c# 根据NPOI 读取一个excel 文件的多个Sheet

using NPOI.HSSF.UserModel;
using NPOI.SS.UserModel;
using NPOI.XSSF.UserModel;
using System;
using System.Collections.Generic;
using System.Data;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
 
 
  public class ExcelHelper : IDisposable
        {
            private string fileName = null; //文件名
            private IWorkbook workbook = null;
            private FileStream fs = null;
            private bool disposed;
 
            public ExcelHelper(string fileName)
            {
                this.fileName = fileName;
                disposed = false;
            }
 
            /// <summary>
            /// 将DataTable数据导入到excel中
            /// </summary>
            /// <param name="data">要导入的数据</param>
            /// <param name="isColumnWritten">DataTable的列名是否要导入</param>
            /// <param name="sheetName">要导入的excel的sheet的名称</param>
            /// <returns>导入数据行数(包含列名那一行)</returns>
            public int DataTableToExcel(DataTable data, string sheetName, bool isColumnWritten)
            {
                int i = 0;
                int j = 0;
                int count = 0;
                ISheet sheet = null;
 
                fs = new FileStream(fileName, FileMode.OpenOrCreate, FileAccess.ReadWrite);
                if (fileName.IndexOf(".xlsx") > 0) // 2007版本
                    workbook = new XSSFWorkbook();
                else if (fileName.IndexOf(".xls") > 0) // 2003版本
                    workbook = new HSSFWorkbook();
 
                try
                {
                    if (workbook != null)
                    {
                        sheet = workbook.CreateSheet(sheetName);
                    }
                    else
                    {
                        return -1;
                    }
 
                    if (isColumnWritten == true) //写入DataTable的列名
                    {
                        IRow row = sheet.CreateRow(0);
                        for (j = 0; j < data.Columns.Count; ++j)
                        {
                            row.CreateCell(j).SetCellValue(data.Columns[j].ColumnName);
                        }
                        count = 1;
                    }
                    else
                    {
                        count = 0;
                    }
 
                    for (i = 0; i < data.Rows.Count; ++i)
                    {
                        IRow row = sheet.CreateRow(count);
                        for (j = 0; j < data.Columns.Count; ++j)
                        {
                            row.CreateCell(j).SetCellValue(data.Rows[i][j].ToString());
                        }
                        ++count;
                    }
                    workbook.Write(fs); //写入到excel
                    return count;
                }
                catch (Exception ex)
                {
                    Console.WriteLine("Exception: " + ex.Message);
                    return -1;
                }
            }
 
            /// <summary>
            /// 将excel中的数据导入到DataTable中
            /// </summary>
            /// <param name="sheetName">excel工作薄sheet的名称</param>
            /// <param name="isFirstRowColumn">第一行是否是DataTable的列名</param>
            /// <returns>返回的DataTable</returns>
            ///
 
 
                public Dictionary<int,string> ReturnSheetList()
            {
                Dictionary<int, string> t = new Dictionary<int, string>();
                ISheet sheet = null;
                DataTable data = new DataTable();
                int startRow = 0;
                    try
                    {
                        fs = new FileStream(fileName, FileMode.Open, FileAccess.Read);
                        if (fileName.IndexOf(".xlsx") > 0) // 2007版本
                            workbook = new XSSFWorkbook(fs);
                        else if (fileName.IndexOf(".xls") > 0) // 2003版本
                            workbook = new HSSFWorkbook(fs);
                        int count = workbook.NumberOfSheets; //获取所有SheetName
                        for(int i=0;i<count;i++)
                        {
                            sheet = workbook.GetSheetAt(i);
                            if (sheet.LastRowNum > 0)
                            {
                                t.Add(i, workbook.GetSheetAt(i).SheetName);
                            }
                        }
                        return t;
                    }
                    catch (Exception ex)
                    {
                        throw new Exception(ex.Message);
                    }
                  
 
            }<br>　　　　　　　　///index excel的第几个sheet
            public DataTable ExcelToDataTable(int index)
            {
                ISheet sheet = null;
                DataTable data = new DataTable();
                int startRow = 0;
                try
                {
                    fs = new FileStream(fileName, FileMode.Open, FileAccess.Read);
                    if (fileName.IndexOf(".xlsx") > 0) // 2007版本
                        workbook = new XSSFWorkbook(fs);
                    else if (fileName.IndexOf(".xls") > 0) // 2003版本
                        workbook = new HSSFWorkbook(fs);
                    //int coutnts = workbook.NumberOfSheets;
 
                    sheet = workbook.GetSheetAt(index);
                    //string names= sheet.SheetName;
                    if (sheet != null)
                    {
                        IRow firstRow = sheet.GetRow(0);
                        int cellCount = firstRow.LastCellNum; //一行最后一个cell的编号 即总的列数
 
 
                        for (int i = firstRow.FirstCellNum; i < cellCount; ++i)
                        {
                            ICell cell = firstRow.GetCell(i);
                            CellType c = cell.CellType;
                            if (cell != null)
                            {
                                string cellValue = cell.StringCellValue;
                                if (cellValue != null)
                                {
                                    DataColumn column = new DataColumn(cellValue);
                                    data.Columns.Add(column);
                                }
                            }
                        }
                        startRow = sheet.FirstRowNum + 1;
 
 
                        //最后一列的标号
                        int rowCount = sheet.LastRowNum;
                        for (int i = startRow; i <= rowCount; ++i)
                        {
                            IRow row = sheet.GetRow(i);
                            if (row == null) continue; //没有数据的行默认是null　　　　　　　
 
                            DataRow dataRow = data.NewRow();
                            for (int j = row.FirstCellNum; j < cellCount; ++j)
                            {
                                if (row.GetCell(j) != null) //同理，没有数据的单元格都默认是null
                                    dataRow[j] = row.GetCell(j).ToString(); 
                            }
                            data.Rows.Add(dataRow);
                        }
                    }
 
                    return data;
                }
                catch (Exception ex)
                {
                    return null;
                    throw new Exception(ex.Message);
 
                }
            }
 
            public void Dispose()
            {
                Dispose(true);
                GC.SuppressFinalize(this);
            }
 
            protected virtual void Dispose(bool disposing)
            {
                if (!this.disposed)
                {
                    if (disposing)
                    {
                        if (fs != null)
                            fs.Close();
                    }
 
                    fs = null;
                    disposed = true;
                }
            }
        }<br><br>