protected virtual void OnContentRemoved(DockContentEventArgs e)
 {
     var handler = (EventHandler<DockContentEventArgs>)Events[ContentRemovedEvent];
     if (handler != null)
         handler(this, e);
 }
        private void DockContainer_ContentAdded(object sender, DockContentEventArgs e)
        {
            if (e.Content is DocumentBase)
            {
                if (Documents.Contains(e.Content as DocumentBase))
                {
                    return;
                }

                Documents.Add(e.Content as DocumentBase);
            }
        }