how to use this code in web application which is window form code ? and error is of dropdown llist

con = new SqlConnection(cs);
            con.Open();


            string ct = "select distinct RTRIM(FeeID) from FeesDetails ";

            cmd = new SqlCommand(ct);
            cmd.Connection = con;

            rdr = cmd.ExecuteReader();

            while (rdr.Read())
            {
                DropDownList2.Items.Add(rdr[0]);

            }
            con.Close();