private void btnLapPhieuMoi_Click(object sender, EventArgs e)
 {
     i                      = 0;
     Tien                   = 0;
     txtTongtien.Text       = "0";
     btnGhiPhieu.Enabled    = true;
     btnLapPhieuMoi.Enabled = false;
     btnThemSach.Enabled    = false;
     cboNXB.Enabled         = true;
     txtSoPN.Text           = PhieuNhapBUS.GetIDPhieuNhap();
     txtMaSach.ResetText();
     txtTenSach.ResetText();
     txtSoLuong.ResetText();
     txtTacgia.ResetText();
     txtGiaNhap.ResetText();
     txtTheloai.ResetText();
     dtkNgayNhap.Value = DateTime.Now;
     dgvDanhSach.Rows.Clear();
 }