private void btnThem_Click(object sender, EventArgs e)
{
MoDieuKien();
SetNull();
themmoi = true;
int ma;
if (bus.CountID() == 1)
{
ma = (int.Parse(bus.GetLastID()) + 1);
}
else
{
ma = 0;
}
txtMaNV.Text = ma.ToString();
}