Why do some classes have an "I" in front of their name? - vba

Question

I'm working with some legacy code in Visual Basic 98, and there are several classes with an "I" in front of their name. Most of the classes don't have this name, however.
Here's the contents of the IXMLSerializable.cls file.
' Serialization XML:
' Create and receive complete split data through XML node
Public Property Let SerializationXML(ByVal p_sXML As String)
End Property
Public Property Get SerializationXML() As String
End Property
Public Property Get SerializationXMLElement() As IXMLDOMElement
End Property

Answer 1

Note that VBA supports interfaces, just as C#/VB.NET do (almost). Interfaces are the only way to provide inheritance mechanisms in VBA.
By convention interfaces start their name with the capital letter I.
Here is an example interface declaration that states an object must define a name property
[File: IHasName.cls, Instancing: PublicNotCreatable]
Option Explicit
Public Property Get Name() As String
End Property
As you can see there is no implementation required.
Now to create an object that uses the interface to advertise that it contains a name property. Of course, the point is that there are multiple classes that use the one interface.
[File: Person.cls, Instancing: Private]
Option Explicit
Implements IHasName
Private m_name As String
Private Sub Class_Initialize()
m_name = "<Empty>"
End Sub
' Local property
Public Property Get Name() as String
Name = m_name
End Property
Public Property Let Name(ByVal x As String)
m_name = x
End Property
' This is the interface implementation that relies on local the property `Name`
Private Property Get IHasName_Name() As String
IHasName_Name = Name
End Property
As a convenience in the UI once you include the Implements statement you can choose the interface properties from the top
And to consume the above code use the following test, which calls a function that can take any object that implements IHasName.
[File: Module1.bas]
Option Explicit
Public Sub TestInterface()
Dim target As New Person
target.Name = "John"
GenReport target
' This prints the name "John".
End Sub
Public Function GenReport(ByVal obj As IHasName)
Debug.Print obj.Name
End Function

Answer 2

The I stands for Interface, like specified in the Microsoft Official Documentation:
IXMLDOMElement Members.
The following tables show the properties, methods, and events.
In C++, this interface inherits from IXMLDOMNode.
That was a pretty common convention and by doing so, you immediately know that it represent an Interface, without looking at the code.
Hope this helps.

Answer 3

I stands for interface. VBA and older Visual Basic dialects up to VB 6.0 are said to be object oriented but a have a very poor support for it. For example, there is no class inheritance. Nevertheless, you can declare and implement interfaces in VBA/VB6; however, there is no Interface keyword as there is a Class keyword. Instead, you just declare a class with empty Subs, Functions and Properties.
Example. In a Class named IComparable, declare a Function CompareTo:
Public Function CompareTo(ByVal other As Object) As Long
'Must return -1, 0 or +1, if current object is less than, equal to or greater than obj.
'Must be empty here.
End Function
Now you can declare classes that implement this interface. E.g. a Class named clsDocument:
Implements IComparer
public Name as String
Private Function IComparable_CompareTo(other As Variant) As Long
IComparable_CompareTo = StrComp(Name, other.Name, vbTextCompare)
End Function
Now, this lets you create search and sorting algorithms that you can apply to different class types that implement this method. Example of a class called Document
Option Explicit
Implements IComparable
Public Name As String
Public FileDate As Date
Public Function IComparable_CompareTo(ByVal other As Object) As Long
Dim doc As Document, comp As Long
Set doc = other
comp = StrComp(Me.Name, doc.Name, vbTextCompare)
If comp = 0 Then
If Me.FileDate < doc.FileDate Then
IComparable_CompareTo = -1
ElseIf Me.FileDate > doc.FileDate Then
IComparable_CompareTo = + 1
Else
IComparable_CompareTo = 0
End If
Else
IComparable_CompareTo = comp
End If
End Function
Here an example of a QuickSort for VBA. It assumes that you pass it an array of IComparables:
Public Sub QuickSort(ByRef a() As IComparable)
'Sorts a unidimensional array of IComparable's in ascending order very quickly.
Dim l As Long, u As Long
l = LBound(a)
u = UBound(a)
If u > l Then
QS a, l, u
End If
End Sub
Private Sub QS(ByRef a() As IComparable, ByVal Low As Long, ByVal HI As Long)
'Very fast sort: n Log n comparisons
Dim i As Long, j As Long, w As IComparable, x As IComparable
i = Low: j = HI
Set x = a((Low + HI) \ 2)
Do
While a(i).CompareTo(x) = -1: i = i + 1: Wend
While a(j).CompareTo(x) = 1: j = j - 1: Wend
If i <= j Then
Set w = a(i): Set a(i) = a(j): Set a(j) = w
i = i + 1: j = j - 1
End If
Loop Until i > j
If Low < j Then QS a, Low, j
If HI > i Then QS a, i, HI
End Sub