How are you adding your list box to the form in your c# code?
Using your new declaration I asuume your constructor is something like:
public class mycontrol( Control FormName, int Left, int Top, int Width, int Height)....
so you should be adding it to the form like this...
FormName.Controls.Add(mc) ,were mc is the name of your actual control(s)( 1 add line for each control that makes up your control..)
This has never failled me and I have used this in my aboutpanel dll, wich contains about 6 diferent controls ...