C#中通过com组件操作excel不能关闭的问题

问题：

当用如下代码操作完Excel，虽然调用了Application的Quit()方法，但发现Excel进程并没退出。

           object missing = System.Reflection.Missing .Value;
            Application app = new Application();
            app.Visible = false ;
            app.UserControl = true ;
            Workbook wb = app.Workbooks.Open(path, missing, true , missing, missing, missing, missing,
                        missing, missing, true , missing, missing, missing, missing, missing);
            Worksheet ws = (Worksheet )wb.Worksheets.get_Item(1);
            Console .WriteLine(ws.Cells[2,1].Value);
            ws.Close();
            ws = null;
            app.Quit();
            app = null;

结果：

        public static void ExcelRead(string path)
        {

            object missing = System.Reflection.Missing.Value;
            Application app = new Application();
            app.Visible = false;
            app.UserControl = true;
            Workbooks wbs = app.Workbooks;
            Workbook wb = wbs.Open(path, missing, true, missing, missing, missing, missing,
                        missing, missing, true, missing, missing, missing, missing, missing);
            Sheets wss = wb.Worksheets; 
            Worksheet ws = (Worksheet)wss.get_Item(1);
            Console.WriteLine(ws.Cells[2,1].Value);
            NAR(ws);
            ws = null;
            NAR(wss);
            wss = null;
            wb.Close();
            NAR(wb);
            wb = null;
            wbs.Close();
            NAR(wbs);
            wbs = null;
            app.Quit();
            NAR(app);
            app = null;
            GC.Collect();
        }
        private static void NAR(object o)
        {
            try
            {
                while (System.Runtime.InteropServices.Marshal.ReleaseComObject(o) > 0) ;

            }
            catch
            { }
            finally
            {
                o = null;
            }
        }