General Info: Class Constructor

Constructors are called when you instantiate an object from a class. This is where you can initialize variables and put code you wish executed each time the class is created. When you initially set the member fields and properties of an object, you are initializing the state of the object. The state of an object is the values of all it's member fields and properties at a given time.

Languages Focus: Constructor

What is the syntax? Can you overload constructors? Is a special method name reserved for constructors?

Public Class Cyborg
  Public CyborgName As String

  Public Sub New(ByVal pName As String)
    CyborgName = pName
  End Sub
End Class

Working WinForms Example

The following example demonstrates using a constructor to initialize a public member field.

Create a form and place a button on it and alter the code as follows:

Public Class Form1

  Private Sub Button1_Click(ByVal sender As System.Object, _

      ByVal e As System.EventArgs) Handles Button1.Click
  

    Dim MyRobot As New Cyborg("Cameron")

    MessageBox.Show("My robot's name is " & MyRobot.CyborgName & ".")
  End Sub
End Class

  
Public Class Cyborg
  Public CyborgName As String

  Public Sub New(ByVal pName As String)
    CyborgName = pName
  End Sub
End Class

Overloading Constructors

Here is a working example of overloading a constructor. In this example, we overload our New() constructor so you can either pass in a name or set the name after creation.

Public Class Form1

  Private Sub Button1_Click(ByVal sender As System.Object, _
    ByVal e As System.EventArgs) Handles Button1.Click

    Dim MyRobot1 As New Cyborg()
    MyRobot1.CyborgName = "John"
    MessageBox.Show("My robot's name is " & MyRobot1.CyborgName & ".")

    Dim MyRobot2 As New Cyborg("Cameron")
    MessageBox.Show("My robot's name is " & MyRobot2.CyborgName & ".")

  End Sub
End Class

 
Public Class Cyborg
  Public CyborgName As String

  Public Sub New()
  End Sub

  Public Sub New(ByVal pName As String)
    CyborgName = pName
  End Sub
End Class