Curl -F equivalent in C#

Question

I am trying to use HTTPClient object in C# to send a post request to an API. This is the CURL command:

curl -X POST https://zzz.zzz.zzz/yyy -F Key=abcd -F media=@"audio.aac"

I wrote the following code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Net.Http;
using System.IO;
using System.Net.Http.Headers;

namespace Speech2Text
{
  class Program
  {
    static void Main(string[] args)
    {
        curl().Wait();
    }

    static async Task curl()
    {
        var client = new HttpClient();

        //Create List of KeyValuePairs
        List<KeyValuePair<string, string>> bodyProperties = new List<KeyValuePair<string, string>>();

        bodyProperties.Add(new KeyValuePair<string, string>("key", "abcd"));
        bodyProperties.Add(new KeyValuePair<string, string>("media", "@audio.aac"));

        var dataContent = new FormUrlEncodedContent(bodyProperties.ToArray());

        HttpResponseMessage response = await client.PostAsync("https://zzz.zzz.zzz/yyy", dataContent);

        HttpContent responseContent = response.Content;

        using (var reader = new StreamReader(await responseContent.ReadAsStreamAsync()))
        {
            Console.WriteLine(await reader.ReadToEndAsync());
        }
    }
  }
}

But I keep getting this:

{"code":400,"message":"Please use [media] for the media file field name in your POST request."}

Is there any issue in the dataContect variable or it's something else?

Answer 1

When sending a POST it is important that the Content-Type you send to server is indeed the one the server expects. You can easily verify what is send with the -trace option from curl:

curl -v --trace - -X POST http://localhost -F Key=abcd -F media=@"image.txt"

When you run this you'll find somewhere in first few lines this:

00b0: 63 6f 6e 74 69 6e 75 65 0d 0a 43 6f 6e 74 65 6e continue..Conten
00c0: 74 2d 54 79 70 65 3a 20 6d 75 6c 74 69 70 61 72 t-Type: multipar
00d0: 74 2f 66 6f 72 6d 2d 64 61 74 61 3b 20 62 6f 75 t/form-data; bou
00e0: 6e 64 61 72 79 3d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d ndary=----------
00f0: 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d ----------------
0100: 2d 2d 61 61 62 64 66 31 66 66 38 35 66 66 0d 0a --aabdf1ff85ff..
0110: 0d 0a                                           ..

So curl is sending an Content-Type of multipart/form-data;

In your code you use the class FormUrlEncodedContent which states in its description:

A container for name/value tuples encoded using application/x-www-form-urlencoded MIME type.

So that is not the correct Conent-Type.

A better match is the MultipartFormDataContent because it states:

Provides a container for content encoded using multipart/form-data MIME type.

With the correct classes found we only need to build up the container:

var dataContent = new MultipartFormDataContent();
var keyValue = new ByteArrayContent( Encoding.ASCII.GetBytes("abcd") );
dataContent.Add(keyValue, "key");

using (var client = new HttpClient())
{
    // open your file
    using (var fs = File.OpenRead(@"c:\path\to\audio.acc"))
    {
        // create StreamContent from the file   
        var fileValue = new StreamContent(fs);
        // add the name and meta-data 
        dataContent.Add(fileValue, "media", "audio.acc");

        HttpResponseMessage response = await client.PostAsync(
                  "http://yoursite.org", 
                  dataContent);

        HttpContent responseContent = response.Content;

        using (var reader = new StreamReader(await responseContent.ReadAsStreamAsync()))
        {
            Console.WriteLine(await reader.ReadToEndAsync());
        }
    }
}

Running this code reveals in Fiddler:

POST / HTTP/1.1
Content-Type: multipart/form-data; boundary="f7335f00-bc16-4518-ae7a-149491403792"
Host: yoursite.org
Content-Length: 1014
Expect: 100-continue
Connection: Keep-Alive

--f7335f00-bc16-4518-ae7a-149491403792
Content-Disposition: form-data; name=key

abcd
--f7335f00-bc16-4518-ae7a-149491403792
Content-Disposition: form-data; name=media; filename=audio.acc; filename*=utf-8''audio.acc

which is more in line with the trace output from curl.