public List<ControlMessages> ControlMessages = new List<ControlMessages>();
public Paragraph PTrace = new Paragraph();

public void WriteControlMessage(string _id, string _message) {

            ControlMessage cm = new ControlMessage(_id, _message);
            ControlMessages.Add(cm);
            PTrace.Inlines.Add(cm.Line);

}

public void UpdateControlMessage(string _id, string _message) {
            ControlMessage cm = ControlMessages.Where(wh => wh.Id == _id).FirstOrDefault();
            cm.SetMessage(_message);
            PTrace.Inlines.InsertAfter(cm.OldLine, cm.Line);
            PTrace.Inlines.Remove(cm.OldLine);

}

public class ControlMessage {

        public string Id { get; set; }
        public Inline Line { get; set; }
        public Inline OldLine { get; set; }

        public ControlMessage(string _id, string _message) {

            Id = _id;
            SetMessage(_message);
        }

        public void SetMessage(string _message) {

            OldLine = Line;
            Paragraph p = new Paragraph();
            p.Inlines.Add(_message);
            Line = p.Inlines.LastInline;
        }
}