通过NPOI插件完成Excel和DataTable的互相转换，兼容xlsx和xls

这次我们通过NPOI插件来完成Excel的导入和导出

这个插件的好处有两个：

1.兼容xls和xlsx，这两种后缀的都能打开

2.服务器可以不用安装office，也就不需要考虑office版本的问题

（下载NPOI插件）

插件下载解压以后有两个版本，Net20和Net40，分别对应的是.net2.0版本和.net4.0版本

这里用的是Net40，打开以后里面有5个dll，需要把他们都引用进来

引用进来以后，需要再代码里添加引用，主要引用下面4个

using NPOI.HSSF.UserModel;
using NPOI.SS.UserModel;
using NPOI.XSSF.UserModel;
using NPOI.SS.Util;

1.导入Excel代码，将Excel内容转换为DataTable

/// <summary>
        /// Excel导入成Datable
        /// </summary>
        /// <param name="file">上传的Excel路径</param>
        /// <returns></returns>
        public static DataTable ExcelToTable(string file)
        {
            DataTable dt = new DataTable();
            IWorkbook workbook;
            string fileExt = Path.GetExtension(file).ToLower();
            using (FileStream fs = new FileStream(file, FileMode.Open, FileAccess.Read))
            {
                //XSSFWorkbook 适用XLSX格式，HSSFWorkbook 适用XLS格式
                if (fileExt == ".xlsx") { workbook = new XSSFWorkbook(fs); } else if (fileExt == ".xls") { workbook = new HSSFWorkbook(fs); } else { workbook = null; }
                if (workbook == null) { return null; }
                ISheet sheet = workbook.GetSheetAt(0);

                //表头  
                IRow header = sheet.GetRow(sheet.FirstRowNum);
                List<int> columns = new List<int>();
                for (int i = 0; i < header.LastCellNum; i++)
                {
                    object obj = GetValueType(header.GetCell(i));
                    if (obj == null || obj.ToString() == string.Empty)
                    {
                        dt.Columns.Add(new DataColumn("Columns" + i.ToString()));
                    }
                    else
                        dt.Columns.Add(new DataColumn(obj.ToString()));
                    columns.Add(i);
                }
                //数据  
                for (int i = sheet.FirstRowNum + 1; i <= sheet.LastRowNum; i++)
                {
                    DataRow dr = dt.NewRow();
                    bool hasValue = false;
                    foreach (int j in columns)
                    {
                        dr[j] = GetValueType(sheet.GetRow(i).GetCell(j));
                        if (dr[j] != null && dr[j].ToString() != string.Empty)
                        {
                            hasValue = true;
                        }
                    }
                    if (hasValue)
                    {
                        dt.Rows.Add(dr);
                    }
                }
            }
            return dt;
        }

/// <summary>
        /// 获取单元格类型
        /// </summary>
        /// <param name="cell"></param>
        /// <returns></returns>
        public static object GetValueType(ICell cell)
        {
            if (cell == null)
                return null;
            switch (cell.CellType)
            {
                case CellType.Blank: //BLANK:  
                    return null;
                case CellType.Boolean: //BOOLEAN:  
                    return cell.BooleanCellValue;
                case CellType.Numeric: //NUMERIC:  
                    return cell.NumericCellValue;
                case CellType.String: //STRING:  
                    return cell.StringCellValue;
                case CellType.Error: //ERROR:  
                    return cell.ErrorCellValue;
                case CellType.Formula: //FORMULA:  
                default:
                    return "=" + cell.CellFormula;
            }
        }

2.导出Excel代码，将DataTable转换为Excel

        /// <summary>
        /// Datable导出成Excel
        /// </summary>
        /// <param name="dt"></param>
        /// <param name="file"></param>
        public static void TableToExcel(DataTable dt, string file, string category)
        {
            IWorkbook workbook;
            string fileExt = Path.GetExtension(file).ToLower();
            if (fileExt == ".xlsx") { workbook = new XSSFWorkbook(); } else if (fileExt == ".xls") { workbook = new HSSFWorkbook(); } else { workbook = null; }
            if (workbook == null) { return; }
            ISheet sheet = string.IsNullOrEmpty(dt.TableName) ? workbook.CreateSheet("Sheet1") : workbook.CreateSheet(dt.TableName);
            
            //Excel在第一行插入一行
            IRow rowTop = sheet.CreateRow(0);
            sheet.AddMergedRegion(new CellRangeAddress(0, 0, 0, 7));//合并单元格(起始行，结束行，起始列，结束列)
            ICell cellTop = rowTop.CreateCell(0);
            cellTop.SetCellValue(category);

            //表头  
            IRow row = sheet.CreateRow(1);
            for (int i = 0; i < dt.Columns.Count; i++)
            {
                ICell cell = row.CreateCell(i);
                cell.SetCellValue(dt.Columns[i].ColumnName);
            }

            //数据  
            for (int i = 0; i < dt.Rows.Count; i++)
            {
                IRow row1 = sheet.CreateRow(i + 2);
                for (int j = 0; j < dt.Columns.Count; j++)
                {
                    ICell cell = row1.CreateCell(j);
                    cell.SetCellValue(dt.Rows[i][j].ToString());
                }
            }

            //设置列的宽度，第一个参数表示第几列（从第0列开始）,第二个参数是字符数长度
            sheet.SetColumnWidth(0, 10 * 256);
            sheet.SetColumnWidth(1, 40 * 256);
            sheet.SetColumnWidth(2, 40 * 256);
            sheet.SetColumnWidth(3, 20 * 256);
            sheet.SetColumnWidth(4, 20 * 256);
            sheet.SetColumnWidth(5, 30 * 256);
            sheet.SetColumnWidth(6, 30 * 256);
            sheet.SetColumnWidth(7, 50 * 256);

            //转为字节数组  
            MemoryStream stream = new MemoryStream();
            workbook.Write(stream);
            var buf = stream.ToArray();

            //保存为Excel文件  
            using (FileStream fs = new FileStream(file, FileMode.Create, FileAccess.Write))
            {
                fs.Write(buf, 0, buf.Length);
                fs.Flush();
            }
        }