private void Button_register_Click(object sender, EventArgs e)
        {
            if (button_register.Text.Equals("ADD"))
            {
                foreach (TextBox box in textBoxes)
                {
                    if (box.Text.Trim().Equals(""))
                    {
                        MessageBox.Show("Please do not leave any empty fields.");
                        return;
                    }
                }

                currentEmployee = new Employee(controller.GetRoleId(comboBox_role.Text), textbox_name.Text, textbox_surname.Text, Char.Parse(comboBox_gender.Text.Substring(0, 1)),
                                               Int32.Parse(textbox_age.Text), textbox_email.Text, long.Parse(textbox_phone_number.Text), textbox_address.Text,
                                               DateTime.Parse(date_birth.Text), DateTime.Parse(date_joined.Text));
                controller.AddEmployee(currentEmployee);

                MessageBox.Show("Employee " + currentEmployee.Name + " registered.");

                EmployeeChanged(comboBox.Items.Count);
            }
            else
            {
                Employee currentEmployee = controller.GetEmployee(int.Parse(comboBox.Text.Split(' ')[0]));
                Employee newEmployee     = new Employee(controller.GetRoleId(comboBox_role.Text), textbox_name.Text, textbox_surname.Text, Char.Parse(comboBox_gender.Text.Substring(0, 1)),
                                                        Int32.Parse(textbox_age.Text), textbox_email.Text, long.Parse(textbox_phone_number.Text), textbox_address.Text,
                                                        DateTime.Parse(date_birth.Text), DateTime.Parse(date_joined.Text));

                if (currentEmployee.GetDifferences(newEmployee).Equals(""))
                {
                    MessageBox.Show("Nothing has changed.");
                    return;
                }

                controller.ChangeEmployee(currentEmployee, newEmployee);

                MessageBox.Show("Employee " + currentEmployee.Name + " changed.");

                EmployeeChanged(comboBox.SelectedIndex);
            }
        }