Export GridView to ReadOnly Excel File in ASP.Net

I have generated a Excel report using C#,which generated fine.now i need to do all of those field is to be noneditable.how could i do it

Here my Export to Excel COde below:

#region Export to Excel

        if (dt.Rows.Count > 0)
        {
            string filename = reportName + ".xls";
            System.IO.StringWriter tw = new System.IO.StringWriter();
            System.Web.UI.HtmlTextWriter hw = new System.Web.UI.HtmlTextWriter(tw);
            GridView dgGrid = new GridView();
            dgGrid.DataSource = dt;
            dgGrid.DataBind();
            for (int i = 0; i < dgGrid.Rows.Count; i++)
            {
                dgGrid.Rows[i].Cells[4].Attributes.Add("style", "mso-number-format:\\@");
            }
            //Get the HTML for the control.
            dgGrid.RenderControl(hw);
            //Write the HTML back to the browser.
            Response.ContentType = "application/vnd.ms-excel";
            Response.AppendHeader("Content-Disposition", "attachment; filename=" + filename + "");
           
            this.EnableViewState = false;
            Response.Write(tw.ToString());
            Response.End();
        }

        #endregion

please let me know as early as possible.

thanks in advance