Does not make sense. Looks like you modified the Inherits attribute of the page. Inherits defines a code-behind class for the page to inherit.
I guess it should just be:
dropdownlist, change and see.
Try:
<%@ Page Language="vb" AutoEventWireup="false" CodeBehind="dropdownlist.aspx.vb" Inherits="dropdownlist" %>
Details:
MSDN: @ Page[
^]