C# Excel操作的问题

我是通过这样的方式实现的的但是 执行完毕以后 excel文件被加了锁  进程里还有一个Excel的进程怎么解决呢
using System;
using System.Collections.Generic;
using System.Text;using Microsoft.Office.Core;
using ExcelAPI=Microsoft.Office.Interop.Excel;
using Microsoft.Office.Interop.Excel;
using System.Runtime.InteropServices;
namespace ExcelOrderReader
{
    public class ExcelDocument
    {
        string filePath;        private ExcelAPI.Workbook workbook;
        private ExcelAPI.Application app;        public ExcelDocument(string filePath)
        {
            this.filePath = filePath;
        }        public ExcelAPI.Workbook OpenDocument()
        {
            return this.openDocument(filePath);
        }        private ExcelAPI.Workbook openDocument(string fileName)
        {
            object MissingValue = Type.Missing;
            this.app = new ExcelAPI.Application();
            workbook = app.Workbooks.Open(fileName, MissingValue,
MissingValue, MissingValue, MissingValue,
MissingValue, MissingValue, MissingValue,
MissingValue, MissingValue, MissingValue,
MissingValue, MissingValue, MissingValue,
MissingValue);
            return workbook;
        }        public void CloseDocument()
        {
            try
            {
                if (workbook!=null)
                {
                    for (int i = 1; i < workbook.Sheets.Count; i++)
                    {
                        ExcelAPI.Worksheet workSheet = (ExcelAPI.Worksheet)workbook.Sheets[i];
                        System.Runtime.InteropServices.Marshal.ReleaseComObject(workSheet);
                        workSheet = null;
                    }
                    workbook.Close(true, null, null);
                    System.Runtime.InteropServices.Marshal.ReleaseComObject(workbook);
                    workbook = null;                    if (app != null)
                    {
                        app.Workbooks.Close();
                        app.Quit();
                        KillSpecialExcel(app);
                        System.Runtime.InteropServices.Marshal.ReleaseComObject(app);
                        app = null;                    }
                }
            }
            catch (Exception)
            {
                throw;
            }
            finally
            {
                GC.Collect();
            }        }        #region 结束EXCEL.EXE进程的方法
        /// <summary>
        /// 结束EXCEL.EXE进程的方法
        /// </summary>
        /// <param name="m_objExcel">EXCEL对象</param>
        [DllImport("user32.dll", SetLastError = true)]
        static extern int GetWindowThreadProcessId(IntPtr hWnd, out int lpdwProcessId);        public void KillSpecialExcel(Microsoft.Office.Interop.Excel.Application m_objExcel)
        {
            try
            {
                if (m_objExcel != null)
                {
                    int lpdwProcessId;
                    GetWindowThreadProcessId(new IntPtr(m_objExcel.Hwnd), out lpdwProcessId);
                    System.Diagnostics.Process.GetProcessById(lpdwProcessId).Kill();
                }
            }
            catch (Exception ex)
            {
                throw;
            }
        }
        #endregion         ~ExcelDocument()
        {
            CloseDocument();
        }
    }
}