Properties

Public fields let anyone set any value. Properties look like fields from the outside but run code when read or written, so a class can validate and compute values.

Auto-implemented properties

Public Class Person
    Public Property Name As String
    Public Property Age As Integer
    Public Property Country As String = "Unknown"
End Class

Module Program
    Sub Main()
        Dim p As New Person() With {.Name = "Ada", .Age = 36}
        p.Age += 1
        Console.WriteLine($"{p.Name}, {p.Age}, {p.Country}")
    End Sub
End Module

Ada, 37, Unknown

Public Property Name As String creates a hidden field plus a getter and setter. This is the normal way to expose data from a class.

Properties with logic

Public Class Thermostat
    Private _target As Double = 20

    Public Property Target As Double
        Get
            Return _target
        End Get
        Set(value As Double)
            _target = Math.Max(5, Math.Min(30, value))
        End Set
    End Property

    Public ReadOnly Property IsHeating As Boolean
        Get
            Return _target > 21
        End Get
    End Property
End Class

Module Program
    Sub Main()
        Dim t As New Thermostat()
        t.Target = 42
        Console.WriteLine($"{t.Target} {t.IsHeating}")
        t.Target = 18
        Console.WriteLine($"{t.Target} {t.IsHeating}")
    End Sub
End Module

30 True
18 False

• Set receives the new value (conventionally named value) and can validate or clamp it.
• A ReadOnly Property has only a Get — a computed value callers can't assign.
• An underscore prefix (_target) is a common convention for the field behind a property.

Read-only auto-properties

Public Class Order
    Public ReadOnly Property Id As Integer
    Public ReadOnly Property Created As Date = New Date(2025, 3, 9)

    Public Sub New(id As Integer)
        Me.Id = id
    End Sub
End Class

Module Program
    Sub Main()
        Dim o As New Order(7)
        Console.WriteLine($"Order {o.Id} created {o.Created:yyyy-MM-dd}")
    End Sub
End Module

Order 7 created 2025-03-09

A ReadOnly auto-property can only be set in its declaration or in a constructor (Sub New, next lesson) — perfect for values that should never change after the object is created.

Common mistakes

• Making everything Public fields. Use properties — they let you add validation later without breaking callers, and data binding in Windows Forms and WPF works with properties only.
• Doing slow work (database calls) in a property getter. Callers expect properties to be cheap; use a method for expensive operations.