关于事件处理的完整框架Event，delegate，event args，无图有真相。

public delegate void SelectItemsChangedEventHandler(ItemChangingEventArgs e);
    public class ItemChangingEventArgs : EventArgs
    {
        public List<string> lstSelItems { get; set; }  //这里存放用户选择的分类信息
    }

    public class Win子窗口
    {
        public event SelectItemsChangedEventHandler SelectItemsChangedEvent;


        public void Win子窗口_Closed()
        {
            ItemChangingEventArgs args = new ItemChangingEventArgs();
            args.lstSelItems = new List<string>();
            //获取用户选择的分类信息...

            //调用自定义事件通知主窗口
            this.SelectItemsChangedEvent(args);
        }

        public void 树形控件SelectIndexChanged事件处理函数()
        {
            ItemChangingEventArgs args = new ItemChangingEventArgs();
            args.lstSelItems = new List<string>();
            //获取用户选择的分类信息...

            //调用自定义事件通知主窗口
            this.SelectItemsChangedEvent(args);
        }
    }

    public class Win主窗口
    {
        Win子窗口 winChild;
        public Win主窗口()
        {
            //主窗口构造函数中初始化子窗口对象，但是不显示，需要的时候调用Show
            winChild = new Win子窗口();
            winChild.SelectItemsChangedEvent += new SelectItemsChangedEventHandler(winChild_SelectItemsChangedEvent);

        }

        //子窗口分类项选择事件处理
        void winChild_SelectItemsChangedEvent(ItemChangingEventArgs e)
        {
            List<string> lstRt = e.lstSelItems;
            //做需要的事情
            //...
        }
        
    }