OpenFileDialog open = new OpenFileDialog();
            open.Filter = "*.xls(*.xls)|*.xls|*.xlsx(*.xlsx)|*.xlsx";
            if (open.ShowDialog() == DialogResult.OK)
            {
               string path = open.FileName;
            }

------解决方案--------------------
C# code

           FileDialog fileDialog =new OpenFileDialog();
           fileDialog.Title = "Open File";
            //是否自动添加默认扩展名
            fileDialog.AddExtension = true;
            fileDialog.Filter = "Excel文件(*.xls)|*.xls";
            fileDialog.DefaultExt = "." + Filetype;
            fileDialog.FilterIndex = 1;
            fileDialog.RestoreDirectory = true;



            //默认路径为“”；
            string path = "";
            if (fileDialog.ShowDialog() == DialogResult.OK)
            {
                //选择文件的路径
                path = fileDialog.FileName;
            }

------解决方案--------------------
改改就能用
using System;
using System.Collections.Generic;
using System.Data;
using System.IO;
using System.Text;
using System.Web;
using NPOI;
using NPOI.HPSF;
using NPOI.HSSF;
using NPOI.HSSF.Record.Formula.Eval;
using NPOI.HSSF.UserModel;
using NPOI.HSSF.Util;
using NPOI.POIFS;
using NPOI.SS.UserModel;
using NPOI.Util;
using NPOI.SS;
using NPOI.DDF;
using NPOI.SS.Util;
using System.Collections;
using System.Text.RegularExpressions;

namespace CommonFunction
{
   public class ExcelHelper
   {
       //private static WriteLog wl = new WriteLog();
      

       #region 从datatable中将数据导出到excel
       /// <summary>
       /// DataTable导出到Excel的MemoryStream
       /// </summary>
       /// <param name="dtSource">源DataTable</param>
       /// <param name="strHeaderText">表头文本</param>
       static MemoryStream ExportDT(DataTable dtSource, string strHeaderText)
       {
           HSSFWorkbook workbook = new HSSFWorkbook();
           HSSFSheet sheet = workbook.CreateSheet() as HSSFSheet;

           #region 右击文件 属性信息

           //{
           //    DocumentSummaryInformation dsi = PropertySetFactory.CreateDocumentSummaryInformation();
           //    dsi.Company = "http://www.yongfa365.com/";
           //    workbook.DocumentSummaryInformation = dsi;

           //    SummaryInformation si = PropertySetFactory.CreateSummaryInformation();
           //    si.Author = "柳永法"; //填加xls文件作者信息
           //    si.ApplicationName = "NPOI测试程序"; //填加xls文件创建程序信息
           //    si.LastAuthor = "柳永法2"; //填加xls文件最后保存者信息
           //    si.Comments = "说明信息"; //填加xls文件作者信息
           //    si.Title = "NPOI测试"; //填加xls文件标题信息
           //    si.Subject = "NPOI测试Demo"; //填加文件主题信息
           //    si.CreateDateTime = DateTime.Now;
           //    workbook.SummaryInformation = si;
           //}

           #endregion

           HSSFCellStyle dateStyle = workbook.CreateCellStyle() as HSSFCellStyle;
           HSSFDataFormat format = workbook.CreateDataFormat() as HSSFDataFormat;
           dateStyle.DataFormat = format.GetFormat("yyyy-mm-dd");

           //取得列宽
           int[] arrColWidth = new int[dtSource.Columns.Count];
           foreach (DataColumn item in dtSource.Columns)
           {