Auto generate Number in Winform TextBox

Question

 

private int SNo = 0;

 private void serial_Enter(object sender, EventArgs e)
        {
           SNo++;
 serial.Text = SNo.ToString();
 serial.Focus();
        }

 

i Will Save Serial No 1 than after Saving show me value 2 

but I will closed Form than Reopen Form show me again value 1 

than how to check already value save and give me 2nd One