sending email using .Net Classes

hi,Realli need help from expert out here..I have the following code which retrieve email address from the text box and send the repective attachment to the email address from the textbox..:<%@ Page Language="C#" %>
<%@ Import Namespace="System.Web.Mail" %>

i like to ask wat is the correct syntax if i want to retrieve the email address from the dropdownlist instead of typing on the textbox....what is the correct syntax to use instead of using txtto.text....

Thanks and Regards