How to Create a Multipart HTTP Response With ASP.NET Core

[RehanSaeed]

I would like to create an action method in my ASP.NET Core controller which returns a Multipart HTTP Response containing several files. I know that using a .zip file is the recommended approach for websites but I am considering using such a request for an API.

The examples I have been able to find in the ASP.NET Core samples are to do with multipart HTTP requests when uploading files. In my case, I want to download files.

StackOverflow Question

[Eilon]

@Tratcher ?

[kichalla]

I don't think we currently have a way to write out multipart responses( @Tratcher can confirm this ). One way I can think of is to use System.Net.Http.MultipartContent. You would need to subclass it to provide the ASP.NET Core's response stream for the SerializeToStreamAsync method. An output formatter for this content (ex: we have a formatter for the string type) which deals with setting content type and triggers serialization should give a not so bad experience.

[Tratcher]

No, we don't have a built in formatter for it. That said, the multipart format is much easier to generate than it is to parse. See https://github.com/dotnet/corefx/blob/bfdd0b7694a99838760fbccc8185ee53c7b451cd/src/System.Net.Http/src/System/Net/Http/MultipartContent.cs#L216-L237

[Tratcher]

Also note that downloads don't normally use multipart. You can just set the content-disposition header on a normal response.

[RehanSaeed]

Thanks @Tratcher @kichalla.

My use case is that I have clients for my API with very low internet bandwidth/connectivity in Africa and need to send them multiple zip files. I'm trying to decide between creating a zip of zips or a Multipart response. I realize that using a multipart response is not a good method of downloading files from a browser due to lack of support but I am writing an API and have a client which I also control.

[kichalla]

Based on what you mention probably byte ranges support is what you are looking for? That said I don't know much about byte ranges with respect to multiparts.

[Tratcher]

This sounds like the old WebApi request batching feature. Is that still in the compat shim?

[kichalla]

No it's not part of the shim. Shim is mostly about the controller, it's helper methods and it's action selection process. Batching in webapi was more involved with a delegating handler splitting into individual requests and combining them back.

[Eilon]

We are not planning to build anything for this at this time. I think in your case because you control the client and the server you have more flexibility around the data format, so you could relatively easily build up a custom way of "packing" multiple files into one response.

We'll have to see if we get more interest from the broader community around generating these responses.

[RehanSaeed]

I've written an MultipartResult class inheriting from ActionResult which simplifies creating multipart responses. Hopefully this will help someone out in the ether.

Usage Example

[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);
    }
}

Implementation

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);
    }
}

[Tratcher]

Looks promising.

• You can't use Environment.NewLine, it will give you the wrong output on unix. The multipart format always requires \r\n.
• Read about filename* for non-ascii file names.
• I don't think that's the correct usage of content-range. It should start with the offset in the file from which you are starting, not the offset in the generated response stream.

[RehanSaeed]

Thanks @Tratcher

• Yep, I forgot that web stuff is all \r\n. Updated the code above to switch from Environment.NewLine to a "\r\n" constant.
• I discovered the ContentDispositionHeaderValue class in ASP.NET Core which is a delight to use, updated the code to use it.
• I'm a bit confused by the Content-range HTTP header and what the start and end values should really mean. I've struggled to find a decent example, also Google does not include it altogether. I based my implementation on this code sample. Any pointers on the subject?