Problem update quantity in ShoppingCart

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C#

ASP.NET

SQL-Server

The quantity of products incorrect after update the product.
Ex:select Product 001->quantity:1
Product 002->add 002,quantity:1
Product 002->001's quantity:2 and 002's quantity:1

My ShopCart class:

DataView dv=new DataView();
    int rIndex=0;
    DataRow dr;

    public DataTable MakeCart()
    {
        DataTable tb = new DataTable("ShopCart");
        tb.Columns.Add("ProductID");
        tb.Columns.Add("quantity");
        tb.Columns.Add("price");
        tb.Columns.Add("total");
        return tb;
    }

    public DataTable AddCart(string ProductID,int price)
    {
        DataTable tbCart = new DataTable();
        tbCart = (DataTable)HttpContext.Current.Session["Cart"];
        if (tbCart == null)
        {
            tbCart = MakeCart();
        }
        dv = tbCart.DefaultView;
        dv.Sort = "ProductID";
        rIndex = dv.Find(ProductID);
        if (rIndex != -1)
        {
            tbCart.Rows[rIndex][1] = Convert.ToInt32(tbCart.Rows[rIndex][1]) + 1;
            tbCart.Rows[rIndex][3] = Convert.ToInt32(tbCart.Rows[rIndex][1]) * Convert.ToInt32(tbCart.Rows[rIndex][2]);
        }
        else
        {
            dr = tbCart.NewRow();
            dr[0] = ProductID;
            dr[1] = 1;
            dr[2] = price;
            dr[3] = price;
            //tbCart.AcceptChanges();
            tbCart.Rows.Add(dr);
        }
        return tbCart;
    }

event when click Add button:

protected void DataList1_ItemCommand(object source, DataListCommandEventArgs e)
    {
        DataTable tb = new DataTable();
        string name = e.CommandName;
        if (name == "Add")
        {
                Label IDP = (Label)e.Item.FindControl("lblProductID");
                IDProduct = IDP.Text;
                tb = scart.AddCart(IDProduct, 20000);
                
        }
        Session["Cart"] = tb;
        Response.Redirect("Cart.aspx");
    }