private void txt_empid_TextChanged(object sender, EventArgs e)
 {
     employeeRepository            = new EmployeeRepository();
     lbl_employee_id_error.Visible = employeeRepository.EmployeeExist(_employee_id, txt_empid.Text);
     btn_save.Enabled = !employeeRepository.EmployeeExist(_employee_id, txt_empid.Text);
 }