private void btnSave_Click(object sender, RoutedEventArgs e)
        {
            String firstname  = txtFirstName.Text;
            String lastname   = txtLastName.Text;
            String username   = txtUsername.Text;
            String password   = txtPassword.Text;
            String type       = "nurse";
            String phone      = txtPhone.Text;
            String experience = txtExperience.Text;

            DateTime dob = new DateTime();

            if (txtDob.SelectedDate != null)
            {
                dob = (DateTime)txtDob.SelectedDate;
            }
            int    salary = Convert.ToInt32(txtSalary.Text);
            String gender = "female";

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

                user u = new user();
                u.username = username;
                u.password = password;
                u.type     = type;


                employee emp = new employee();
                emp.firstname = firstname;
                emp.lastname  = lastname;
                emp.dob       = dob;
                emp.salary    = salary;
                emp.gender    = gender;
                emp.phone     = phone;
                emp.user      = u;

                nurse n = new nurse();
                n.experience = experience;
                n.employee   = emp;


                hms.addNurse(n);
                NurseList.Add(n);


                this.Close();
            }
            else
            {
                MessageBox.Show("All fields are required.", "Reminder", MessageBoxButton.OK, MessageBoxImage.Warning);
            }
        }