如何使用ASP.NET Core创建多部分HTTP响应

Question

我想在我的ASP.NET核心控制器中创建一个动作方法,它返回一个包含多个文件的多部分HTTP响应.我知道使用.zip文件是网站的推荐方法,但我正在考虑使用这样的API请求.

我在ASP.NET Core示例中能够找到的示例与上传文件时的多部分HTTP请求有关.就我而言,我想下载文件.

UPDATE

我提出了以下GitHub问题:#4933

Answer 1

我写了一个更通用的MultipartResult类,它继承自ActionResult:

用法示例

[Route("[controller]")]
public class MultipartController : Controller
{
    private readonly IHostingEnvironment hostingEnvironment;

    public MultipartController(IHostingEnvironment hostingEnvironment)
    {
        this.hostingEnvironment = hostingEnvironment;
    }

    [HttpGet("")]
    public IActionResult Get()
    {
        return new MultipartResult()
        {
            new MultipartContent()
            {
                ContentType = "text/plain",
                FileName = "File.txt",
                Stream = this.OpenFile("File.txt")
            },
            new MultipartContent()
            {
                ContentType = "application/json",
                FileName = "File.json",
                Stream = this.OpenFile("File.json")
            }
        };
    }

    private Stream OpenFile(string relativePath)
    {
        return System.IO.File.Open(
            Path.Combine(this.hostingEnvironment.WebRootPath, relativePath),
            FileMode.Open,
            FileAccess.Read);
    }
}

履行

public class MultipartContent
{
    public string ContentType { get; set; }

    public string FileName { get; set; }

    public Stream Stream { get; set; }
}

public class MultipartResult : Collection<MultipartContent>, IActionResult
{
    private readonly System.Net.Http.MultipartContent content;

    public MultipartResult(string subtype = "byteranges", string boundary = null)
    {
        if (boundary == null)
        {
            this.content = new System.Net.Http.MultipartContent(subtype);
        }
        else
        {
            this.content = new System.Net.Http.MultipartContent(subtype, boundary);
        }
    }

    public async Task ExecuteResultAsync(ActionContext context)
    {
        foreach (var item in this)
        {
            if (item.Stream != null)
            {
                var content = new StreamContent(item.Stream);

                if (item.ContentType != null)
                {
                    content.Headers.ContentType = new System.Net.Http.Headers.MediaTypeHeaderValue(item.ContentType);
                }

                if (item.FileName != null)
                {
                    var contentDisposition = new ContentDispositionHeaderValue("attachment");
                    contentDisposition.SetHttpFileName(item.FileName);
                    content.Headers.ContentDisposition = new System.Net.Http.Headers.ContentDispositionHeaderValue("attachment");
                    content.Headers.ContentDisposition.FileName = contentDisposition.FileName;
                    content.Headers.ContentDisposition.FileNameStar = contentDisposition.FileNameStar;
                }

                this.content.Add(content);
            }
        }

        context.HttpContext.Response.ContentLength = content.Headers.ContentLength;
        context.HttpContext.Response.ContentType = content.Headers.ContentType.ToString();

        await content.CopyToAsync(context.HttpContext.Response.Body);
    }
}

Answer 2

来自MSDN

MSDN有一个列出许多多部分子类型的文档.这multipart/byteranges似乎最适合在HTTP响应中发送多个文件以供客户端应用程序下载.大胆的部分特别相关.

multipart/byteranges内容类型被定义为HTTP消息协议的一部分.它包含两个或多个部分,每个部分都有自己的Content-Type和Content-Range字段.使用MIME边界参数分隔部件.它允许将二进制以及7位和8位文件作为多个部分发送,并在每个部分的标题中指定部分的长度.请注意,虽然HTTP规定了为HTTP文档使用MIME,但HTTP并不严格符合MIME.(重点补充.)

来自RFC2068

RFC2068,第19.2节提供了对multipart/byteranges.同样,大胆的部分是相关的.每个字节范围都可以有自己的Content-type,结果也可以有自己的Content-disposition.

multipart/byteranges媒体类型包括两个或多个部分,每个部分都有自己的Content-Type和Content-Range字段.使用MIME边界参数分隔部件.(重点补充.)

RFC还提供了这个技术定义:

Media Type name:           multipart
Media subtype name:        byteranges
Required parameters:       boundary
Optional parameters:       none
Encoding considerations:   only "7bit", "8bit", or "binary" are permitted
Security considerations:   none

RFC的最佳部分是它的示例,下面的ASP.NET Core示例说明了这一点.

HTTP/1.1 206 Partial content
Date: Wed, 15 Nov 1995 06:25:24 GMT
Last-modified: Wed, 15 Nov 1995 04:58:08 GMT
Content-type: multipart/byteranges; boundary=THIS_STRING_SEPARATES

--THIS_STRING_SEPARATES
Content-type: application/pdf
Content-range: bytes 500-999/8000

...the first range...
--THIS_STRING_SEPARATES
Content-type: application/pdf
Content-range: bytes 7000-7999/8000

...the second range
--THIS_STRING_SEPARATES--

请注意,他们正在发送两个PDF!这正是你所需要的.

一种ASP.NET核心方法

这是一个适用于Firefox的代码示例.也就是说,Firefox会下载三个图像文件,我们可以使用Paint打开它们.源代码在GitHub上.

样本使用app.Run().要使样本适应控制器操作,请注入IHttpContextAccessor控制器并写入_httpContextAccessor.HttpContext.Response操作方法.

using System.IO;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.DependencyInjection;

public class Startup
{
    private const string CrLf = "\r\n";
    private const string Boundary = "--THIS_STRING_SEPARATES";
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddMvc();
    }

    public void Configure(IApplicationBuilder app)
    {
        app.Run(async context =>
        {
            var response = context.Response;
            response.ContentType = $"multipart/byteranges; boundary={Boundary}";

            // TODO Softcode the 'Content-length' header.            
            response.ContentLength = 13646;
            var contentLength = response.ContentLength.Value;

            await response.WriteAsync(Boundary + CrLf);

            var blue = new FileInfo("./blue.jpg");
            var red = new FileInfo("./red.jpg");
            var green = new FileInfo("./green.jpg");

            long start = 0;
            long end = blue.Length;
            await AddImage(response, blue, start, end, contentLength);

            start = end + 1;
            end = start + red.Length;
            await AddImage(response, red, start, end, contentLength);

            start = end + 1;
            end = start + green.Length;
            await AddImage(response, green, start, end, contentLength);

            response.Body.Flush();
        });
    }

    private async Task AddImage(HttpResponse response, FileInfo fileInfo,
        long start, long end, long total)
    {
        var bytes = File.ReadAllBytes(fileInfo.FullName);
        var file = new FileContentResult(bytes, "image/jpg");

        await response
            .WriteAsync($"Content-type: {file.ContentType.ToString()}" + CrLf);

        await response
            .WriteAsync($"Content-disposition: attachment; filename={fileInfo.Name}" + CrLf);

        await response
            .WriteAsync($"Content-range: bytes {start}-{end}/{total}" + CrLf);

        await response.WriteAsync(CrLf);
        await response.Body.WriteAsync(
            file.FileContents,
            offset: 0,
            count: file.FileContents.Length);
        await response.WriteAsync(CrLf);

        await response.WriteAsync(Boundary + CrLf);
    }
}

注意:此示例代码在到达生产之前需要重构.