private void resetform()
{
txtHoTen.Enabled = false;
txtVaiTro.Enabled = false;
cbSach.Enabled = false;
cbTacGia.Enabled = false;
btnThemTacGia.Enabled = true;
btnHuyTacGia.Enabled = false;
btnLuuTacGia.Enabled = false;
btnXoaTacGia.Enabled = false;
btnThemThamGia.Enabled = true;
btnHuyThamGia.Enabled = false;
btnLuuThamGia.Enabled = false;
btnXoaThamGia.Enabled = false;
txtHoTen.Text = "";
txtVaiTro.Text = "";
cbSach.DataSource = client.listAllBook();
cbSach.DisplayMember = "TieuDeSach";
cbSach.ValueMember = "MaSach";
cbTacGia.DataSource = client.listAllAuthor();
cbTacGia.DisplayMember = "HoTen";
cbTacGia.ValueMember = "MaTacGia";
dgvTacGia.DataSource = client.listAllAuthor();
dgvThamGia.DataSource = client.listAllAttend();
}