[DllImport("User32.dll" , CharSet = CharSet.Auto)]
        private static extern int GetWindowThreadProcessId(IntPtr hwnd , out int ID);

        //End the process of xlApp
        public void KillExcelProcess()
        {
            //get the hwnd(t) of excel, it is the entrance of memory used by excel
            IntPtr t = new IntPtr(xlApp.Hwnd);
            int k = 0;
            //get the id(k) of process by hwnd(t)
            GetWindowThreadProcessId(t , out k);
            //get the reference of process(p) by id(k)
            Process p = Process.GetProcessById(k);
            //kill the process
            p.Kill();
        }

        public void KillExcelProcess2(string fileName)
        {
            Process[] procList = Process.GetProcesses();
            IntPtr t = new IntPtr(xlApp.Hwnd);
            int k = 0;
            //get the id(k) of process by hwnd(t)
            GetWindowThreadProcessId(t, out k);

            foreach (Process proc in procList)
            {
                if (k == proc.Id)
                    proc.Kill();
            }
        }