How to insert DataValueField into database.... from gridview ?

i am inserting multiple data into gridview from dropdown list ,i have bind dropdownlist data from database where DataTextField and DataValueField  are coming....Now i want to store DatavalueField in session of multiple row of gridview data,,then i insert bulk data into database...

here is my code

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Configuration;
using System.Data;
using System.Data.SqlClient;

namespace WebApplication1
{
    public partial class WebForm1 : System.Web.UI.Page
    {
        SqlConnection con = new SqlConnection("Data Source=DESKTOP-5PJ76B9;Integrated Security=SSPI;Initial Catalog=SPS;MultipleActiveResultSets=True;");
        protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                BindGrid();
                itemload();
            }
        }

        private void itemload()
        {
          
            con.Open();
            SqlDataAdapter adpr1 = new SqlDataAdapter("select * from ItemMasterFile ", con);
            DataSet dspr1 = new DataSet();
            adpr1.Fill(dspr1);
            DropDownList1.DataSource = dspr1.Tables[0];
            DropDownList1.DataTextField = "Descriptionitem";
            DropDownList1.DataValueField = "Codeitem";
            DropDownList1.DataBind();
        }

        private void BindGrid()
        {
            GridView1.DataSource = DataSource;
            GridView1.DataBind();
        }

        private DataTable DataSource
        {
            //-- this is the property that will retain the data source and data will keep on adding in there on button click
            get
            {
                DataTable dtSource = new DataTable();
                if (null != Session["__DataSource"])
                    dtSource = Session["__DataSource"] as DataTable;
                else
                    dtSource = GetDataFromDatabase();

                return dtSource;
            }

            set
            {
                Session["__DataSource"] = value;
            }
        }

        private DataTable GetDataFromDatabase()
        {
            //-- here is a sample code, that just generate data for Gridview, you will require to write code here to fetch data from Database.
            DataTable resultDt = new DataTable();
            resultDt.Columns.AddRange(
                new DataColumn[] {
        new DataColumn("Descriptionitem"),
        new DataColumn("QTY"),
            });

            for (int i = 0; i < 0; i++)
            {
                DataRow dr = resultDt.NewRow();
                //dr["Field1"] = "Field1 " + i.ToString();
                //dr["Field2"] = "Field2 " + i.ToString();
                //dr["Field3"] = "Field3 " + i.ToString();
                resultDt.Rows.Add(dr);
            }

            return resultDt;
        }

        protected void btnSave_Click(object sender, EventArgs e)
        {
            DataTable dtSource = DataSource;
            DataRow dr = dtSource.NewRow();
            dr["Descriptionitem"] = DropDownList1.SelectedItem.Text.Trim();

           // dr["Descriptionitem"] = DropDownList1.Text.Trim();
            dr["QTY"] = txtqty.Text.Trim();

            dtSource.Rows.Add(dr);
            DataSource = dtSource;

            BindGrid();

            txtqty.Text = string.Empty;
        }
    }
}

Please Consider