combobox select with taxand without ta

with tax and with out tax same total view that is error:

public void calc_total()

{

double ser_tax = 0, vat = 0, total_tax = 0, sub_tot = 0, dis_amt = 0, sc = 0, discount = 0, balance = 0;

if (textBox5.Text != "" && textBox5.Text != null && textBox5.Text != "0")

{

connection.Close();

connection.Open();

MySqlCommand cmd_tax = new MySqlCommand("select tax_cat from item_master where cat='1' limit 0,3", connection);

MySqlDataReader dread_tax = cmd_tax.ExecuteReader();

int x = 0;

sub_tot = Convert.ToDouble(textBox5.Text);

if (textBox6.Text == null || textBox6.Text == "." || textBox6.Text == "")

{

discount = 0.0;

}

else

{

discount = Convert.ToDouble(textBox6.Text);

}

if (discount <= 100)

{

dis_amt = ((discount / 100) * sub_tot);

balance = sub_tot - dis_amt;

if (balance < 0)

{

discount = 0;

dis_amt = ((discount / 100) * sub_tot);

}

sub_tot -= dis_amt;

while (dread_tax.Read())

{

if (x == 0)

{

x = 1;

ser_tax = ((Convert.ToDouble(dread_tax.GetValue(0).ToString()) / 100) * sub_tot);

textBox10.Text = ser_tax.ToString();

}

else if (x == 1)

{

x = 2;

vat = ((Convert.ToDouble(dread_tax.GetValue(0).ToString()) / 100) * sub_tot);

textBox11.Text = vat.ToString();

}

else

{

sc = ((Convert.ToDouble(dread_tax.GetValue(0).ToString()) / 100) * sub_tot);

textBox15.Text = sc.ToString();

}

dread_tax.Close();

connection.Close();

}

double tt2 = 0;

if (comboBox4.SelectedIndex == 1)

{

ser_tax = vat = sc = 0;

}

// total_tax = ser_tax + vat + sc;

// total_tax += sub_tot;

int rowcount = dataGridView1.Rows.Count;

double total = 0;

double service = 0, vat1 = 0;

int i;

for (i = 0; i < rowcount; i++)

{

double val3 = Convert.ToDouble(dataGridView1.Rows[i].Cells[13].Value);

total += val3;

double tt = Convert.ToDouble(dataGridView1.Rows[i].Cells[11].Value);

service += tt;

double tt1 = Convert.ToDouble(dataGridView1.Rows[i].Cells[12].Value);

vat1 += tt1;

}

i = 0;

// textBox5.Text = total.ToString();

//textBox10.Text = service.ToString();

//textBox11.Text = vat1.ToString();

textBox10.Text = string.Format("{0:0.00}", service);

textBox11.Text = string.Format("{0:0.00}", vat1);

textBox15.Text = string.Format("{0:0.00}", sc);

//textBox10.Text = string.Format("{0:0.00}", ser_tax);

//textBox11.Text = string.Format("{0:0.00}", vat);

//textBox15.Text = string.Format("{0:0.00}", sc);

total_tax = service + vat1 + sc;

total_tax += sub_tot;

if (textBox8.Text != null && textBox8.Text != "")

{

dis_amt = Convert.ToDouble(textBox8.Text);

}

double roundoff = 0;

double voucher = 0;

if (textBox14.Text != null && textBox14.Text != "")

{

if (textBox14.Text == "-" || textBox14.Text == "-." || textBox14.Text == ".")

{

roundoff = 0;

}

else

{

roundoff = Convert.ToDouble(textBox14.Text);

}

if (textBox22.Text != null && textBox22.Text != "")

{

if (textBox22.Text == "-" || textBox22.Text == "-." || textBox22.Text == ".")

{

voucher = 0;

}

else

{

voucher = Convert.ToDouble(textBox22.Text);

}

double tot_val = ((total - ((voucher)) + roundoff) - dis_amt);

textBox12.Text = string.Format("{0:0.00}", tot_val);

get_cash_card();

}