ASP.NET WebApi: how to perform a multipart post with file upload using WebApi HttpClient

Question

I have a WebApi service handling an upload from a simple form, like this one:

    <form action="/api/workitems" enctype="multipart/form-data" method="post">
        <input type="hidden" name="type" value="ExtractText" />
        <input type="file" name="FileForUpload" />
        <input type="submit" value="Run test" />
    </form>

However, I can't figure out how to simulate the same post using the HttpClient API. The FormUrlEncodedContent bit is simple enough, but how do I add the file contents with the name to the post?

Do you mind sharing the code in the WebApi side that consumes the MultipartFormDataContent? — granadaCoder, Jan 25 '16 at 16:49
Example send parameters as multipeart dotnet 3.0 in answer https://stackoverflow.com/a/59308653/5333683 — D.Oleg, Dec 12 '19 at 16:33

score 134 · Accepted Answer · edited Jun 05 '13 at 07:12

134

After much trial and error, here's code that actually works:

using (var client = new HttpClient())
{
    using (var content = new MultipartFormDataContent())
    {
        var values = new[]
        {
            new KeyValuePair<string, string>("Foo", "Bar"),
            new KeyValuePair<string, string>("More", "Less"),
        };

        foreach (var keyValuePair in values)
        {
            content.Add(new StringContent(keyValuePair.Value), keyValuePair.Key);
        }

        var fileContent = new ByteArrayContent(System.IO.File.ReadAllBytes(fileName));
        fileContent.Headers.ContentDisposition = new ContentDispositionHeaderValue("attachment")
        {
            FileName = "Foo.txt"
        };
        content.Add(fileContent);

        var requestUri = "/api/action";
        var result = client.PostAsync(requestUri, content).Result;
    }
}

edited Jun 05 '13 at 07:12

Brijesh Mishra

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answered May 24 '12 at 19:27

Michael Teper

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Use StreamContent for large files: var fileContent = new StreamContent(File.OpenRead(fileName)); – Jalal El-Shaer Jun 03 '12 at 22:16
Woohoo -- I had a totally different issue, but needed to post content nonetheless. I was able to modify to fit my needs. Thanks a lot!! – Matthew Pitts Oct 28 '12 at 07:36
3

Strangely this does not work for me. I had to add quotes around the content names, see this: http://stackoverflow.com/questions/15638622/how-to-upload-files-to-asp-net-mvc-4-0-action-running-in-iis-express-with-httpcl/15638623#15638623 – deerchao Mar 27 '13 at 08:53
@deerchao I think the difference is in how you process the post on the back-end. In my case, I was using WebApi and `MultipartFormDataStreamProvider`. – Michael Teper Mar 27 '13 at 14:35
6

FYI, there is a constant for "attachment" under System.Net.Mime.DispositionTypeNames – Robert Jan 10 '15 at 23:47
2

I too had to add quotes around the name value: FileName = "\"TestImage.jpg\"". I am using ServiceStack on the server instead of WebAPI not sure if that is related or not. – Rob Bird Jan 30 '15 at 20:56
I keep getting `StatusCode: 415, ReasonPhrase: 'Unsupported Media Type'`, any ideas ? – djack109 Apr 23 '20 at 18:17

score 14 · Answer 2 · answered Sep 03 '14 at 00:34

Thank you @Michael Tepper for your answer.

I had to post attachments to MailGun (email provider) and I had to modify it slightly so it would accept my attachments.

var fileContent = new ByteArrayContent(System.IO.File.ReadAllBytes(fileName));
fileContent.Headers.ContentDisposition = 
        new ContentDispositionHeaderValue("form-data") //<- 'form-data' instead of 'attachment'
{
    Name = "attachment", // <- included line...
    FileName = "Foo.txt",
};
multipartFormDataContent.Add(fileContent);

Here for future reference. Thanks.

score 11 · Answer 3 · edited Mar 12 '23 at 07:52

11

You need to look for various subclasses of HttpContent.

You create a multiform http content and add various parts to it. In your case you have a byte array content and form url encoded along the lines of:

HttpClient c = new HttpClient();
var fileContent = new ByteArrayContent(new byte[100]);
fileContent.Headers.ContentDisposition 
  = new ContentDispositionHeaderValue("attachment")
{
  FileName = "myFilename.txt"
};

var formData = new FormUrlEncodedContent(new[]
{
  new KeyValuePair<string, string>("name", "ali"),
  new KeyValuePair<string, string>("title", "ostad")
}); 
        
MultipartContent content = new MultipartContent();
content.Add(formData);
content.Add(fileContent);
c.PostAsync(myUrl, content);

edited Mar 12 '23 at 07:52

Konrad Viltersten

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answered Apr 26 '12 at 20:24

Aliostad

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That doesn't work, unfortunately, although the `ContentDisposition` header does seems to take me a step forward. If I use your sample code as is, I get an `Unsupported Media Type` exception. However, if I replace `MultipartContent` with `MultipartFormDataContent` the exception goes away. On the server, however, the value of the hidden field is missing. – Michael Teper Apr 26 '12 at 22:42
@MichaelTeper I have not tested the code. I will play a bit with it and update the post, but my point was to show you a snippet of what you need to look for and not a working code. – Aliostad Apr 27 '12 at 00:06
I just get `StatusCode: 415, ReasonPhrase: 'Unsupported Media Type'` – djack109 Apr 23 '20 at 18:17

ASP.NET WebApi: how to perform a multipart post with file upload using WebApi HttpClient

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