MultipartActionResult

This is an implementaion of an asynchronous streaming multipart IActionResult for ASP.NET Core MVC.

Usage

The multipart action executor needs to be registered in the ConfigureServices method of the Startup class.

public void ConfigureServices(IServiceCollection services)
{
    // ...
    services.AddMultipartSupport();
    // ...
}

Then use the Multipart extension method to return the multipart content to the client.

public IActionResult GetMultipartContent()
{
    return this.Multipart(new[]
    {
        Task.FromResult<(IHeaderDictionary, Stream)>(
            (
                GetHeaders("text/plain", "file1.txt"),
                new MemoryStream(Encoding.UTF8.GetBytes("Some text content"))
            )),
        Task.FromResult<(IHeaderDictionary, Stream)>(
            (
                GetHeaders("text/csv", "file2.csv"),
                new MemoryStream(Encoding.UTF8.GetBytes("A,B\r\n1,2\r\n"))
            ))
    });
}

private static IHeaderDictionary GetHeaders(string contentType, string fileName)
{
    IHeaderDictionary result = new HeaderDictionary();

    var contentDisposition = new ContentDispositionHeaderValue("attachment");
    contentDisposition.SetHttpFileName(fileName);
    result[HeaderNames.ContentDisposition] = contentDisposition.ToString();

    result[HeaderNames.ContentType] = new MediaTypeHeaderValue(contentType)
    {
        Charset = "utf-8"
    }.ToString();

    return result;
}

The resulting HTTP message is:

HTTP/1.1 200 OK
Content-Type: multipart/mixed; boundary=93f0339c-3919-4842-9e2c-f656d6b05420
Server: Kestrel
X-Powered-By: ASP.NET
Date: Sun, 26 Aug 2018 18:03:45 GMT
Content-Length: 399

--93f0339c-3919-4842-9e2c-f656d6b05420
Content-Disposition: attachment; filename=file1.txt; filename*=UTF-8''file1.txt
Content-Type: text/plain; charset=utf-8

Some text content
--93f0339c-3919-4842-9e2c-f656d6b05420
Content-Disposition: attachment; filename=file2.csv; filename*=UTF-8''file2.csv
Content-Type: text/csv; charset=utf-8

A,B
1,2

--93f0339c-3919-4842-9e2c-f656d6b05420--

Parsing the multipart content can be done using the ReadAsMultipartAsync extension methods in Microsoft.AspNet.WebApi.Client package.

 HttpClient httpClient = new HttpClient();
var response = await httpClient.GetAsync(
    "http://localhost:19701/api/multipart/example",
    HttpCompletionOption.ResponseHeadersRead).ConfigureAwait(false);

var tempFolder = Path.Combine(Path.GetTempPath(), $"multipart_{Guid.NewGuid()}");

try
{
    Directory.CreateDirectory(tempFolder);

    var results = await response.Content.ReadAsMultipartAsync(
        new MultipartFileStreamProvider(tempFolder)).ConfigureAwait(false);

    foreach (var fileData in results.FileData)
    {
        Console.WriteLine($"Local file: {fileData.LocalFileName}");
        foreach (var header in fileData.Headers)
        {
            Console.WriteLine($"{header.Key}: {String.Join(",", header.Value)}");
        }

        Console.WriteLine("-------------------------------------");
    }
}
finally
{
    if (Directory.Exists(tempFolder))
    {
        Directory.Delete(tempFolder, true);
    }
}