user control event not firing after overriding Render method

I've added a CheckBoxList user control (which inherits CheckBoxList) to my aspx page - in the user control I override the Render method so that when a user clicks on an item it is highlighted - this part works fine. However, I cannot get the SelectedIndexChanged event of the user control checkboxlist to fire.  Nothing happens.
Here is my user control code:
public class ULCheckBoxList : CheckBoxList
{
private string _onClick;
public string OnClick
{< ...