[MSCE C#]单元中的子对象添加属性，会崩溃

  /// <summary>
/// 给单元对象的子对象添加自定义属性
/// </summary>
/// <param name="cellEl"></param>
private void AddCellChildProperty(CellHeaderElement cellEl)
{
ChildElementCollection elements = cellEl.GetChildren();
foreach (var el in elements)
{
var itemlibrary = ItemTypeLibrary.FindByName("库名1", Session.Instance.GetActiveDgnFile());
var itemType = itemlibrary.GetItemTypeByName("属性类型");
var host = new CustomItemHost(el, true);
var instance = host.GetCustomItem("库名1", "属性类型");
if (null == instance)
{
instance = host.ApplyCustomItem(itemType);
}
instance.IsReadOnly = true;
// instance.WriteChanges();
instance.ScheduleChanges(el);
el.ReplaceInModel(el);//崩溃
cellEl.ReplaceInModel(el);//也会崩溃
}

}