Classes and Objects

A class is a blueprint that bundles data (fields and properties) with behavior (methods). An object is one instance created from it with New.

Public Class BankAccount
    Public Owner As String
    Private balance As Decimal

    Public Sub Deposit(amount As Decimal)
        If amount <= 0 Then Throw New ArgumentException("Amount must be positive.")
        balance += amount
    End Sub

    Public Function Withdraw(amount As Decimal) As Boolean
        If amount > balance Then Return False
        balance -= amount
        Return True
    End Function

    Public Function GetBalance() As Decimal
        Return balance
    End Function
End Class

Module Program
    Sub Main()
        Dim account As New BankAccount()
        account.Owner = "Ada"
        account.Deposit(100D)
        Console.WriteLine(account.Withdraw(30D))
        Console.WriteLine(account.Withdraw(500D))
        Console.WriteLine($"{account.Owner}: {account.GetBalance()}")
    End Sub
End Module

True
False
Ada: 70

• Dim account As New BankAccount() declares a variable and creates the object in one step (the same as Dim account = New BankAccount()).
• Private members are hidden from code outside the class — here, balance can only change through Deposit and Withdraw. This is encapsulation.
• Methods are just Subs and Functions inside a class.

Objects are references

A class variable holds a reference to an object. Assigning it copies the reference, not the object:

Public Class Counter
    Public Value As Integer
End Class

Module Program
    Sub Main()
        Dim a As New Counter()
        Dim b = a
        b.Value = 5
        Console.WriteLine(a.Value)
        Console.WriteLine(a Is b)

        Dim c As New Counter() With {.Value = 5}
        Console.WriteLine(a Is c)
    End Sub
End Module

5
True
False

a and b point at the same object, so changing it through b shows up in a. c is a different object even though its value is the same. With {.Value = 5} is an object initializer that sets members right after creating the object.

Me

Inside a class, Me refers to the current object — useful when a parameter has the same name as a field:

Public Class Dog
    Private name As String

    Public Sub Rename(name As String)
        Me.name = name
    End Sub

    Public Function Speak() As String
        Return $"{Me.name} says woof"
    End Function
End Class

Module Program
    Sub Main()
        Dim d As New Dog()
        d.Rename("Rex")
        Console.WriteLine(d.Speak())
    End Sub
End Module

Rex says woof

Nothing

A class variable that doesn't point to any object is Nothing. Calling a member on it throws a NullReferenceException:

Public Class Dog
    Public Name As String = "Rex"
End Class

Module Program
    Sub Main()
        Dim d As Dog = Nothing
        Console.WriteLine(d Is Nothing)
        Console.WriteLine(d?.Name)
        Console.WriteLine(d.Name)
    End Sub
End Module

True

Unhandled exception. System.NullReferenceException: Object reference not set to an instance of an object.
   at Demo.Program.Main()

d?.Name returns Nothing (printed as an empty line) instead of crashing; d.Name throws.

Practice

Write a Rectangle class with public Width and Height fields and methods Area() and Perimeter(). Create one with an object initializer and print both values.

Public Class Rectangle
    Public Width As Double
    Public Height As Double

    Public Function Area() As Double
        Return Width * Height
    End Function

    Public Function Perimeter() As Double
        Return 2 * (Width + Height)
    End Function
End Class

Module Program
    Sub Main()
        Dim r As New Rectangle() With {.Width = 3, .Height = 4.5}
        Console.WriteLine($"Area {r.Area()}, perimeter {r.Perimeter()}")
    End Sub
End Module

Area 13.5, perimeter 15