Sumation of item quantity and total sales column

Thank you team, it is working now 

public void ADDToLIST()
{
    try
    {
        using (SqlConnection cn = new SqlConnection(dbcon.MyConnection()))
        {
            cn.Open();
            using (SqlCommand cm = new SqlCommand("select ProductId, PCode, ProductDate, ProductName, Category, CostPrice, Profit, SalesPrice, Tax, TotalPrice, StockIn, Barcode from tblProduct where Barcode like @searchBarcode", cn))
            {
                cm.Parameters.AddWithValue("@searchBarcode", "%" + txt_Search_Product_Barcode.Text + "%");
                using (SqlDataReader dr = cm.ExecuteReader())
                {
                    if (txt_Search_Product_Barcode.Text == String.Empty)
                    {
                        return;
                    }

                    while (dr.Read())
                    {
                        string productId = dr["ProductId"].ToString();
                        // Check if the product already exists in the dataGridView1
                        bool productExists = dataGridView1.Rows.Cast().Any(row => row.Cells[0].Value.ToString() == productId);

                        if (productExists)
                        {
                            // If the product exists, increase the quantity and calculate the new total price
                            int rowIndex = dataGridView1.Rows.Cast().ToList().FindIndex(row => row.Cells[0].Value.ToString() == productId);
                            decimal currentQuantity = Convert.ToDecimal(dataGridView1.Rows[rowIndex].Cells[7].Value);
                            dataGridView1.Rows[rowIndex].Cells[7].Value = currentQuantity + 1;
                            decimal saleprice = Convert.ToDecimal(dr["SalesPrice"].ToString());
                            decimal tax = Convert.ToDecimal(dr["Tax"].ToString());
                            decimal newTotalPrice = Convert.ToDecimal(saleprice * (tax) / 100) + saleprice;
                            dataGridView1.Rows[rowIndex].Cells[8].Value = newTotalPrice.ToString("#,##0.00");
                        }
                        else
                        {
                            // If the product doesn't exist, add a new row
                            string pcode = dr["PCode"].ToString();
                            string productname = dr["ProductName"].ToString();
                            string category = dr["Category"].ToString();
                            decimal saleprice = Convert.ToDecimal(dr["SalesPrice"].ToString());
                            decimal tax = Convert.ToDecimal(dr["Tax"].ToString());
                            string totalprice = dr["TotalPrice"].ToString();
                            decimal totalqtyprice = Convert.ToDecimal(saleprice * (tax) / 100) + saleprice;
                            dataGridView1.Rows.Add(productId, pcode, productname, category, saleprice, tax, totalprice, 1, totalqtyprice.ToString("#,##0.00"));
                        }

                        txt_Search_Product_Barcode.Clear();
                        txt_Search_Product_Barcode.Focus();
                    }
                }
            }
        }
    }
    catch (Exception ex)
    {
        if (dr != null && !dr.IsClosed)
        {
            dr.Close();
        }
        if (cn.State == ConnectionState.Open)
        {
            cn.Close();
        }
        MessageBox.Show(ex.Message, "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
    }
}

The issue is that you're directly adding a new row to your dataGridView1 every time you scan a barcode, without checking if the product already exists in the grid.

To fix this, you need to check if the product already exists in the grid before adding a new row. If it does, you can simply increase the quantity and calculate the new total price.

This code first checks if the product already exists in the grid by comparing the ProductId of the scanned product with the ProductId of the products in the grid. If the product exists, it increases the quantity and calculates the new total price. If the product doesn't exist, it adds a new row to the grid. 

I was searching on how to go about and I found this code in C++ but I didn't know how to convert it  into C#

Dim exist As Booleen = false, numrow As Integer = 0, numtext As Integer
For Each item As DataGridViewRow In DataGridView.Rows
if item.Cells(0).Value IsNot Nothing Then
if item.Cells(1).Value.ToString = txt_SearchProduct_Barcode.Text then
exist = True
numtext = CInt(item.Cells(7)
Exit for
End If
End If
Next
If exist = False then
Try
cn.Open()
cm = new SqlCommand()
End Try
End If
End Sub