Select and export SQL table to Excel file from Web page

SqlDataAdapter da = new SqlDataAdapter("select * from Sample", con); // SqlDataAdapter da = new SqlDataAdapter( "select * from"+"'Dropdownlist1.selecteditem.text'"+"" ", con);
    DataSet ds = new DataSet();
    da.Fill(ds);

    System.IO.StringWriter stringWrite = new System.IO.StringWriter();

   System.Web.UI.HtmlTextWriter htmlWrite = new HtmlTextWriter(stringWrite);


   dgrid.DataSource = ds;


   htmlWrite.WriteLine("hello");

   dgrid.DataBind();//Data Grid Bind

   dgrid.RenderControl(htmlWrite);

   Response.Clear();
    string str="Student";
    Response.AddHeader("content-disposition", "attachment;filename='"+str+"'"+".xls");

    Response.Charset = "";

   Response.Cache.SetCacheability(HttpCacheability.NoCache);

   Response.ContentType = "application/vnd.xls";

   Response.Write(stringWrite.ToString());
   Response.End();