What does a class property do?: get, set, accessor, class, property

What does a class property do?

Asked by: prebek

I'm having trouble understanding the purpose of class properties. I just made a class called NumberChanger. Ultimately, all I want to do is make a class that takes an integer sent over my a module and multiplies it by 2. What does the property in this class do exactly?

Public Class NumberChanger
Public Number As Integer

Public Property Changer() As Single
Get
Return Number
End Get
Set(ByVal Value As Single)
Number = Value
End Set
End Property

Public Function ChangeNum()
Number = Number * 2
Return Number
End Function
End Class

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Almost, yes.

Public Number As Integer

This is a "dumb" property. The routine using the class can manipulate <obj>.Number just like a variable -- it is just a variable; the class can modify the value as well.

Private MyNumber As Integer

Public Property Number() As Integer
Get: Return MyNumber: End Get
Let (Val As Integer): MyNumber = Val: End Let
End Property

This is the same in more words. The hidden variable is manipulated through gateways, automatically invoked when the calling code issues:

WhatIs = <obj>.Number ' get the value
Let <obj>.Number = 101 ' 'let' is usually omitted

The entire idea of Property Let/Get functions is that they can have side effects, perform validation, or implement calculated properties.

The class below (untested air code) has an auto-validating °K temperature property, and correlated °C and °F properties. You could also implement a single property Temp(Optional °X = °K) As Single, a parametrized property, defaulting to Kelvin, but managing Celsius and Fahrenheit as well.

(°v°)

Private Temp As Single   ' always >= 0!
 
' property with validation
Public Property Temp°K() As Single
  Get:   Return Temp:   End Get
  Let(K As Single) 
    If K < 0! Then Temp = 0! Else Temp = K
  End Let
End Property
 
' calculated read/write property
Public Property Temp°C() As Single
  Get:   Return Temp - 273.15!:   End Get
  Let(C As Single)
    ' use Temp°K, not Temp, for validation!
    Temp°K = C + 273.15!
  End Let
End Property
 
' Compatibility: support °C/°F conversion
Public Property Temp°F() As Single
  Get:   Return Temp°C * 9! / 5! + 32!:   End Get
  Let Temp°F(F As Single)
    Temp°C = (F - 32!) * 5! / 9!
  End Let
End Property

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What does a class property do?

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