public string Save(Book aBook)
{
if (bookGateway.HasBookName(aBook.Title))
{
throw new Exception("This system has this book name.Please try again.");
}
else if (bookGateway.HasISBNNo(aBook.ISbnNo))
{
throw new Exception("This system already have this ISBN No.Please try again.");
}
else
{
return(bookGateway.Save(aBook));
}
}