Using Constructors and Destructors
Constructors and destructors control the creation and destruction of objects.
Constructors
To create a constructor for a class, create a procedure named Sub New anywhere in the class definition. To create a parameterized constructor, specify the names and data types of arguments to Sub New just as you would specify arguments for any other procedure, as in the following code:
Sub New(ByVal s As String)
Constructors are frequently overloaded, as in the following code:
Sub New(ByVal s As String, i As Integer)
When you define a class derived from another class, the first line of a constructor must be a call to the constructor of the base class, unless the base class has an accessible constructor that takes no parameters. A call to the base class that contains the above constructor, for example, would be MyBase.New(s). Otherwise, MyBase.New is optional, and the Visual Basic runtime calls it implicitly.
After you write the code to call the parent object's constructor, you can add any additional initialization code to the Sub New procedure. Sub New can accept arguments when called as a parameterized constructor. These parameters are passed from the procedure calling the constructor, for example, Dim AnObject As New ThisClass(X).
Destructors
The following code shows how to use Dispose and Finalize to release resources in a base class.
Note
You should follow the guidelines for implementing IDisposable set out in Object Lifetime: How Objects Are Created and Destroyed.
' Design pattern for a base class.
Public Class Base
Implements IDisposable
' Keep track of when the object is disposed.
Protected disposed As Boolean = False
' This method disposes the base object's resources.
Protected Overridable Sub Dispose(ByVal disposing As Boolean)
If Not Me.disposed Then
If disposing Then
' Insert code to free managed resources.
End If
' Insert code to free unmanaged resources.
End If
Me.disposed = True
End Sub
#Region " IDisposable Support "
' Do not change or add Overridable to these methods.
' Put cleanup code in Dispose(ByVal disposing As Boolean).
Public Sub Dispose() Implements IDisposable.Dispose
Dispose(True)
GC.SuppressFinalize(Me)
End Sub
Protected Overrides Sub Finalize()
Dispose(False)
MyBase.Finalize()
End Sub
#End Region
End Class
The following code shows how to use Dispose and Finalize to release resources in a derived class.
' Design pattern for a derived class.
Public Class Derived
Inherits Base
' This method disposes the derived object's resources.
Protected Overrides Sub Dispose(ByVal disposing As Boolean)
If Not Me.disposed Then
If disposing Then
' Insert code to free managed resources.
End If
' Insert code to free unmanaged resources.
End If
MyBase.Dispose(disposing)
End Sub
' The derived class does not have a Finalize method
' or a Dispose method with parameters because it inherits
' them from the base class.
End Class
The following code shows a common design pattern for the Dispose destructor, using a Using block and an equivalent Try...Finally block.
Sub DemonstrateUsing()
Using d As New Derived
' Code to use the Derived object goes here.
End Using
End Sub
Sub DemonstrateTry()
Dim d As Derived = Nothing
Try
d = New Derived
' Code to use the Derived object goes here.
Finally
' Call the Dispose method when done, even if there is an exception.
If Not d Is Nothing Then
d.Dispose()
End If
End Try
End Sub
The next example creates an object by using a parameterized constructor, and then calls destructors when the object is no longer needed.
Note
Although this example uses Collect to demonstrate which methods the garbage collector calls to dispose methods, in general you should let the common language runtime (CLR) manage garbage collection.
Sub TestConstructorsAndDestructors()
' Demonstrate how the Using statement calls the Dispose method.
Using AnObject As New ThisClass(6)
' Place statements here that use the object.
MsgBox("The value of ThisProperty after being initialized " & _
" by the constructor is " & AnObject.ThisProperty & ".")
End Using
' Demonstrate how the garbage collector calls the Finalize method.
Dim AnObject2 As New ThisClass(6)
AnObject2 = Nothing
GC.Collect()
End Sub
Public Class BaseClass
Sub New()
MsgBox("BaseClass is initializing with Sub New.")
End Sub
Protected Overrides Sub Finalize()
MsgBox("BaseClass is shutting down with Sub Finalize.")
' Place final cleanup tasks here.
MyBase.Finalize()
End Sub
End Class
Public Class ThisClass
Inherits BaseClass
Implements IDisposable
Sub New(ByVal SomeValue As Integer)
' Call MyBase.New if this is a derived class.
MyBase.New()
MsgBox("ThisClass is initializing with Sub New.")
' Place initialization statements here.
ThisPropertyValue = SomeValue
End Sub
Private ThisPropertyValue As Integer
Property ThisProperty() As Integer
Get
CheckIfDisposed()
ThisProperty = ThisPropertyValue
End Get
Set(ByVal Value As Integer)
CheckIfDisposed()
ThisPropertyValue = Value
End Set
End Property
Protected Overrides Sub Finalize()
MsgBox("ThisClass is shutting down with Sub Finalize.")
Dispose(False)
End Sub
' Do not add Overridable to this method.
Public Overloads Sub Dispose() Implements IDisposable.Dispose
MsgBox("ThisClass is shutting down with Sub Dispose.")
Dispose(True)
GC.SuppressFinalize(Me)
End Sub
Private disposed As Boolean = False
Public Sub CheckIfDisposed()
If Me.disposed Then
Throw New ObjectDisposedException(Me.GetType().ToString, _
"This object has been disposed.")
End If
End Sub
Protected Overridable Overloads Sub Dispose( _
ByVal disposing As Boolean)
MsgBox("ThisClass is shutting down with the Sub Dispose overload.")
' Place final cleanup tasks here.
If Not Me.disposed Then
If disposing Then
' Dispose of any managed resources.
End If
' Dispose of any unmanaged resource.
' Call MyBase.Finalize if this is a derived class,
' and the base class does not implement Dispose.
MyBase.Finalize()
End If
Me.disposed = True
End Sub
End Class
When you run this example, the class ThisClass calls the Sub New constructor of the class BaseClass. After the constructor in the base class is finished, the class ThisClass runs the remaining statements in Sub New that initialize a value for the property ThisProperty.
When the class is no longer needed, the Dispose destructor is called in ThisClass.
This example displays the following:
BaseClass is initializing with Sub New.
ThisClass is initializing with Sub New.
The value of ThisProperty after being initialized by the constructor is 6.
ThisClass is shutting down with Sub Dispose.
ThisClass is shutting down with the Sub Dispose overload.
BaseClass is shutting down with Sub Finalize.
BaseClass is initializing with Sub New.
ThisClass is initializing with Sub New.
ThisClass is shutting down with Sub Finalize.
ThisClass is shutting down with the Sub Dispose overload.
BaseClass is shutting down with Sub Finalize.
See Also
Concepts
Object Lifetime: How Objects Are Created and Destroyed
Finalize Methods and Destructors
How New and Finalize Methods Work in a Class Hierarchy