//  Buttons:
        private void Btn_Login_Click(object sender, RoutedEventArgs e)
        {
            try
            {
                Client.Login(Txt_Email.Text, Txt_Password.Password.ToString());
                while (Client.loginstate == false)
                {
                }

                int userId = Client.GetLoginId();
                if (userId != -1)
                {
                    // hide current form
                    this.Hide();
                    // open new form
                    FormMain form = new FormMain(userId, Client);
                    form.ShowDialog();
                    // close current form
                    this.Close();
                }
                else
                {
                    throw new InvalidLoginCombination();
                }
            }
            catch (InvalidLoginCombination ex)
            {
                Lbl_Warning.Content    = ex.Message;
                Lbl_Warning.Visibility = Visibility.Visible;
            }
            catch (EmptyField ex)
            {
                Lbl_Warning.Content    = ex.Message;
                Lbl_Warning.Visibility = Visibility.Visible;
            }
            catch (InvalidEmail ex)
            {
                Lbl_Warning.Content    = ex.Message;
                Lbl_Warning.Visibility = Visibility.Visible;
            }
        }