Create a class modules with parent and child classes in VBA - vba

Question

I am a novice user of class module.
I don't understand the concept of a class module well.
I want to configure a class module similar to the basic objects of Excel like Worksheet or Cells or ETC..
So I want to control it by creating a parent object and creating its child objects.
Child Class - Defect
Option Explicit
Private pDefectSymptom As String
Private pDefectLevel As Integer
Property Get DefectSymptom() As String
DefectSymptom = pDefectSymptom
End Property
Property Let DefectSymptom(ByVal vDefectSymptom As String)
pDefectSymptom = vDefectSymptom
End Property
Property Get DefectLevel() As Integer
DefectLevel = pDefectLevel
End Property
Property Let DefectLevel(ByVal vDefectLevel As Integer)
pDefectLevel = vDefectLevel
End Property
Function Delete()
'???
End Function
Property Get Parent() As Object
'???
End Property
Parent Class - Defects
Private Defects As New Collection
Function Add(DefectSymptom As String, Optional DefectLevel As Integer) As Defect
Dim NewDefect As Defect
Set NewDefect = New Defect
NewDefect.DefectSymptom = DefectSymptom
NewDefect.DefectLevel = DefectLevel
Defects.Add NewDefect
'Add = NewDefect 'Error! Like the open command of workbook, I want to return an object or just command
End Function
Property Get Count() As Long
Count = Defects.Count
End Property
Property Get Item(Index As Long) As Defect
Item = Defects(Index) 'Error! I don't know what raise Error.
End Property
My question are.
How to add Command like Open Command of workbook. Return or just command.
Why Raise Error Item Property? how to fix that?
Hiding Private variable. Because office office objects seem to be hidden.
enter image description here
If you have time, please help with Delete command and Parent command.

Answer 1

How to add Command like Open Command of workbook. Return or just
command.
You just need to return the newly created object in the function. Keep in mind since we're dealing with objects, we need to Set the object's reference.
Public Function Add(DefectSymptom As String, Optional DefectLevel As Integer) As Defect
Dim NewDefect As Defect
Set NewDefect = New Defect
NewDefect.DefectSymptom = DefectSymptom
NewDefect.DefectLevel = DefectLevel
Defects.Add NewDefect
Set Add = NewDefect '<- here
End Function
Why Raise Error Item Property? how to fix that?
Same as the above, you need to Set the object's reference.
Property Get Item(Index As Long) As Defect
Set Item = Defects(Index)
End Property
To delete, simply supply the index to the function. However, this method must reside where the collection is (parent) since a Defect object cannot delete itself.
Function Delete(ByVal Index As Long)
Defects.Remove Index
End Function
Lastly, to hold a reference to the parent, each child must hold a reference to it in a private variable. Then you need to set the parent when creating a new item using the keyword Me.
So in the Defect class, create a private field.
Private mParent As Defects
Property Set Parent(ByVal objDefects As Defects)
Set mParent = objDefects
End Property
Property Get Parent() As Defects
Set Parent = mParent
End Property
With this done, amend the Add() method to store the reference.
Public Function Add(DefectSymptom As String, Optional DefectLevel As Integer) As Defect
Dim NewDefect As Defect
Set NewDefect = New Defect
NewDefect.DefectSymptom = DefectSymptom
NewDefect.DefectLevel = DefectLevel
Set NewDefect.Parent = Me '<- here
Defects.Add NewDefect
Set Add = NewDefect '<- here
End Function
Not sure this is a good idea though. I tend to avoid circular references altogether, since a child can hold the parent in memory by holding a reference to it. You will need to make sure to clear the reference to the Parent when deleting the item.
Lastly, you should avoid creating the Defects collection like this. Instead, you should make use of the class constructor and destructor.
This method is called automatically when a new class is created:
Private Sub Class_Initialize()
Set Defects = New VBA.Collection
End Sub
This method is called just before the class is destroyed from memory.
Private Sub Class_Terminate()
Set Defects = Nothing
End Sub