system.web.ui not found when creating a class library?

Hello everyone, I am trying to create a custom dropdownlist class as suggested in this post:
https://lazyloaders.com/threads/cascading-drop-down-lists-invalid-postback-or-callback-argument-error.607/
http://johanleino.wordpress.com/2009/11/17/cascadingdropdown-causes-invalid-postback-or-callback-argument-error/
to implement CascadingDropdownList. This is what I did by:
1. Create a new class library project
2. Implement this code:
Namespace CustomControls Public Class NoValidationDropdownList Inherits System.Web.UI.WebControls.DropDownList End Class End Namespace However, System.Web.UI.WebControls.DropDownList can't be inherited.  A ...