Radgrid Excel导出循环

我尝试使用radgrid选项将数据导出到excel，它在单个导出上工作良好。但我试着在每个循环中导出，最后一个文件数据只导出。是否可以在循环中导出?

protected void btnexport_Click(object sender, EventArgs e)
{
            string selectedimportfilesid = "";
            IList<RadListBoxItem> collection = ListBoxFileNames.CheckedItems;
            string selectedfilename = "";
            DataTable dtselect = new DataTable();
            foreach (RadListBoxItem item in collection)
            {
                Label filename = (Label)item.FindControl("lblfilename");
                Label Importfileid = (Label)item.FindControl("lblimportfileid");
                selectedfilename = filename.Text;
                selectedimportfilesid = Importfileid.Text;
                dtselect = selectedexportfilelist(selectedimportfilesid);
                testgrid.DataSource = dtselect; 
                testgrid.DataBind();
                testgrid.ExportSettings.IgnorePaging = true;
                testgrid.ExportSettings.FileName = selectedfilename 
                testgrid.ExportSettings.OpenInNewWindow = true;
                testgrid.ExportSettings.Excel.FileExtension = "xls";
                testgrid.MasterTableView.ExportToExcel();
            }
}