Event Handling of text boxes using MVVM

private void txtUsername_LostFocus(object sender, RoutedEventArgs e)
        {
            string usergrouptxt = txtUsername.Text;
            try
            {
                if (string.IsNullOrEmpty(usergrouptxt))
                {
                    txtUsername.BorderBrush = new SolidColorBrush(Colors.Red);                    
                }
                else
                {
                }
            }
            catch (SqlException usrgrpexc)
            {
                MessageBox.Show(usrgrpexc.ToString());
            }
        }