textbox in datagrid rows and set it for any rows

Question

hi
i have a datagrid that include a textbox in every rows

datagrid is a shopping cart and textbox is for get Quantity of each product

how can i do that users set Quantity in the textbox and sum that with total amount?

here is my code
but there is error

code of buyproducts page

 using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Data.OleDb;

public partial class product_details : System.Web.UI.Page
{
    protected static string productName, productcost, productQuantity;
    protected DataTable objDT;
    protected DataRow objDR;
    protected string product;
    protected Boolean blnMatch = false;
    protected int quantity = 1;

    protected void Page_Load(object sender, EventArgs e)
    {

        if (!IsPostBack)
            makeCart();

        string m;
        m = Request["id"];

        string c = "provider=microsoft.jet.oledb.4.0;data source=" + Server.MapPath("shop.mdb");
        OleDbConnection con = new OleDbConnection(c);
        string k;
        k = "select * from MyProducts where code =" + m;
        OleDbCommand cmd = new OleDbCommand(k, con);
        OleDbDataReader dr;
        con.Open();
        dr = cmd.ExecuteReader();
        dr.Read();
        Page.Header.Title = dr["title"].ToString();
        productName = dr["title"].ToString();
        productcost = dr["cost"].ToString();
        con.Close();
    }

    protected void makeCart()
    {
        objDT = new System.Data.DataTable("Cart");
        objDT.Columns.Add("ID", typeof(int));
        objDT.Columns["ID"].AutoIncrement = true;
        objDT.Columns["ID"].AutoIncrementSeed = 1;

        objDT.Columns.Add("Quantity", typeof(int));
        objDT.Columns.Add("Product", typeof(string));
        objDT.Columns.Add("Cost", typeof(decimal));

        Session["Cart"] = objDT;
    }

    public Decimal GetItemTotal()
    {
        DataTable objDT = (DataTable)Session["Cart"];
        Decimal costprod;
        int intCounter, quantityprod;
        Decimal decRunningTotal = 0;
        for (intCounter = 0; intCounter <= objDT.Rows.Count - 1; intCounter++)
        {
            objDR = objDT.Rows[intCounter];
            costprod = (Decimal)objDR["Cost"];
            quantityprod = int.Parse(objDR["Quantity"].ToString());
            decRunningTotal += costprod * quantityprod;
        }
        return decRunningTotal;
    }
    protected void AddToCart(object sender, ImageClickEventArgs e)
    {
        UserControl cart = (UserControl)Master.FindControl("cart1");
        DataGrid mydg = (DataGrid)cart.FindControl("dg");
        Label lbl = (Label)cart.FindControl("lblTotal");
        TextBox txtQuantity = (TextBox)mydg.FindControl("TextBox1");
        
        //product = productName;
        DataTable objDT = (DataTable)Session["Cart"];
       
        foreach (DataRow objDR in objDT.Rows)
        {
            txtQuantity.Text = "1";
            // TextBox txtQuantity = (TextBox)mydg.FindControl("TextBox1");
            objDR["Quantity"] = txtQuantity;
            if (objDR["product"] == productName)
            {
              //  objDR["Quantity"] = int.Parse(objDR["Quantity"].ToString()) + int.Parse(TextBox1.Text);
               // txtQuantity.Text = objDR["Quantity"].ToString();
               // objDR["Quantity"] += TextBox1.Text;
                objDR["Quantity"] = int.Parse(objDR["Quantity"].ToString()) + int.Parse(txtQuantity.Text);
                blnMatch = true;
           }
        }
        if (blnMatch == false)
        {
            objDR = objDT.NewRow();
            objDR["Quantity"] =1;
          //  txtQuantity.Text = objDR["Quantity"].ToString();
            objDR["Product"] = productName;
            objDR["Cost"] = Decimal.Parse(productcost);
            objDT.Rows.Add(objDR);
        }

        Session["Cart"] = objDT;
        if (objDT != null)
        {
            mydg.DataSource = objDT;
            mydg.DataBind();
            lbl.Text = "????" + GetItemTotal();
        }
        else
        {
            Response.Write("Cart is Null");
        }
    }

i has problem with Quantity

here is cart usercontrol that is my shopping cart

 using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;

public partial class cart : System.Web.UI.UserControl
{
    DataTable objDT;
    DataRow objDR;
    string product;
    Boolean blnMatch = false;
    protected static string Reservation_Number,Redirect_URL,Total_Amount,Merchant_ID;

    protected void Page_Load(object sender, EventArgs e)
    {
      //  foreach (DataRow objDR in objDT.Rows)
       // {
           // TextBox txt = (TextBox)dg.FindControl("TextBox1");
           // txt.Text = "100";
        //}
    }

    public Decimal GetItemTotal()
    {
        DataTable objDT = (DataTable)Session["Cart"];
        Decimal costprod, quantityprod;
        int intCounter;
        Decimal decRunningTotal = 0;
        for (intCounter = 0; intCounter <= objDT.Rows.Count - 1; intCounter++)
        {
            objDR = objDT.Rows[intCounter];
            costprod = (Decimal)objDR["Cost"];
            quantityprod = int.Parse(objDR["Quantity"].ToString());
            decRunningTotal += costprod * quantityprod;
        }
        return decRunningTotal;
    }


    protected void TextBox1_TextChanged(object sender, EventArgs e)
    {
        foreach (DataRow objDR in objDT.Rows)
        {
            TextBox txt = (TextBox)dg.FindControl("TextBox1");
            objDR["Quantity"] = txt.Text;
            lblTotal.Text = "????" + GetItemTotal();
        }
      
    }
    protected void Delete_Item(object source, DataGridCommandEventArgs e)
    {
        objDT = (DataTable)Session["Cart"];
        //  objDT.Rows[e.RowIndex].Delete();
        objDT.Rows[e.Item.ItemIndex].Delete();
        Session["Cart"] = objDT;
        dg.DataSource = objDT;
        dg.DataBind();
        lblTotal.Text = "????" + GetItemTotal();
    }
}

my cart design is here

 <%@ Control Language="C#" AutoEventWireup="true" CodeFile="cart.ascx.cs" Inherits="cart" %>
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>
<asp:DataGrid ID="dg" runat="server" OnDeleteCommand="Delete_Item" ShowHeader="False">
    <Columns>
        <asp:TemplateColumn>
            <ItemTemplate>
                <asp:ImageButton ID="ImageButton1" runat="server" CommandName="Delete" ImageUrl="~/picture/delrecord.png" />
            </ItemTemplate>
        </asp:TemplateColumn>
        <asp:TemplateColumn>
            <ItemTemplate>
                <asp:TextBox ID="TextBox1" runat="server" OnTextChanged="TextBox1_TextChanged" Style="font-size: 9pt;
                    color: #000000; direction: rtl; font-family: tahoma; text-align: center" Width="65px"></asp:TextBox>
            </ItemTemplate>
        </asp:TemplateColumn>
    </Columns>
</asp:DataGrid>
Total Amount :&nbsp;
<asp:Label ID="lblTotal" runat="server"></asp:Label>
<br />
<br />

i can't set the txtQuantity

in this code i set it to 1 that there isn't any problem in add first product
but if i add same product again
error : object not set to an instance ....