本来代码已经写好了，传递给前台文件路径就可以，后来同事告诉我，给路径可能下载不了，文件可能放在另一台服务器，告诉我说用流进行下载，然后就开始百度加上找现在公司的代码，做出来了，感觉怪怪的。

//判断路径是否存在，不存在则创建

var basePath = Path.Combine(ProjectConfig.BaseFolder));

if(!File.Exists(basePath )){System.IO.Directory.CreateDirectory(basePath);}

//待读取文件路径

string filePath = Path.Combine(basePath,"xxx.xls");

//获取HSSFWorkbook对象

HSSFWorkbook workbook = (HSSFWorkbook)OpenWorkbook(filePath);

//操作sheet

ISheet sheet = workbook.GetSheetAt(0);

//设置生成下拉框的行和列;绑定下拉框和作用区域
sheet.AddValidationData(new HSSFDataValidation(new CellRangeAddressList(4, 30, 0, 0),
DVConstraint.CreateExplicitListConstraint(字符串类型数组)));

//设置标题
ICell titleRow = sheet.GetRow(0).GetCell(0);
titleRow.SetCellValue(titleRow.StringCellValue.Replace("x", year));

//导出Excel

FileStream stream = new FileStream(filePath, FileMode.Create, System.IO.FileAccess.ReadWrite);
workbook.Write(stream);//设置模板写入流中用于导出

//流转换成二进制数组

BinaryReader r = new BinaryReader(stream);
r.BaseStream.Seek(0, SeekOrigin.Begin);//将文件指针设置到文件开
byte[] pReadByte = r.ReadBytes((int)r.BaseStream.Length);
//关闭流

stream.Close();
stream.Dispose();

//返回待下载文件

Stream stream = new MemoryStream(buffer);
stream.Seek(0, SeekOrigin.Begin);//设置stream的position为流的开始
return stream;

 

//Controller代码

public HttpResponseMessage xxx()
{
            HttpResponseMessage result = new HttpResponseMessage(HttpStatusCode.OK);
            var res = _aviationProtectionService.xxx();//返回的流
            result.Content = new StreamContent(res);
            result.Content.Headers.ContentType = new System.Net.Http.Headers.MediaTypeHeaderValue("application/octet-stream");
            result.Content.Headers.ContentDisposition = new ContentDispositionHeaderValue("attachment");
            result.Content.Headers.ContentDisposition.FileName = "xxx.xls";
            return result;
  }