68

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?

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2
  • 1
    Do you mind sharing the code in the WebApi side that consumes the MultipartFormDataContent? Commented Jan 25, 2016 at 16:49
  • Example send parameters as multipeart dotnet 3.0 in answer stackoverflow.com/a/59308653/5333683 Commented Dec 12, 2019 at 16:33

3 Answers 3

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136

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;
    }
}
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7 Comments

Use StreamContent for large files: var fileContent = new StreamContent(File.OpenRead(fileName));
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!!
Strangely this does not work for me. I had to add quotes around the content names, see this: stackoverflow.com/questions/15638622/…
FYI, there is a constant for "attachment" under System.Net.Mime.DispositionTypeNames
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.
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14

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.

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Comments

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);
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3 Comments

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.
@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.
I just get StatusCode: 415, ReasonPhrase: 'Unsupported Media Type'

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