C# – way to overload the constructor / initialize procedure for a class in VBA

c++excelms-accessoopvba

In C#, I know that I can overload the constructor for a class by specifying it in the body of the class:

public class MyClass()
{
    public MyClass(String s) { ... }
}

This overrides the default constructor (which has no parameters) and forces the class to be initialized with parameter s.

I know that in VBA I can initialize my class with Private Sub Class_Initialize(), but I don't know if there's a way to force my class to be initialized with parameters. Can this be done?

Best Solution

As Jtolle indicated, this is simply not possible in VBA/VB6. There is no perfect way to work around this but, what I personally do is create a Public/Friend sub call Initialize with the parameters I want (in VBA/VB6 you use "Optional" parameters for overloading) and then put a quick check in all exposed members of the class that throws an exception if you try to access them without running the initialize method. A basic example might look like this:

Option Explicit

Private m_blnInitialized As Boolean
Private m_lngID As Long
Private m_strFirstName As String

Public Sub Initialize(ByVal ID As Long, Optional ByVal someOtherThing As String = vbNullString)
    If m_blnInitialized Then Me.Clear
    m_lngID = ID
    m_strFirstName = SomeLookUp()
    If LenB(someOtherThing) Then
        ''Do something here.
    End If
    m_blnInitialized = True
End Sub

Public Property Get ID() As Long
    If Not m_blnInitialized Then Err.Raise eStandardErrors.eNotInitialized
    ID = m_lngID
End Property

Public Property Get FirstName() As String
    If Not m_blnInitialized Then Err.Raise eStandardErrors.eNotInitialized
    FirstName = m_strFirstName
End Property

Private Function SomeLookUp() As String
    ''perform magic on Me.ID
End Function

Public Sub LoadPicture()
    If Not m_blnInitialized Then Err.Raise eStandardErrors.eNotInitialized
    ''More magic
End Sub

Public Sub Clear()
    If Not m_blnInitialized Then Err.Raise eStandardErrors.eNotInitialized
    m_strFirstName = vbNullString
    m_lngID = 0&
    m_blnInitialized = False
End Sub

It's not great, but it's about as good as it's going to get with VBA/VB6.

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