After adding a few items in RadMenu, and changing their visibility settings:
1.When turned back to visible, they does not appear
2. When they are collapsed, their empty spots are still there (although AutoSize = true)

int i = 1;
        void radMenuItem1_DropDownOpening(object sender, CancelEventArgs e)
        {
            if (i == 0)
            {
                radMenuButtonItem2.Visibility = Telerik.WinControls.ElementVisibility.Collapsed;
                radMenuButtonItem3.Visibility = Telerik.WinControls.ElementVisibility.Collapsed;
                i++;
                
            }
            else
            {
                radMenuButtonItem2.Visibility = Telerik.WinControls.ElementVisibility.Visible;
                radMenuButtonItem3.Visibility = Telerik.WinControls.ElementVisibility.Visible;
                i--;
            }
            radMenuItem1.DropDown.PopupElement.InvalidateMeasure();
            radMenuItem1.DropDown.PopupElement.InvalidateArrange();
            radMenuItem1.DropDown.PopupElement.UpdateLayout();
        }