How can I ignore certain fields conditionally on XML serialization

Question

I'm trying to write a program that should serialize a class to an XML file, but recently my specification changed and now I have a previously unique and not null field changed like this:

• Normal case, like before, fields are not changed [a,b,c]
• n fields, with a link to another field id [a,b,c,id]

How can I change my class to allow this kind of behavior?

Actual:

    <Serializable>
    Public Class DataType
        Public FieldToChange As New FieldType 'FieldType to change
    End Class

    <Serializable>
    Public class FieldType
        Public a As Integer
        Public b As Integer
        Public c As Integer
    End Class

Future:

    <Serializable>
    Public Class ClassWithElemToChangeType
        <XmlElement("FieldType")> ' Is it right?
        Public FieldToChange As New List(Of FieldType)
    End Class

    <Serializable>
    Public class FieldType
        Public a As Integer
        Public b As Integer
        Public c As Integer
        Public id As Integer
        ' How can i add a field that gets ignored whenever is null
        ' or how can I programmatically tell my serializer to ignore
    End Class