public override int GetHashCode()
        {
            int hash = 13;

            hash = hash * 7 + Title.GetHashCode();
            hash = hash * 7 + Author.GetHashCode();
            hash = hash * 7 + ISBN.GetHashCode();
            hash = hash * 7 + Publisher.GetHashCode();
            hash = hash * 7 + PublishYear.GetHashCode();
            hash = hash * 7 + Price.GetHashCode();
            hash = hash * 7 + Pages.GetHashCode();

            return(hash);
        }
 public override int GetHashCode()
 {
     return(PublishYear.GetHashCode());
 }