private void btnUpdate_Nurse_Click(object sender, RoutedEventArgs e)
        {
            if (nurseSelected == null)
            {
                return;
            }



            String   firstname  = txtFirstName_Nurse.Text;
            String   lastname   = txtLastName_Nurse.Text;
            String   username   = txtUsername_Nurse.Text;
            String   password   = txtPassword_Nurse.Text;
            String   experience = txtExperience.Text;
            String   phone      = txtPhone_Nurse.Text;
            DateTime dob        = (DateTime)txtDob_Nurse.SelectedDate;
            int      salary     = Convert.ToInt32(txtSalary_Nurse.Text);
            String   gender     = "female";

            if (!string.IsNullOrEmpty(firstname) && !string.IsNullOrEmpty(lastname) &&
                !string.IsNullOrEmpty(username) && !string.IsNullOrEmpty(password) &&
                !string.IsNullOrEmpty(phone) && !string.IsNullOrEmpty(txtDob_Nurse.Text) &&
                !string.IsNullOrEmpty(txtSalary_Nurse.Text) && !string.IsNullOrEmpty(experience))
            {
                if (username != nurseSelected.employee.user.username && hms.getUser(username) != null)
                {
                    MessageBox.Show("Username already exists!", "Warning!", MessageBoxButton.OK, MessageBoxImage.Warning);
                    return;
                }

                nurseSelected.employee.firstname = firstname;
                nurseSelected.employee.lastname  = lastname;
                nurseSelected.employee.dob       = dob;
                nurseSelected.employee.salary    = salary;
                nurseSelected.employee.phone     = phone;
                nurseSelected.employee.gender    = gender;

                nurseSelected.employee.user.username = username;
                nurseSelected.employee.user.password = password;

                nurseSelected.experience = experience;

                hms.updateNurse(nurseSelected);
            }
            else
            {
                MessageBox.Show("All fields are required.", "Reminder", MessageBoxButton.OK, MessageBoxImage.Warning);
                return;
            }
            MessageBox.Show("Record updated success.", "Success", MessageBoxButton.OK, MessageBoxImage.Information);
            dataGrid_Nurse.Items.Refresh();
            RoomList = hms.getRooms();
            dataGrid_Room.ItemsSource = RoomList;
        }