ASP.NET

<td>
            <asp:UpdatePanel ID="UpdatePanel2" runat="server" UpdateMode="Conditional">
                <ContentTemplate>                
            <asp:DropDownList ID="CompanyList" runat="server" 
                Height="22px" Width="155px" AutoPostBack="True" AppendDataBoundItems="true"                
                        onselectedindexchanged="CompanyList_SelectedIndexChanged" >
                        <asp:ListItem Selected ="True" Text="--Please Select--"></asp:ListItem>
            </asp:DropDownList>  
            <br />
            <asp:UpdateProgress ID="UpdateProgress2" AssociatedUpdatePanelID="UpdatePanel2"  runat="server" >
            <ProgressTemplate>
            <span class="ProgressBar">
            Data Loading. Please Wait....</span></ProgressTemplate>
            </asp:UpdateProgress>
                   
             <asp:DropDownList ID="ProjectList" runat="server" AppendDataBoundItems="true"
                Height="23px" Width="155px" AutoPostBack="True" 
                        onselectedindexchanged="ProjectList_SelectedIndexChanged" >    
                <asp:ListItem Selected ="True" Text="--Please Select--"></asp:ListItem>         
            </asp:DropDownList>
            <br />           
            <asp:DropDownList ID="UserTypeList" runat="server" AppendDataBoundItems="true" 
                Width="155px">
                <asp:ListItem Selected="True" Text="--Please Select--"></asp:ListItem>
            </asp:DropDownList>
           
            <br />
             </ContentTemplate>
            </asp:UpdatePanel>                      
           </td>

protected void BtnUpdate_Click(object sender, EventArgs e)
{
    dt = (DataTable)Session["Cart"];
    CheckBox chkSelected = new CheckBox();

    foreach (DataGridItem dgItem in DataGrid2.Items)
    {
        chkSelected = (CheckBox)dgItem.FindControl("chkSelection");

        if (chkSelected.Checked == true)
        {


            TextBox TxtQTY = (TextBox)dgItem.Cells[2].FindControl("TxtQTY");
            int Txt2 = Convert.ToInt32(TxtQTY.Text);

            Label lblProduct_Id = (Label)dgItem.Cells[0].FindControl("lblProduct_Id");
            string id2 = lblProduct_Id.Text;

            Label lblDescription = (Label)dgItem.Cells[1].FindControl("lblDescription");
            string desc2 = lblDescription.Text;

            Label lblPrice = (Label)dgItem.Cells[3].FindControl("lblPrice");
            float price2 = Convert.ToSingle(lblPrice.Text);

            Label lblCost = (Label)dgItem.Cells[3].FindControl("lblCost");
            float cost2 = Convert.ToSingle(lblCost.Text);

            float final_cost = price2 * Txt2;

              DataRow myrow = dt.NewRow();
              myrow["REF"] = id2;
              myrow["Description"] = desc2;
              myrow["QTY"] = Txt2;
              myrow["Price"] = price2;
              myrow["Cost"] = final_cost;

              dt.Rows.Add(myrow);
              dt.AcceptChanges();
              Session["data"] = dt;
        }

    }
    DataGrid2.DataSource = dt;
    DataGrid2.DataBind();
}

if (file.Exists)
{
 Response.Clear();
 Response.AddHeader("Content-Disposition", "attachment; filename=" + fileName);
 Response.AddHeader("Content-Length", file.Length.ToString());
 Response.ContentType = "application/octet-stream";
 Response.WriteFile(file.FullName);            
 Response.End();
}