add multiple rows to gridview

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ASP.NET

hello sir.
i have a gridview with 15 boundfields. i want fill gridview with textboxes with submit button. i filled one row. but i want to create a new row and add that new row to gridview below first row. to do that how can i write the code.
plz see my code and help me.
thanking you.

DataTable dt = new DataTable();
    protected void GVProductOrderBind()
    {
        GVProductOrder.DataSource = dt;
        GVProductOrder.DataBind();
        Session["GVTable"] = dt;
        
    }
    protected void Page_Load(object sender, EventArgs e)
    {
        try
        {
            int i = GVProductOrder.Rows.Count;
            if (i == 0)
            {
                //dt.Columns.Add("ItemCode", System.Type.GetType("System.String"));
                DataRow dr = dt.NewRow();
                dt.Columns.Add("ItemCode");
                dt.Columns.Add("ItemName");
                dt.Columns.Add("EanCode");
                dt.Columns.Add("Cost");
                dt.Columns.Add("MRP");
                dt.Columns.Add("SP");
                dt.Columns.Add("MBQ");
                dt.Columns.Add("Quantity");
                dt.Columns.Add("FreeQty");
                dt.Columns.Add("FixedDisc");
                dt.Columns.Add("Tax");
                dt.Columns.Add("Value");
                dt.Columns.Add("OnOrder");
                dt.Columns.Add("Remarks");
                dt.Columns.Add("Type");

                dt.Rows.Add(dr);
                GVProductOrderBind();

                //Session["tab"] = dt;
            }
            else
            {
                //dt.Rows.Add(row);               
                if (Session["GVTable"] != null)
                {
                    ((DataTable)Session["GVTable"]).Merge(dt);
                }

                //for (int j = 0; j <= i + 1; j++)
                //{
                //    if (j == (i + 1))
                //    {
                //        DataRow dr1;
                //        dr1 = dt.NewRow();
                //        dr1["slno"] = j;
                //        dt.Rows.Add(dr1);
                //        GVProductOrder.DataSource = dt;
                //        GVProductOrder.DataBind();
                //    }

                //}
            }
        }
        catch (Exception ex)
        {
            throw ex;
        }
        
        //DataSet ds = new DataSet();
        //ds = BusinessLogic.SP_GridDetails();
        //GVProductOrder.DataSource = ds;
        //GVProductOrder.DataBind();
    }
    protected void BtnClose_Click(object sender, EventArgs e)
    {
        Response .Redirect ("~/Welcome.aspx");
    }
    protected void BtnSave_Click(object sender, EventArgs e)
    {
        try
        {
            
                DataRow drNew = dt.NewRow();
                
                drNew["ItemCode"] = TxtItemCode.Text;
                drNew["ItemName"] = TxtItemDescription.Text;
                drNew["EanCode"] = TxtEanCode.Text;
                drNew["Cost"] = TxtCP.Text;
                drNew["MRP"] = TxtMRP.Text;
                drNew["SP"] = TxtSP.Text;
                drNew["MBQ"] = TxtACP.Text;
                drNew["Quantity"] = TxtQty.Text;
                drNew["FreeQty"] = TxtFreeQty.Text;
                drNew["FixedDisc"] = TxtDiscPercentage.Text;
                drNew["Tax"] = TxtTaxPercentage.Text;
                drNew["Value"] = TxtValue.Text;
                drNew["OnOrder"] = TxtOnOrder.Text;
                drNew["Remarks"] = TxtQOH.Text;
                drNew["Type"] = TxtUOP.Text;
               
                dt.Rows.Add(drNew);
                GVProductOrderBind();
            }
           
        
        catch (Exception ex)
        {
            throw ex;
        }