private void addBook_Load(object sender, EventArgs e)
{
lc.getAuthorAndCategory(out dt1, out dt2);
categorycb.DataSource = dt1;
categorycb.ValueMember = dt1.Columns[0].ToString();
categorycb.DisplayMember = dt1.Columns[1].ToString();
authorcb.DataSource = dt2;
authorcb.ValueMember = dt2.Columns[0].ToString();
authorcb.DisplayMember = dt2.Columns[1].ToString();
}