Advantage or Disadvantage between Two Class Modules - vba

Question

What would be the advantages or disadvantages of using Class1 instead of Class2?
The quantity information stored in each instance of the class will be adjusted up and down as needed (via the functions, and while it seems to make sense to me that I would only need to make these variables public so that they are visible from outside the class, I feel that there is most likely some reason that this shouldn't been done.
Class1
Option Explicit
Public Sequence As String
Public Quantity As Double
Public Sub AddQty(sAddQty As Double)
Quantity = Quantity + AddQty
End Sub
Public Sub SubQty(sSubQty As Double)
Quantity = Quantity - sSubQty
End Sub
Class2
Option Explicit
Private iSeq As String
Private iQty As Double
Public Property Get Qty() As Double
Qty = iQty
End Property
Public Property Let Qty(lQty As Double)
iQty = lQty
End Property
Public Property Get Sequence() As String
Sequence = iSeq
End Property
Public Property Let Sequence(lSeq As String)
iSeq = lSeq
End Property
Public Sub AddQty(sAddQty As Double)
iQty = iQty + AddQty
End Sub
Public Sub SubQty(sSubQty As Double)
iQty = iQty - sSubQty
End Sub

Answer 1

In terms of interfaces, the two are exactly equivalent, because public fields are exposed as Property members. If you added a 3rd class module and wrote this:
Implements Class1
You would be forced by the compiler to add these members:
Private Property Get Class1_Sequence() As String
End Property
Private Property Let Class1_Sequence(ByVal RHS As String)
End Property
Private Property Get Class1_Quantity() As Double
End Property
Private Property Let Class1_Quantity(ByVal RHS As Double)
End Property
Private Sub Class1_AddQty(sAddQty As Double)
End Sub
Private Sub Class1_SubQty(sSubQty As Double)
End Sub
If you added another class module and wrote this:
Implements Class2
You would be forced by the compiler to have essentially the exact same members:
Private Property Get Class2_Sequence() As String
End Property
Private Property Let Class2_Sequence(ByVal RHS As String)
End Property
Private Property Get Class2_Qty() As Double
End Property
Private Property Let Class2_Qty(ByVal RHS As Double)
End Property
Private Sub Class2_AddQty(sAddQty As Double)
End Sub
Private Sub Class2_SubQty(sSubQty As Double)
End Sub
When properties do nothing and there's no incentive to properly encapsulate their values, go ahead and have public fields.
However there's little need for AddQty or SubQty instance methods when the backing field exposes a Property Let accessor - one could simply do foo.Quantity = foo.Quantity + 2 instead. An API that appears to provide multiple ways to do the same thing, is a confusing API.
So what you do, is you define an explicit interface that defines the API you want to work with:
Public Property Get Quantity() As Double
End Property
Public Property Get Sequence() As String
End Property
Public Sub AddQty(ByVal value As Double)
End Sub
Public Sub SubQty(ByVal value As Double)
End Sub
And then make your class Implements this interface (say, ISomething), and the rest of the code works with this ISomething interface that only exposes the members you want it to be able to work with - and that excludes the class' Property Let members; the rest of the code only sees what it needs to see, and can only access what it needs to access.
Dim foo As ISomething
Set foo = New Something
'foo.Quantity = 42 ' illegal
Dim bar As Something
Set bar = foo
bar.Quantity = 42 ' ok
bar.AddQty 2
Debug.Print foo.Quantity ' should be 44