[SOLVED] How to implement a Default property usage in nested class hierarchy inside VBA?

How to implement a Default property usage in nested class hierarchy inside VBA?

Consider the following code

parent.cls

Public Child as New ChildClass

ChildClass.cls

Private m_internal As New Collection

Public Property Get Item(index As Integer) As Variant
    Attribute Item.VB_UserMemId = 0          <-- used to mark the default property
    If IsObject(m_internal(Item)) = True Then
        Set Item = m_internal(index)
    Else
        Item = m_internal(index)
    End If
End Property

Public Property Let Item(index As Integer, val As Variant)
    m_internal(index) = val
End Property

Public Property Set Item(index As Integer, val As Object)
    Set m_internal(index) = val
End Property

with the above files...

Dim p As Parent
Set p = New Parent

' these lines are just fine    
Dim pChild As ChildClass
Set pChild = p.Child
pChild(2) = 2

' this throws compilation error "Wrong number of arguments or invalid property assignment"
p.Child(2) = 2

Can you explain why the second option to access the Child internal member's default property is not compiling?

Solution

Because you are not using properties. Public Child as New ChildClass is a field.

Make it a property:

Private mChild As New ChildClass

Public Property Get Child() As ChildClass
  Set Child = mChild
End Property

and p.Child(2) = 2 will compile.