public class ExcelHelper
    {
        /// <summary>
        /// List转Excel
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="list">数据</param>
        /// <param name="sheetName">表名</param>
        /// <param name="overwrite">true,覆盖单元格，false追加内容(list和创建的excel或excel模板)</param>
        /// <param name="xlsx">true-xlsx，false-xls</param>
        /// <returns>返回文件</returns>
        public static MemoryStream ParseListToExcel<T>(List<T> list, string sheetName = "sheet1", bool overwrite = true, bool xlsx = true) where T : class
        {
            var mapper = new Mapper();
            MemoryStream ms = new MemoryStream();
            mapper.Save<T>(ms, list, sheetName, overwrite, xlsx);
            return ms;
        }
        /// <summary>
        /// Excel转为List
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="fileStream"></param>
        /// <param name="sheetname"></param>
        /// <returns></returns>
        public static List<T> ParseExcelToList<T>(Stream fileStream, string sheetname = "") where T : class
        {
            List<T> ModelList = new List<T>();
            var mapper = new Mapper(fileStream);
            List<RowInfo<T>> DataList = new List<RowInfo<T>>();
            if (!string.IsNullOrEmpty(sheetname))
            {
                DataList = mapper.Take<T>(sheetname).ToList();
            }
            else
            {
                DataList = mapper.Take<T>().ToList();
            }

            if (DataList != null && DataList.Count > 0)
            {
                foreach (var item in DataList)
                {
                    ModelList.Add(item.Value);
                }
            }
            return ModelList;
        }
    }

 public class User
    {
        [Column("用户ID")]
        public int UserId { get; set; }
        [Column("用户名")]
        public string UserName { get; set; }
        [Column("手机号")]
        public string Phone { get; set; }
        [Column("邮箱")]
        public string Email { get; set; }
    }

 public class FileController : Controller
    {
        public IActionResult Index()
        {
            return View();
        }
        [HttpPost]
        public IActionResult Upload()
        {
            //用户List
            List<User> users = new List<User>();
            //获取到上传的文件
            var file = Request.Form.Files[0];
            //文件转成流
            using (var fileStream = file.OpenReadStream())
            {
                //excel转实体
                users = ExcelHelper.ParseExcelToList<User>(fileStream, "Sheet1");
            }
            return Content(JsonConvert.SerializeObject(users));

        }
        public IActionResult DownLoad()
        {
            //生成List
            List<User> usrs = new List<User>();
            for(int i=1;i<=10;i++)
            {
                User user = new User()
                {
                    UserId = i,
                    UserName = "用户" + i,
                    Phone = "13800000000",
                    Email = $"user{i}@qq.com"
                };
                usrs.Add(user);
            }
            //List转为Excel文件
            var fileStream = ExcelHelper.ParseListToExcel(usrs);
            return File(fileStream.ToArray(), "application/vnd.ms-excel", "用户信息.xlsx");
        }
    }

@{
}
<a href="/File/DownLoad">导出excel文件</a>
<form action="/File/Upload" method="post" enctype="multipart/form-data" >
  选择excel文件:  <input type="file" name="file" />
    <input type="submit" value="上传excel" />
</form>