Dim Liste As New List(Of Hauptklasse)
 
        'Definiere Unterelement
        Dim UElement As New Unterklasse
        UElement.Var = "Hallo"
 
        'Füge Element zur Liste hinzu
        Liste.Add(New Hauptklasse With {.UK = UElement})
 
 
        'Serialisierung, hier tritt der Fehler auf
        Dim ser As Xml.Serialization.XmlSerializer = New _
          Xml.Serialization.XmlSerializer(GetType(List(Of Hauptklasse)))
 
        Dim writer As IO.StringWriter = New IO.StringWriter
        ser.Serialize(writer, Liste)
        MsgBox(writer.ToString)
        writer.Close()

Public Class Hauptklasse
 
    'Ggf. ignorieren beim Serialisieren
    '<Xml.Serialization.XmlIgnore()>   
    Public UK As Unterklasse
 
    Public ....
    Public ....
    Public ....
 
End Class
 
Public Class Unterklasse
 
    Public Var As String
    Public ....
    Public ....
 
End Class