formatting_xml_5.cs

public class PhoneNumber 
{
    [XmlIgnore]
    public string AreaCode { get; set; }

    [XmlIgnore]
    public string Number { get; set; }

    [XmlText]
    public string FormattedNumber
    {
        get { return $"{AreaCode}-{Number}"; }
        set {}
    }
}