ASP.NET How read a multipart form data in Web API?

This is code i've used before to receive json data + an optional file:

var result = await Request.Content.ReadAsMultipartAsync();

var requestJson = await result.Contents[0].ReadAsStringAsync();
var request = JsonConvert.DeserializeObject<MyRequestType>(requestJson);

if (result.Contents.Count > 1)
{
    var fileByteArray = await result.Contents[1].ReadAsByteArrayAsync();
    ...
}

Its really neat that you can combine different types of data in a request like this.

Edit: an example of how to send this request:

let serialisedJson = JSON.stringify(anyObject);
let formData = new FormData();
formData.append('initializationData', serialisedJson);
// fileObject is an instance of File
if (fileObject) {
    // the 'jsonFile' name might cause some confusion: 
    // in this case, the uploaded file is actually a textfile containing json data
    formData.append('jsonFile', fileObject);
}

return new Promise((resolve, reject) => {
    let xhr = new XMLHttpRequest();
    xhr.open('POST', 'http://somewhere.com', true);
    xhr.onload = function(e: any) {
        if (e.target.status === 200) {
            resolve(JSON.parse(e.target.response));
        }
        else {
            reject(JSON.parse(e.target.response));
        }
    };
    xhr.send(formData);
});

You can read content and get all file information (in my example image) without copying to local disk in this way:

public async Task<IHttpActionResult> UploadFile()
{
    if (!Request.Content.IsMimeMultipartContent())
    {
        return StatusCode(HttpStatusCode.UnsupportedMediaType);
    }        

    var filesReadToProvider = await Request.Content.ReadAsMultipartAsync();

    foreach (var stream in filesReadToProvider.Contents)
    {
        // Getting of content as byte[], picture name and picture type
        var fileBytes = await stream.ReadAsByteArrayAsync();
        var pictureName = stream.Headers.ContentDisposition.FileName;
        var contentType = stream.Headers.ContentType.MediaType;
    }
}

For sending more than one file

        System.Web.HttpFileCollection hfc = System.Web.HttpContext.Current.Request.Files;

        //// CHECK THE FILE COUNT.
        for (int iCnt = 0; iCnt <= hfc.Count - 1; iCnt++)
        {
            System.Web.HttpPostedFile hpf = hfc[iCnt];
            string Image = UploadDocuments.GetDocumentorfileUri(hpf);
            UploadDocuments.UploadDocumentsIntoData(Image, hpf.FileName, id);

        }

Sending HTML Form Data in ASP.NET Web API: File Upload and Multipart MIME

 // read the file content without copying to local disk and write the content byte to file 
       try
       {
           var filesReadToProvider = await Request.Content.ReadAsMultipartAsync();
           JavaScriptSerializer json_serializer = new JavaScriptSerializer();

           foreach (var stream in filesReadToProvider.Contents)
           {

               //getting of content as byte[], picture name and picture type
               var fileBytes = await stream.ReadAsByteArrayAsync();
               var fileName = stream.Headers.ContentDisposition.Name;

               var pictureName = stream.Headers.ContentDisposition.FileName;
               var contentType = stream.Headers.ContentType.MediaType;
               var path = Path.Combine(HttpContext.Current.Server.MapPath("~/Images/Upload/"), json_serializer.Deserialize<string>(pictureName));

               File.WriteAllBytes(path, fileBytes);
           }

           return Request.CreateResponse(HttpStatusCode.OK);
       }catch(Exception ex)
       {
           return Request.CreateErrorResponse(HttpStatusCode.InternalServerError, ex);
       }

//Web User Interface Method protected void btnPrescAdd_Click(object sender, EventArgs e) {

        NameValueCollection collection = new NameValueCollection();

        collection.Set("c1", Session["CredID"].ToString());
        collection.Set("p1", "");
        collection.Set("p2", Request.Form["ctl00$MainContent$hdnHlthId"]);
        collection.Set("p3", Request.Form["ctl00$MainContent$PresStartDate"]);
        collection.Set("p4", Request.Form["ctl00$MainContent$PrescEndDate"]);

        FileUpload fileUpload = PrescUpload;

        ApiServices<Status> obj = new ApiServices<Status>();
        Status objReturn = obj.FetchObjectUploadAPI("POSTUHRPL", collection, 
         fileUpload, ApiServices<Status>.ControllerType.DU);

    }

//Request Method

  public T1 FetchObjectUploadAPI(string strAPIMethod, NameValueCollection collection, FileUpload file, ControllerType enObj)
    {
        T1 objReturn;
        try
        {
            string url = strWebAPIUrl + getControllerName(enObj) + strAPIMethod;

            MultipartFormDataContent content = new MultipartFormDataContent();



            int count = collection.Count;
            List<string> Keys = new List<string>();
            List<string> Values = new List<string>();

            //MemoryStream filedata = new MemoryStream(file);
            //Stream  stream = filedata;
            for (int i = 0; i < count; i++)
            {
                Keys.Add(collection.AllKeys[i]);
                Values.Add(collection.Get(i));

            }

            for (int i = 0; i < count; i++)
            {
                content.Add(new StringContent(Values[i], Encoding.UTF8, "multipart/form-data"), Keys[i]);
            }

            int fileCount = file.PostedFiles.Count();

            HttpContent filecontent = new StreamContent(file.PostedFile.InputStream);


            content.Add(filecontent, "files");

            HttpClient client = new HttpClient();


            HttpResponseMessage response = client.PostAsync(url, content).Result;

            if (response.IsSuccessStatusCode)
            {
                objReturn = (new JavaScriptSerializer()).Deserialize<T1>(response.Content.ReadAsStringAsync().Result);
            }
            else
                objReturn = default(T1);

        }
        catch (Exception ex)
        {
            Logger.WriteLog("FetchObjectAPI", ex, log4net_vayam.Constants.levels.ERROR);
            throw (ex);
        }
        return objReturn;

    }

https://stackoverflow.com/users/9600164/gaurav-sharma

Super late to the party, but anyone else having to work with ASP.NET Web API (1/2):

    [HttpPost, Route("img/up")]
    public async Task<IHttpActionResult> ItemImage()
    {
        var data = HttpContext.Current.Request.Form["example"];
        var item = JsonConvert.DeserializeObject<Example>(data);
        if (item == null) return BadRequest("Invalid request: Example cannot be null");

        var path = HostingEnvironment.MapPath("/") + @"img";
        if (!Directory.Exists(path))
            Directory.CreateDirectory(path);

        try
        {
            var image = HttpContext.Current.Request.Files["stream"];
            if (image == null) return BadRequest("Invalid request: no image received.");
            path = $@"{path}\{DateTime.Now:MMddyyHHmmss}.png";
            image.SaveAs(path);
        }
        catch (Exception e)
        {
            // TODO: Document Exception
        }
        return Ok();
    }

Here you can see that we're accepting HttpPost requests to /api/img/up endpoint. Currently we're not checking Mime type but you can if desired.

First we get the Form Data for your "example" and deserialize it from json to the Example class (replace this with what you're using as your model)

Then we make sure the img directory exists (modify the path if you want it somewhere outside of your API files)

Then we get the stream object from the HttpContext.Current.Request.Files["stream"] I took this snippet from an endpoint where the filename is a timestamp (to support multiple images for the same identifier)

I tried to keep it simple and sweet, it can be a tad abstract because of the Client / Server layering.

If you need to test I recommend PostMan: