流对象列表作为来自 API 的文件

Question

API 端点是否可以从实体框架填充的 IQueryable 对象列表中流式传输数据，并链接到 SQL 数据库以创建和返回 csv 文件？

到目前为止，我循环遍历列表中的所有项目并创建一个临时文件。然后，我将该文件作为我的 GET API 调用的结果进行流式传输。

Directory.CreateDirectory($"{Environment.CurrentDirectory}/TmpData");

string tmpFileName = $"{Environment.CurrentDirectory}/TmpData/{Guid.NewGuid().ToString()}.csv";

using (FileStream file = new FileStream(tmpFileName, FileMode.CreateNew))
{
    using (StreamWriter fileStream = new StreamWriter(file))
    {
        LookupItem liTmp = new();

        await fileStream.WriteAsync(nameof(liTmp.LookupItemId));
        await fileStream.WriteAsync(",");
        await fileStream.WriteAsync(nameof(liTmp.Code));
        await fileStream.WriteAsync(",");
        await fileStream.WriteAsync(nameof(liTmp.Label));
        await fileStream.WriteLineAsync();

        foreach (var li in items)
        {
            await fileStream.WriteAsync(li.LookupItemId.ToString());
            await fileStream.WriteAsync(",");
            await fileStream.WriteAsync(li.Code?.ToString());
            await fileStream.WriteAsync(",");
            await fileStream.WriteAsync(li.Label?.ToString());
            await fileStream.WriteLineAsync();
        }
    }
}

this.Response.StatusCode = 200;
this.Response.Headers.Add(HeaderNames.ContentDisposition, $"attachment; filename=\"{ request.LookupTableType.ToString() } Data { DateTime.Now.ToString("yyyy-mm-dd hh-MM-ss")}.csv\"");
this.Response.Headers.Add(HeaderNames.ContentType, "application/octet-stream");
var inputStream = new FileStream(tmpFileName, FileMode.Open, FileAccess.Read);
var outputStream = this.Response.Body;
const int bufferSize = 1 << 10;
var buffer = new byte[bufferSize];

while (true)
{
    var bytesRead = await inputStream.ReadAsync(buffer, 0, bufferSize);
    if (bytesRead == 0) break;
    await outputStream.WriteAsync(buffer, 0, bytesRead);
}
await outputStream.FlushAsync();

System.IO.File.Delete(tmpFileName);

return new EmptyResult();

这很好，我得到了一个 csv 文件，但我在想是否可以将数据直接写入输出流而不是创建一个临时文件？我不想将所有项目加载到内存中，因为我想在大型数据集上使用此方法以允许用户下载表格数据。

Answer 1

您可以添加EPPlus 包。下面的代码我已经测试过了，它对我有用。

using dotnetcoreMVC.Models;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Extensions.Logging;
using OfficeOpenXml;
using OfficeOpenXml.Style;
using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Linq;
using System.Threading.Tasks;

namespace dotnetcoreMVC.Controllers
{
public class ForTestController : Controller
{
    public IActionResult ExportData()
    {
        //TODO read data from db
        // Mock data
        List<TestModel> li = new List<TestModel>();
        
        for (int i = 0; i < 10; i++)
        {
            TestModel m = new TestModel();
            m.id = 1+i;
            m.name = "test name"+ i;
            li.Add(m);
        }

        var data = li;

        if (data?.Any() != true)
        {
            return new ContentResult() { Content = "no data" };
        }

        ExcelPackage.LicenseContext = LicenseContext.NonCommercial;
        using (var ep = new ExcelPackage())
        {
            using (var worksheet = ep.Workbook.Worksheets.Add("export data for test"))
            {
                var x = 1;
                var y = 1;

                var columnTitles = new List<string>()
               {
               "id",
               "alias"
               };

                foreach (var columnTitle in columnTitles)
                {
                    var cell = worksheet.Cells[x, y++];
                    cell.Style.Font.Bold = true;
                    cell.Style.HorizontalAlignment = ExcelHorizontalAlignment.Center;
                    cell.Style.VerticalAlignment = ExcelVerticalAlignment.Center;
                    cell.Value = columnTitle;
                }

                foreach (var item in data)
                {
                    x++;
                    y = 1;
                    var cell = worksheet.Cells[x, y++];
                    cell.Value = item.id;
                    cell = worksheet.Cells[x, y++];
                    cell.Value = item.name;
                }

                using (var stream = new MemoryStream())
                {
                    ep.SaveAs(stream);
                    return new FileContentResult(stream.ToArray(), "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet")
                    {
                        FileDownloadName = Guid.NewGuid()+"test.csv"
                    };
                }
            }
        }
    }
}
public class TestModel { 
    public int id { get; set; }
    public string name { get; set; }
}
}