Here is a simple utility method that will take a string argument such as
"ctl00$ContentPlaceHolder1$DropDownList1" and return "DropDownList1"
public static string GetFriendlyControlId(string renderedControlId)
{
// PageIdSeparator is a property returning Page.IdSeparator
int indexOfSeparator = renderedControlId.LastIndexOf(PageIdSeparator);
if (indexOfSeparator >= 0)
{
renderedControlId = renderedControlId.Substring(indexOfSeparator + 1);
}
return renderedControlId;
}