export gridview with columns hide

Make enableEventValidation="false"

 protected void Imageexcel_Click(object sender, ImageClickEventArgs e)
    {


        GridView1.Columns[0].Visible = false;
        GridView1.Columns[8].Visible = false;
    

        string attachment = "attachment; filename=patientlist.xls";
        Response.ClearContent();
        Response.AddHeader("content-disposition", attachment);
        Response.ContentType = "application/ms-excel";
        StringWriter sw = new StringWriter();
        HtmlTextWriter htw = new HtmlTextWriter(sw);
        // HttpContext.Current.Response.Write("<img src='http://ghrce.edu.in/pic/adminlogin.JPG'>");
        //HttpContext.Current.Response.Write("<br><br><br><br><br><br><br>");    
        HttpContext.Current.Response.Write("<br>");
        // Create a form to contain the grid  

        HtmlForm frm = new HtmlForm();
     
        GridView1.Parent.Controls.Add(frm);
        frm.Attributes["runat"] = "server";
        frm.Controls.Add(GridView1);
        frm.RenderControl(htw);
        //GridView1.RenderControl(htw);
        Response.Write(sw.ToString());
        Response.End();
    }