NPOI导入Excel文件程序报错,请指点,多谢.

using System;
using using System.Windows.Forms;
using System.Data.SQLite;
using using NPOI.SS.UserModel;
using NPOI.XSSF.UserModel;
using NPOI.HSSF.UserModel;

public System.Data.DataTable ExcelUp(string excelName)
        {
            string filePath = excelName;//读取excel文件路径；

            //打开Excel文件,取得Sheet名
            if (filePath.IndexOf(".xlsx") > 0) // 2007版本
            {
                XSSFWorkbook workbook = new XSSFWorkbook(filePath);
            }
            else if (filePath.IndexOf(".xls") > 0) // 2003版本
            {
                HSSFWorkbook workbook = new HSSFWorkbook(filePath);  //此处报错
            }

            System.Data.DataTable dt = GetDataTable("Sheet1", filePath);

            return dt;

        }