为什么Excel在生成PDF后不关闭

我使用此代码将excel文件转换为PDF。问题是Excel进程未关闭。我错过了什么？

        protected void Indexchanged_ConvertPDF(Object sender, EventArgs e)
    {
        Microsoft.Office.Interop.Excel.Application xlApp = new Microsoft.Office.Interop.Excel.Application();
        xlApp.DisplayAlerts = false;
        xlApp.Visible = false;
        xlApp.ScreenUpdating = false;
        string path = CertificadosPresion.SelectedRow.Cells[0].Text;
        string CertName = CertificadosPresion.SelectedDataKey.Value.ToString();
        Workbook xlWorkbook = xlApp.Workbooks.Open(path);
        xlWorkbook.ExportAsFixedFormat(Microsoft.Office.Interop.Excel.XlFixedFormatType.xlTypePDF, @"C:'pdf'" + SALESID.Text + "_CertPres.pdf", 0, false, true);
        xlWorkbook.Close();
        xlApp.Quit();
        xlApp = null;
        xlWorkbook = null;
        DisposeCOMObject(xlWorkbook);
        DisposeCOMObject(xlApp);
    }