VBA / VB6 Interface implementation

Few months ago I blogged about interface implementation in VB6. You can find the post here
However, I discovered that the code had a memory leak caused by a circulare reference.
You can find a solution here.

The Keyword Implements in VBA/VB6 allows you to do Interface Implementation, which in turns allows for Polymorphism. This is indeed a great capability of the VB6 language that is often overlooked.
While the VB6 version of interface implementation is not so elegant as the one from .NET environment, it still allows you to implement most of the pattern of the GoF books
I will show you a very easy example on how to use it in VBA.
The Idea is to create an interface called IInstrumet with just one property "Id", and have a Security class that implements its interface. You could also have a Fund, Porftolio or a Security Class that implements this interface. This wil allow for polymorphism.

Dim inst as IInstrument
Dim sec as Security
Dim fn as Fund

Set sec = new Security
Set fn = new Fund

Set inst = sec
Set inst = fn

As you can see, bot a security and a fund can be assigned to an Instruments object!
We first define a Class called IInstrument. The code is here

'This is an Interface for the Generic Financial Instrument

Public Property Get Id() As String
'Only Signature
End Property

Public Property Let Id(value As String)
'Only Signature
End Property

We now create a new Class called Security that Implements the IInstrument one. This is a bit more tricky.
Once we implement an Interface, the Class that implements it in VB6 will declare those method as Private like that.

Private Property Get IInstrument_Id() As String
   IInstrument_Id = mId
End Property

This is a kind of unsual behaviour, because if I know that the Security class implements the IInstrument interface, I expect to have access to the same methods and property made available by the interface obejcts. For this reason, I usually expose as public member the same properties and functions that are available in the interface. When I implement them, I delegate the job to the interface method


3) I implement the interface methods as I normally do. a declare a module level variable mId

Private Property Get IInstrument_Id() As String

    IInstrument_Id = mId  
End Property

Private Property Let IInstrument_Id(value As String)
   mId =value
End Property

4) I crate public properties / methods mirroring the interface delegating their implementation to the
    mInstrument object
Public Property Get Id() As String

   Id =IInstrument_Id
End Property

Public Property Let Id(value As String)
  IInstrument_Id = value
End Property

Here you can find the Security Class Code

Implements IInstrument

Private mId As String
Public Ticker as String



Private Sub Class_Initialize()

End Sub

Private Property Get IInstrument_Id() As String
       IInstrument_Id = mId
End Property


Private Property Let IInstrument_Id(value As String)
  mId = value
End Property


'Public Interface

Public Property Get Id() As String
     Id = IInstrument_Id
End Property

Public Property Let Id(value As String)
   IInstrument_Id = value
End Property

We can now test the code

Sub TestSecurity()
 Dim Sec1 As Security
 Dim Inst As IInstrument
 Dim Sec2 As Security
 Set Sec1 = New Security
 
 Sec1.Id = 10
 Sec1.Ticker = "MXEU"
 
 Set Inst = Sec1 'Upcast: A Security in an Instruments
 Debug.Print Inst.Id
 
 'DownCast, this should have been done explicit, but VBA does not support CType.
 'VB6 does. So instead of CType(Inst, "Security") we can do
 If TypeName(Inst) = "Security" Then
    Set Sec2 = Inst
    End If
 
 
 Set Sec2 = Inst

 Debug.Print Sec2.Id
 Debug.Print Sec2.Ticker
End Sub