protected void Page_Load(object sender, EventArgs e)
{
    handleExport();
}

void handleExport()
{
    // ...
    // Code to produce spreadsheet is here...
    // ...

    // Write the Excel spreadsheet from the MemoryStream and send it to the browser
    string saveAsFileName = sFileName + ".xlsx";

    Response.Clear();
    Response.ContentType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";
    Response.AddHeader("Content-Disposition", string.Format("attachment; filename={0}", saveAsFileName));
    workbook.Write(Response.OutputStream);
    Response.Flush();
    Response.Close();

    Page.ClientScript.RegisterOnSubmitStatement(typeof(Page), "closePage", "window.onunload = CloseWindow();");
}