C# 实现NPOI的Excel导出

技术点：

1.自定义attribute属性

2.通过反射取类及其属性的attribute属性值

3.NPOI包常用属性及方法(我也仅仅知道用到过的，陌生的要么见名知意，要么百度查)

实现功能点：

List类对象的模板导出，实用场景例子见最后代码块

（emm...还是比较抽象，代码见）

EXCEL导出类DTO超类

public abstract class ExcelSuper
{
  // 所有excel导出类DTO必须要继承的方法
  // 限制比较弱，主要还是用来区分DTO用在何处
}

定义继承导出类DTO的特性说明类

/// <summary>
    /// 导出类 类特性
    /// </summary>
    [AttributeUsage(AttributeTargets.Class)]
    public class ExcelExpClassAttribute : Attribute
    {
        public ExcelExpClassAttribute(int colSplit, int rowSplit, int leftmostColumn, int topRow)
        {
            this.colSplit = colSplit;
            this.rowSplit = rowSplit;
            this.leftmostColumn = leftmostColumn;
            this.topRow = topRow;
        }

        /// <summary>
        /// 冻结的列数
        /// </summary>
        public int colSplit { get; set; }
        /// <summary>
        /// 冻结的行数
        /// 只冻结列时为0
        /// </summary>
        public int rowSplit { get; set; }
        /// <summary>
        /// 右边区域可见的首列序号，从1开始计算
        /// </summary>
        public int leftmostColumn { get; set; }
        /// <summary>
        /// 边区域可见的首行序号，从1开始计算，
        /// 只冻结列时为0
        /// </summary>
        public int topRow { get; set; }
    }

    /// <summary>
    /// 导出类 属性特性
    /// </summary>
    [AttributeUsage(AttributeTargets.Property)]
    public class ExcelExpAttribute : Attribute
    {
        /// <summary>
        /// 是否隐藏，与SortIndex搭配使用
        /// </summary>
        public bool IsHidden { get; set; } = false;
        /// <summary>
        /// 排序索引（保持连贯性、不可重复）
        ///   - 对应dataTable的列排序 [0,1...]
        ///   - 对应Excel的列索引 [0,1...]
        /// </summary>
        public int SortIndex { get; set; }
        /// <summary>
        /// Excel列名
        /// </summary>
        public string ColName { get; set; }
        /// <summary>
        /// 是否动态列
        /// </summary>
        public bool IsdynamicColName { get; set; }

        /// <summary>
        /// 是否合并行 -- 预留，后补
        /// </summary>
        public bool IsRowMerge { get; set; } = false;
        /// <summary>
        /// 合并行依据 -- 预留，后补
        /// </summary>
        public string RowMergeBasis { get; set; }
    }

Excel帮助类

    /// <summary>
    /// Excel帮助类
    /// </summary>
    public static class ExcelHelper
    {
        /// <summary>
        ///
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="workbook"></param>
        /// <param name="dtSource"></param>
        /// <param name="sheetNum"></param>
        /// <param name="useAttributeColName"> 添加一行列名，取类自定义属性列名称：ExcelExpAttribute.ColName</param>
        /// <returns></returns>
        private static IWorkbook OutputExcel<T>(this IWorkbook workbook, IEnumerable<T> dtSource, int sheetNum, bool useAttributeColName = false)
        {
            // 读取sheet
            ISheet sheet = workbook.GetSheetAt(sheetNum);
            int rowIndex = sheet.LastRowNum + 1;// 获取写入行初始值

            if (useAttributeColName)
            {
                // 待补充
            }

            Type objType = typeof(T);

            // 取类上的自定义特性
            object[] objs = objType.GetCustomAttributes(typeof(ExcelExpClassAttribute), true);
            foreach (object obj in objs)
            {
                ExcelExpClassAttribute attr = obj as ExcelExpClassAttribute;
                if (attr != null)
                {
                    sheet.CreateFreezePane(attr.colSplit, attr.rowSplit, attr.leftmostColumn, attr.topRow);// 设置冻结行、列
                    break;
                }
            }

            // 循环添加数据
            foreach (T item in dtSource)
            {
                IRow rowi = sheet.CreateRow(rowIndex);

                // 设置自适应宽度,9为Excel列数，根据需要自已修改
                for (int columnNum = 0; columnNum <= rowi.LastCellNum; columnNum++)
                {
                    int columnWidth = sheet.GetColumnWidth(columnNum) / 256;

                    sheet.SetColumnWidth(columnNum, columnWidth * 256);
                }

                // 取属性上的自定义特性
                foreach (PropertyInfo propInfo in objType.GetProperties())
                {
                    object[] objAttrs = propInfo.GetCustomAttributes(typeof(ExcelExpAttribute), true);
                    if (objAttrs.Length > 0)
                    {
                        ExcelExpAttribute attr = objAttrs[0] as ExcelExpAttribute;
                        if (attr != null)
                        {
                            if (attr.IsHidden)
                                continue;

                            int colIndex = attr.SortIndex;
                            var name = propInfo.Name;// 实例名称
                            var value = propInfo.GetValue(item);// 实例值

                            #region 判断值类型并填充
                            var newCell = rowi.CreateCell(colIndex);
                            switch (propInfo.PropertyType.ToString())
                            {
                                case "System.String"://字符串类型
                                    newCell.SetCellValue(value == null ? "" : value.ToString());
                                    break;
                                case "System.DateTime"://日期类型
                                    DateTime.TryParse(value.ToString(), out DateTime dateV);
                                    newCell.SetCellValue(dateV);
                                    break;
                                case "System.Boolean"://布尔型
                                    bool.TryParse(value.ToString(), out bool boolV);
                                    newCell.SetCellValue(boolV);
                                    break;
                                case "System.Int16"://整型
                                case "System.Int32":
                                case "System.Int64":
                                case "System.Byte":
                                    int.TryParse(value.ToString(), out int intV);
                                    newCell.SetCellValue(intV);
                                    break;
                                case "System.Decimal"://浮点型
                                case "System.Double":
                                    double.TryParse(value.ToString(), out double doubV);
                                    newCell.SetCellValue(doubV);
                                    break;
                                case "System.DBNull"://空值处理
                                    newCell.SetCellValue("");
                                    break;
                                default:
                                    newCell.SetCellValue("");
                                    break;
                            }
                            #endregion
                        }
                    }
                }

                rowIndex++;
            }

            return workbook;
        }

        /// <summary>
        /// 导出模板Excel
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="iEnumerable">数据源</param>
        /// <param name="fileFullPath">文件全路径（包含文件及后缀名）</param>
        /// <returns>文件流</returns>
        public static MemoryStream ExportToExcel<T>(IEnumerable<T> iEnumerable, string fileFullPath)
            where T : ExcelSuper
        {
            // 打开模板文件并写入
            var workbook = GetIWorkbook(fileFullPath, out ExcelTypeEnum type);

            if (type == ExcelTypeEnum.XLS)
            {
                workbook = (HSSFWorkbook)workbook.OutputExcel(iEnumerable, 0, false);
            }
            else
            {
                workbook = (XSSFWorkbook)workbook.OutputExcel(iEnumerable, 0, false);
            }

            try
            {
                using (MemoryStream ms = new MemoryStream())
                {
                    workbook.Write(ms);
                    ms.Flush();
                    ms.Position = 0;

                    // sheet.Dispose();
                    // workbook.Dispose();//一般只用写这一个就OK了，他会遍历并释放所有资源，但当前版本有问题所以只释放sheet
                    return ms;
                }
            }
            catch (Exception ex)
            {
                throw ex;
            }

        }

        #region Private method
        private static IWorkbook GetIWorkbook(string fileFullPath, out ExcelTypeEnum type)
        {
            string filename = Path.GetFileNameWithoutExtension(fileFullPath);// 文件名称
            string extension = Path.GetExtension(fileFullPath);// 后缀名 带点(.)
            try
            {
                if (!File.Exists(fileFullPath))
                {
                    throw new Exception($"模板：{filename + extension}不存在");
                }

                FileStream fs = new FileStream(fileFullPath, FileMode.Open, FileAccess.ReadWrite);
                if (".xls".Equals(extension.ToLower()))
                {
                    type = ExcelTypeEnum.XLS;
                    return new HSSFWorkbook(fs);// Excel2003以前版本
                }
                else
                {
                    type = ExcelTypeEnum.XLSX;
                    return new XSSFWorkbook(fs);// Excel2007后的版本
                }
            }
            catch (Exception ex)
            {

                throw new Exception("获取文件错误：" + ex);
            }
        }
        #endregion

        enum ExcelTypeEnum
        {
            XLS,
            XLSX
        }
    }

这部分要讲的点其实挺多的，关键就是EXCEL导出所用到的数据源是强类型的。

可以看出来list其实是EF的Queryable toList()后的类集合，作为数据源存在；// 比较喜欢强类型，直接点出来的属性让人放心ヾ(•ω•`)o

里面的DTO DesWeeklyReportExcExp继承ExcelSuper，特性分别加在类及属性上。

public class XXXXController : CoreController
{
    // 控制器内部

    [HttpPost]
    public ActionResult export()
    {
        // 控制器接口
        var list = op
                    .GetPagedQuery(PageModel)
                    .Select(s => new DesWeeklyReportExcExp
                    {
                        col1 = s.Project.ProjName,
                        col2 = s.ColAttROPDate1?.ToString("yyyy.MM.dd"),
                        col3 = (s.ColAttROPDate2 == null ? "无" : s.ColAttROPDate2.Value.ToString("yyyy.MM.dd"))
                                                       + "/"
                                                       + (s.ColAttROPDate3 == null ? "无" : s.ColAttROPDate3.Value.ToString("yyyy.MM.dd")),
                        col4 = s.ColAttROPDate4?.ToString("yyyy.MM.dd")
                    }).ToList();
         string filePath = Server.MapPath("~/download/[这是模板名称].xlsx");
         string filename = Path.GetFileNameWithoutExtension(filePath);// 文件名称
         string extension = Path.GetExtension(filePath);// 后缀名 带点(.)
         string fileDownloadName = filename + extension;

         var fs = ExcelHelper.ExportToExcel(list, filePath).ToArray();
         return File(fs, "application/ms-excel", fileDownloadName);
    }
}

[ExcelExpClassAttribute(2, 0, 2, 0)]
public class DesWeeklyReportExcExp : ExcelSuper
{
    /// <summary>
    /// 列1
    /// </summary>
    [ExcelExp(SortIndex = 0, ColName = "列1")]
    public string col1 { get; set; }

    /// <summary>
    /// 列2
    /// </summary>
    [ExcelExp(SortIndex = 0, ColName = "列2")]
    public string col2 { get; set; }

    /// <summary>
    /// 列3
    /// </summary>
    [ExcelExp(SortIndex = 0, ColName = "列3")]
    public string col3 { get; set; }

    /// <summary>
    /// 列4
    /// </summary>
    [ExcelExp(SortIndex = 0, ColName = "列4")]
    public string col4 { get; set; }
}

部分拙见，大部分还需要补充，比如设置合并列，比如数据源支持DataTable导出等等，还有现有的代码可能不够完善，看到的多多提下宝贵意见吧