BindingList<IGanttAppointment> list = new BindingList<IGanttAppointment>();
                foreach (var BuildingSpeciality in BuildingSpecialities)
                {
                    foreach (var DesignDocument in BuildingSpeciality.DesignDocuments)
                    {
                        list.Add(DesignDocument);
                    }
                }

var list = new BindingList<IGanttAppointment>(BuildingSpecialities.SelectMany(x=>x.DesignDocuments).ToList());