گرفتن خروجی اکسل از گرید تلریک

void btnExport_Click(object sender, RoutedEventArgs e){

    string extension = "xls";

    SaveFileDialog dialog = new SaveFileDialog()

    {

        DefaultExt = extension,

        Filter = String.Format("{1} files (.{0})|.{0}|All files (.)|.", extension, "Excel"),

        FilterIndex = 1

    };

    if (dialog.ShowDialog() == true)

    {

        using (Stream stream = dialog.OpenFile())

        {

            gridViewExport.Export(stream,

             new GridViewExportOptions()

             {

                 Format = ExportFormat.Html,

                 ShowColumnHeaders = true,

                 ShowColumnFooters = true,

                 ShowGroupFooters = false,

             });

        }

    }

}