Mdi程序，无法遍历子窗口。

  vipAddForm = new VipAddForm();
     vipAddForm.MdiParent = this;
        vipAddForm.Left = (this.splitContainer1.Panel2.Width - vipSearch.Width) / 2;
            vipAddForm.Top = (this.splitContainer1.Panel2.Height - vipSearch.Height) / 2;
            splitContainer1.Panel2.Controls.Add(vipAddForm);
            vipAddForm.Show();
已打开了子窗口，为啥this.ActiveMdiChild 还是为空？