PrintPrefix As Boolean Shared Sub New() PrintPrefix = True End Sub End Class