How does one correctly implement a MediaTypeFormatter to handle requests of type 'multipart/mixed'?

Question

Consider a web service written in ASP.NET Web API to accept any number files as a \'multipart/mixed\' request. The helper method mat look as follows (assuming _client

Answer 1

Have a look at this forum: http://forums.asp.net/t/1777847.aspx/1?MVC4+Beta+Web+API+and+multipart+form+data

Here is a snippet of code (posted by imran_ku07) that might help you implement a custom formatter to handle the multipart/form-data:

public class MultiFormDataMediaTypeFormatter : FormUrlEncodedMediaTypeFormatter
{
    public MultiFormDataMediaTypeFormatter() : base()
    {
        this.SupportedMediaTypes.Add(new MediaTypeHeaderValue("multipart/form-data"));
    }

    protected override bool CanReadType(Type type)
    {
        return true;
    }

    protected override bool CanWriteType(Type type)
    {
        return false;
    }

    protected override Task<object> OnReadFromStreamAsync(Type type, Stream stream, HttpContentHeaders contentHeaders, FormatterContext formatterContext)
    {
        var contents = formatterContext.Request.Content.ReadAsMultipartAsync().Result;
        return Task.Factory.StartNew<object>(() =>
        {
            return new MultiFormKeyValueModel(contents);
        });
    }

    class MultiFormKeyValueModel : IKeyValueModel
    {
        IEnumerable<HttpContent> _contents;
        public MultiFormKeyValueModel(IEnumerable<HttpContent> contents)
        {
            _contents = contents;
        }


        public IEnumerable<string> Keys
        {
            get
            {
                return _contents.Cast<string>();
            }
        }

        public bool TryGetValue(string key, out object value)
        {
            value = _contents.FirstDispositionNameOrDefault(key).ReadAsStringAsync().Result;
            return true;
        }
    }
}

You then need to add this formatter to your application. If doing self-host you can simply add it by including:

config.Formatters.Insert(0, new MultiFormDataMediaTypeFormatter());

before instantiating the HttpSelfHostServer class.

-- EDIT --

To parse binary streams you'll need another formatter. Here is one that I am using to parse images in one of my work projects.

class JpegFormatter : MediaTypeFormatter
{
    protected override bool CanReadType(Type type)
    {
        return (type == typeof(Binary));
    }

    protected override bool CanWriteType(Type type)
    {
        return false;
    }

    public JpegFormatter()
    {
        SupportedMediaTypes.Add(new MediaTypeHeaderValue("image/jpeg"));
        SupportedMediaTypes.Add(new MediaTypeHeaderValue("image/jpg"));
        SupportedMediaTypes.Add(new MediaTypeHeaderValue("image/png"));
    }

    protected override Task<object> OnReadFromStreamAsync(Type type, Stream stream, HttpContentHeaders contentHeaders, FormatterContext formatterContext)
    {
        return Task.Factory.StartNew(() =>
            {
                byte[] fileBytes = new byte[stream.Length];
                stream.Read(fileBytes, 0, (int)fileBytes.Length);

               return (object)new Binary(fileBytes);
            }); 
    }

    protected override Task OnWriteToStreamAsync(Type type, object value, Stream stream, HttpContentHeaders contentHeaders, FormatterContext formatterContext, TransportContext transportContext)
    {
        throw new NotImplementedException();
    }
}

In your controller/action you'll want to do something along the lines of:

public HttpResponseMessage UploadImage(Binary File) {
 //do something with your file
}

Answer 2

Take a look at this post https://stackoverflow.com/a/17073113/1944993 the answer of Kiran Challa is really nice!

The essential part :

Custom In-memory MultiaprtFormDataStreamProvider:

    public class InMemoryMultipartFormDataStreamProvider : MultipartStreamProvider
{
    private NameValueCollection _formData = new NameValueCollection();
    private List<HttpContent> _fileContents = new List<HttpContent>();

    // Set of indexes of which HttpContents we designate as form data
    private Collection<bool> _isFormData = new Collection<bool>();

    /// <summary>
    /// Gets a <see cref="NameValueCollection"/> of form data passed as part of the multipart form data.
    /// </summary>
    public NameValueCollection FormData
    {
        get { return _formData; }
    }

    /// <summary>
    /// Gets list of <see cref="HttpContent"/>s which contain uploaded files as in-memory representation.
    /// </summary>
    public List<HttpContent> Files
    {
        get { return _fileContents; }
    }

    public override Stream GetStream(HttpContent parent, HttpContentHeaders headers)
    {
        // For form data, Content-Disposition header is a requirement
        ContentDispositionHeaderValue contentDisposition = headers.ContentDisposition;
        if (contentDisposition != null)
        {
            // We will post process this as form data
            _isFormData.Add(String.IsNullOrEmpty(contentDisposition.FileName));

            return new MemoryStream();
        }

        // If no Content-Disposition header was present.
        throw new InvalidOperationException(string.Format("Did not find required '{0}' header field in MIME multipart body part..", "Content-Disposition"));
    }

    /// <summary>
    /// Read the non-file contents as form data.
    /// </summary>
    /// <returns></returns>
    public override async Task ExecutePostProcessingAsync()
    {
        // Find instances of non-file HttpContents and read them asynchronously
        // to get the string content and then add that as form data
        for (int index = 0; index < Contents.Count; index++)
        {
            if (_isFormData[index])
            {
                HttpContent formContent = Contents[index];
                // Extract name from Content-Disposition header. We know from earlier that the header is present.
                ContentDispositionHeaderValue contentDisposition = formContent.Headers.ContentDisposition;
                string formFieldName = UnquoteToken(contentDisposition.Name) ?? String.Empty;

                // Read the contents as string data and add to form data
                string formFieldValue = await formContent.ReadAsStringAsync();
                FormData.Add(formFieldName, formFieldValue);
            }
            else
            {
                _fileContents.Add(Contents[index]);
            }
        }
    }

    /// <summary>
    /// Remove bounding quotes on a token if present
    /// </summary>
    /// <param name="token">Token to unquote.</param>
    /// <returns>Unquoted token.</returns>
    private static string UnquoteToken(string token)
    {
        if (String.IsNullOrWhiteSpace(token))
        {
            return token;
        }

        if (token.StartsWith("\"", StringComparison.Ordinal) && token.EndsWith("\"", StringComparison.Ordinal) && token.Length > 1)
        {
            return token.Substring(1, token.Length - 2);
        }

        return token;
    }}

You can then the "MemoryMultiPartDataStreamProvider" in you webapi like this :

var provider = await Request.Content.ReadAsMultipartAsync<InMemoryMultipartFormDataStreamProvider>(new InMemoryMultipartFormDataStreamProvider());

    //access form data
    NameValueCollection formData = provider.FormData;

    //access files
    IList<HttpContent> files = provider.Files;