如何在Silverlight中获取<PasswordBox的值？

<Grid x:Name="LayoutRoot" Background="White">
   <PasswordBox x:Name="passwordBox" Width="200" Height="50" />
   <Button Content="获取密码" Click="Button_Click" Width="100" Height="50" Margin="0,50,0,0" />
</Grid>

private void Button_Click(object sender, RoutedEventArgs e)
{
    string password = passwordBox.Password;
    MessageBox.Show("密码是：" + password);
}