Problem in searching in database

public class HomeController : Controller

{

SerialPOS db = new SerialPOS();

public ActionResult Index(string Searchby, string search)

{

var Serial = from SerialPOS in db.SERIAL_NUMBER select SerialPOS;

if (Searchby == "SERIAL_NUMBER")

{

var model = db.SERIAL_NUMBER.Where(s => s.SERIAL_NUMBER(search)).ToList();

return View(model);

}

else

{

var model = db.ACPTR_NAME.Where(s => s.ACPTR_NAME.StartsWith(search) || search == null).ToList();

return View(model);

}

}