how to serialize data fom wcf to windows phone

public List<customer> login(string salesmanname, string usercode)
{
    List<customer> customers = new List<customer>();
    con.Open();
    using (SqlCommand cmd = new SqlCommand("Select Id, CompanyRefId from SalesMan where salesmanname=@salesmanname and code=@usercode", con))
    {
        cmd.Parameters.AddWithValue("@salesmanname", salesmanname);
        cmd.Parameters.AddWithValue("@usercode", usercode);
        using (SqlDataReader reader = cmd.ExecuteReader())
        {
            while (reader.Read())
            {
                customer c1 = new customer();
                c1.id = (int)reader["Id"];
                c1.companyrefid = (Guid)reader["CompanyRefId"];
                customers.Add(c1);

            }
            return customers;
        }

    }
}

string username = salesmannames.SelectedItem.ToString();
               string password = textBox1.Text.ToString();
                proxy.loginAsync(username, password);
                proxy.loginCompleted += new EventHandler<loginCompletedEventArgs>(loginfn);

void loginfn(object sender, loginCompletedEventArgs e)
       {
           if (e.Result != null)
           {
               customer ad = new customer();


               NavigationService.Navigate(new Uri("/customernames.xaml", UriKind.Relative));
           }
       }