try this.
private void MaskedtxtTaxId_LostKeyboardFocus(object sender, KeyboardFocusChangedEventArgs e)
{
if (((MaskedTextBox)sender).MaskedTextProvider.AssignedEditPositionCount < ((MaskedTextBox)sender).MaskedTextProvider.AvailableEditPositionCount)
{
System.Windows.MessageBox.Show("Not valid");
}
}