使用C＃和ASP.NET的服务器端ListBox可见性

I am interested in using a "onmouseover" event to make a listbox appear and disappear. I am fairly new with ASP.NET and I do not want to write javascript just yet. I am trying to use the following code, and the color change portion of it works, but the listbox visibility doesn't work:I am interested in using a "onmouseover" event