how to resolve this error in my code for Export to Excel from GridView using c# asp.net?

public void ExportDetails()
{
 SqlConnClass connClass = new SqlConnClass();
        DataTable dt = connClass.GetQrCodeDetails();
        GridView gv = new GridView();
        gv.AutoGenerateColumns = false;

        BoundField ID = new BoundField();
        ID.HeaderText = "ID";
        ID.DataField = "ID";
        gv.Columns.Add(ID);

        BoundField QRIMAGE = new BoundField();
        IMAGE.HeaderText = "IMAGE";
        IMAGE.DataField = "IMAGE";
        gv.Columns.Add(IMAGE);

        StreamWriter sw = new StreamWriter(); //Error is showing in this line.
        HtmlTextWriter htw = new HtmlTextWriter(sw);
        gv.RenderControl(htw);
        Response.Write(htw);
ExportTableData(sw.ToString());
}

public void ExportTableData(string data)
    {
        string attach = "attachment;filename=CodeDetails.xls";
        Response.ClearContent();
        Response.AddHeader("content-disposition", attach);
        Response.ContentType = "application/ms-excel";

        Response.Write(data);
        Response.End();
    }