// https://social.msdn.microsoft.com/Forums/windows/en-US/68c30f52-bba3-48c9-bf59-84304b75068d/maskedtextbox-and-selectall

private void maskedTextBox1_Enter(object sender, EventArgs e)
{
 this.BeginInvoke((MethodInvoker) delegate() {
   txtbox.SelectAll();
 });
}