protected override void OnClosing(CancelEventArgs e)
        {
            base.OnClosing(e);

            while (viewContentCollection.Count > 0)
            {
                IViewContent content = viewContentCollection[0];
                content.CloseView(false);
                if (viewContentCollection.IndexOf(content) >= 0)
                {
                    e.Cancel = true;
                    return;
                }
            }
        }