Implementing Cumulative total and Select All functionality with Events in ASP.NET GridView

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.SqlClient;

public partial class Default3 : System.Web.UI.Page
{
    SqlConnection conn = null;
    double lgTots = 0;

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            BindGridView();
        }
    }
    protected void GridView1_PageIndexChanging(object sender, GridViewPageEventArgs e)
    {
        GridView1.PageIndex = e.NewPageIndex;
        BindGridView();
    }

    protected void GridView1_RowEditing(object sender, GridViewEditEventArgs e)
    {
        GridView1.EditIndex = e.NewEditIndex;
        BindGridView();
    }
    protected void GridView1_RowCancelingEdit(object sender, GridViewCancelEditEventArgs e)
    {
        GridView1.EditIndex = -1;
        BindGridView();
    }
    protected void GridView1_RowDeleting(object sender, GridViewDeleteEventArgs e)
    {
        int id = (int)GridView1.DataKeys[e.RowIndex].Value;
        try
        {
            using (conn = new SqlConnection(ConfigurationManager.ConnectionStrings["tempdbConn"].ConnectionString))
            {
                using (SqlCommand cmd = conn.CreateCommand())
                {
                    cmd.CommandType = CommandType.Text;
                    cmd.CommandText = "Delete from tmp_table where id=@id";
                    cmd.Parameters.AddWithValue("@id", id);
                    conn.Open();
                    cmd.ExecuteNonQuery();
                }
            }
        }
        catch (Exception ex)
        {
            throw new Exception(ex.Message);
        }
        finally
        {
            conn.Close();
        }
        BindGridView();
    }
    protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
    {
        if (e.Row.RowType == DataControlRowType.DataRow)
        {
            lgTots += Convert.ToDouble(DataBinder.Eval(e.Row.DataItem, "totalMarks"));
        }
        if (e.Row.RowType == DataControlRowType.Footer)
        {
            Label lgTotal = (Label)e.Row.FindControl("lblGrandTotal");
            lgTotal.Text = lgTots.ToString();
        }
    }
    protected void BindGridView()
    {
        DataTable dt = null;
        using (conn = new SqlConnection(ConfigurationManager.ConnectionStrings["tempdbConn"].ConnectionString))
        {
            using (SqlCommand cmd = conn.CreateCommand())
            {
                cmd.CommandType = CommandType.Text;
                cmd.CommandText = "select row_number() OVER (ORDER BY id) AS sno,id,uname,totalMarks,selectedItem from tmp_table";
                using (SqlDataAdapter da = new SqlDataAdapter(cmd))
                {
                    dt = new DataTable();
                    da.Fill(dt);
                }
            }
        }
        GridView1.DataSource = dt;
        GridView1.DataBind();
    }

    protected void GridView1_RowUpdating(object sender, GridViewUpdateEventArgs e)
    {
        int sid = Convert.ToInt32(GridView1.DataKeys[e.RowIndex].Value.ToString());
        string nme = string.Empty;
        TextBox tb = GridView1.Rows[e.RowIndex].Cells[3].FindControl("txtName") as TextBox;

        if (tb != null)
        {
            nme = tb.Text.Trim();
        }

        string mrks = string.Empty;
        tb = null;
        tb = GridView1.Rows[e.RowIndex].Cells[4].FindControl("txtTotalMarks") as TextBox;
        if (tb != null)
        {
            mrks = tb.Text.Trim();
        }

        CheckBox chk = GridView1.Rows[e.RowIndex].Cells[0].FindControl("chekSelect") as CheckBox;

        using (conn = new SqlConnection(ConfigurationManager.ConnectionStrings["tempdbConn"].ConnectionString))
        {
            using (SqlCommand cmd = conn.CreateCommand())
            {
                cmd.CommandType = CommandType.Text;
                cmd.CommandText = "UPDATE tmp_table SET uname = @nme, totalMarks = @mrks,selectedItem=@selectItem WHERE id = @sid";
                cmd.Parameters.AddWithValue("@nme", nme);
                cmd.Parameters.AddWithValue("@mrks", mrks);
                cmd.Parameters.AddWithValue("@selectItem", (chk != null) ? chk.Checked : false);
                cmd.Parameters.AddWithValue("@sid", sid);
                conn.Open();
                cmd.ExecuteNonQuery();
            }
        }
        GridView1.EditIndex = -1;
        BindGridView();
    }

    protected void chekSelectALL_CheckedChanged(object sender, EventArgs e)
    {
        CheckBox chkA = GridView1.HeaderRow.FindControl("chekSelectALL") as CheckBox;
        foreach (GridViewRow gv in GridView1.Rows)
        {
            CheckBox chkS = gv.FindControl("chekSelect") as CheckBox;
            if (chkA.Checked)
            {
                chkS.Checked = true;
            }
            else
            {
                chkS.Checked = false;
            }
        }        
    }
}