private void buttonAdd_Click(object sender, EventArgs e)
{
b = new Books();
AddForm f = new AddForm(b);
if (f.ShowDialog() == DialogResult.OK)
{
Books tmp = bd.Books.Where(x => x.Id_Theme == b.Id_Theme && x.Id_Author == b.Id_Author && x.Id_Publisher == b.Id_Publisher &&
x.NameBook == b.NameBook && x.Pages == b.Pages && x.Year == b.Year && x.ProductPrice == b.ProductPrice).FirstOrDefault();
if (tmp == null)
{
bd.Books.Add(b);
bd.SaveChanges();
}
}
}