Win8 在TextBox中设置提示信息，当TextBox获得焦点时消失，失去时验证

private void email_GotFocus(object sender, RoutedEventArgs e)
{
if (this.email.Text.Equals("email@example.com"))
{
this.email.Text = "";
}
this.email.Foreground = new SolidColorBrush(Colors.Black);
}

private void email_LostFocus(object sender, RoutedEventArgs e)
{
if (0 == this.email.Text.Trim().Length)
{
this.email.Text = "email@example.com";
this.email.Foreground = new SolidColorBrush(Colors.Gray);
}
}