yes I have solved this myself, take a look here :
private void insertitem_toBill()
{
DataGridViewRow row = new DataGridViewRow();
row.CreateCells(this.dgvBill, ID, NameTxtBox.Text, outPriceTxtBox.Text, PriceTax.Text, QuantityTxtBox.Text, TaxTxtBox.Text);
if (dgvBill.Rows.Count == 0)
{
this.dgvBill.Rows.Add(row);
}
else
{
bool SAME = false;
for (int i = 0; i < dgvBill.Rows.Count; ++i)
{
if (dgvBill.Rows[i].Cells[0].Value.ToString() == ID)
{
dgvBill.Rows[i].Cells[4].Value = (Convert.ToDouble(dgvBill.Rows[i].Cells[4].Value) + Convert.ToDouble(QuantityTxtBox.Text)).ToString();
SAME = true;
}
try
{
decimal value1 = Convert.ToDecimal(dgvBill.Rows[i].Cells[3].Value);
decimal value2 = Convert.ToDecimal(dgvBill.Rows[i].Cells[4].Value);
dgvBill.Rows[i].Cells[6].Value = value1 * value2;
}
catch{}
}
if (SAME == false)
{
this.dgvBill.Rows.Add(row);
}
}
decimal sum = 0;
for (int i = 0; i < dgvBill.Rows.Count; ++i)
{
sum += Convert.ToDecimal(dgvFatura.Rows[i].Cells[6].Value);
}
TotalTxtBox.Text = sum.ToString();
SearchProductTxtBox.Text = "";
}
[Edit]Code block added[/Edit]