How to use comparison methods between class object modules in VBA in a similar manner as VB.NET?

Question

Due to new project in VBA I moved from VB.NET, to be honest don\'t really know how to deal between objects classes here. What I want to reach is comparing objects between di

Answer 1

VBA doesn't support class polymorphism so I recommend to change the way you're thinking about the Employee and Manager classes.

You can't have an Employee class as a base class and then a separate Manager class that derives from Employee. They could be 2 separate classes implementing a common interface.

I will talk about it in detail in a bit. Let's now go through a few examples...

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↓ Easy approach ↓

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A base class (Person) and child classes which derive from the base class. (applies to C#, VB.NET, etc)

but in VBA you have to think of it like this:

A base class which exposes an enum property describing the position.

Something like

This is the easiest way to have a class exposing some properties. It allows you to add your Person objects into a collection and iterate over using the easy for each loop with Intellisense!

Properties comparison system would be very very easy

note: same applies to enum as its implicitly converted to a number

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↓ More complex approach ↓

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Two separate classes which both expose public properties. For instance you have an Employee and Manager classes which both implement a Person Interface and an additional Comparer class exposing a Compare() method

In your VBA Project you need 4 class modules and a standard module

Person (this is your Interface)

Public Property Get Name() As String
End Property

Public Property Get Age() As Long
End Property

this class is the interface which both the Employee and Manager both need to implement to share some common functions (getters for Names and Ages). Having the interface allows you to do the for each loop using the interface type variable as the enumerator. You will see in a minute.

Employee and Manager are identical. Obviously you may modify them to suit your real life solution.

Implements Person

Private name_ As String
Private age_ As Long

Public Property Get Name() As String
    Name = name_
End Property

Public Property Let Name(ByVal Value As String)
    name_ = Value
End Property

Public Property Get Age() As Long
    Age = age_
End Property

Public Property Let Age(ByVal Value As Long)
    age_ = Value
End Property

Private Property Get Person_Name() As String
    Person_Name = Name
End Property

Private Property Get Person_Age() As Long
    Person_Age = Age
End Property

ComparerCls you will use an instance of this class to compare two objects properties or references. You do not necessarily need to have a class for this but I prefer it that way.

Public Enum ComparisonMethod
    Names = 0 ' default
    Ages = 1
    References = 2
End Enum

' makes names the default comparison method
Public Function Compare(ByRef obj1 As Person, _
                        ByRef obj2 As Person, _
                        Optional method As ComparisonMethod = 0) _
                        As Boolean

    Select Case method
        Case Ages
            Compare = IIf(obj1.Age = obj2.Age, True, False)
        Case References
            Compare = IIf(obj1 Is obj2, True, False)
        Case Else
            Compare = IIf(obj1.Name = obj2.Name, True, False)
    End Select

End Function

And your Module1 code

Option Explicit

Sub Main()

    Dim emp As New Employee
    emp.Name = "person"
    emp.Age = 25

    Dim man As New Manager
    man.Name = "manager"
    man.Age = 25

    Dim People As New Collection
    People.Add emp
    People.Add man

    Dim individual As Person
    For Each individual In People
        Debug.Print TypeName(individual), individual.Name, individual.Age
    Next

End Sub

run the Main() sub and check out the results in the Immediate Window

The best part of the above code is the fact that you are creating a reference variable of the Person interface. It allows you to loop through all items in the collection that implement the interface. Also, you can use the Intellisense which is great if you have had many more properties and functions.

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Comparison

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Take a look again at the code in the ComparerCls class

I hope you see now why I have separated this to be a class. Its purpose is just to take care of the way the objects are being compared. You can specify the Enum order and modify the Compare() method itself to compare differently. Note the Optional parameter which allows you to call the Compare method without the method of comparison.

Now you can play around passing different parameters to the compare function. See what the results are like.

Try combinations:

emp.Name = "name"
man.Name = "name"

Comparer.Compare(emp, name, Names)
Comparer.Compare(emp, name, References)

Comparer.Compare(emp, emp, References)

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If something is still unclear refer to this answer about the Implements keyword in VBA