Problems using classes in vb.net-CodePudding

I am implementing a procedure for creating an XML file for the transmission of some data, using VS 2019 vb.net. To do this, I have built a series of classes as suggested by Microsoft. The procedure compiles correctly but at run time generates an error like 'Object reference not set on an object instance.' to the line

FileAbaco.utenti(0).utente(0).strutture(0).struttura.Add(stra)

I can't understand where I'm wrong. Thank you in advance for any help you will be able to provide.

Dim FileAbaco As New dichiarazioni
        FileAbaco.comune = "I608"
        Dim uti As New utenti
        FileAbaco.utenti.Add(uti)
        Dim ute As New utente
        FileAbaco.utenti(0).utente.Add(ute)
        Dim stre As New strutture
        FileAbaco.utenti(0).utente(0).strutture.Add(stre)
        Dim stra As New struttura
        FileAbaco.utenti(0).utente(0).strutture(0).struttura.Add(stra)
        Dim dr As DataRow
        dr = DTTrimestrale.Rows(0)
        With FileAbaco.utenti(0).utente(0)
            .auth = dr.Item("auth".ToString)
            .gest_codice_fiscale = dr.Item("gest_codice_fiscale")
            .codice_utente = dr.Item("codice_utente")
            .gest_cognome = dr.Item("gest_cognome")
            .gest_nome = ""
        End With

These are the classes:

Imports System.Xml.Serialization

Public Class dichiarazioni
    Public comune As String
    <XmlElement()>
    Public Property utenti As New List(Of utenti)
End Class

Public Class utenti
    <XmlElement()>
    Public Property utente As New List(Of utente)
End Class

Public Class utente
    Public Property auth As String
    Public Property gest_codice_fiscale As String
    Public Property codice_utente As String
    Public Property gest_cognome As String
    Public Property gest_nome As String
    Public Property strutture As New List(Of strutture)
    Public Sub New()

    End Sub

    Public Sub New(auth As String, gest_codice_fiscale As String, codice_utente As String, gest_cognome As String, gest_nome As String, strutture As strutture)
        Me.auth = auth
        Me.gest_codice_fiscale = gest_codice_fiscale
        Me.codice_utente = codice_utente
        Me.gest_cognome = gest_cognome
        Me.gest_nome = gest_nome
        Me.strutture.Add(strutture)
    End Sub
End Class
Public Class strutture
    <XmlElement()>
    Public Property struttura As List(Of struttura)
    Public Sub New()

    End Sub
    Public Sub New(struttura As struttura)
        Me.struttura.Add(struttura)
    End Sub
End Class
Public Class struttura
    <XmlElement()>
    Public Property ID_struttura As String
    Public Property insegna As String
    Public Sub New()

    End Sub
    Public Sub New(ID_struttura As String, insegna As String)
        Me.ID_struttura = ID_struttura
        Me.insegna = insegna
    End Sub
End Class

CodePudding user response：

Obviously it’s right because you have to initialize struttura as it is always Nothing.

Cambia questa classe come segue:

Public Class strutture
    <XmlElement()>
    Public Property struttura As List(Of struttura)
    Public Sub New()
        If Me.struttura Is Nothing Then Me.struttura = New List(Of struttura)
    End Sub
    Public Sub New(struttura As struttura)
        If Me.struttura Is Nothing Then Me.struttura = New List(Of struttura)
        Me.struttura.Add(struttura)
    End Sub
End Class

Otherwise: Altrimenti fai come sopra (nella classe che precede)

Public Property whatEver As New List(Of Whatever)