How do you set the Content-Type header for an HttpClient request with MultipartFormDataContent?

Question

I looked at the MS Source code according to their interpretation the HttpClient itself does not have "Content-Type" only the content should have content-type. Seems logical except when you're dealing with MultipartFormDataContent. MultipartFormDataContent completely ignores the following code:

string boundary = "--" + GenerateRandomString();
using (var content = new MultipartFormDataContent(boundary))
{
    content.Headers.ContentType = new MediaTypeHeaderValue($"multipart/form-data");
    content.Headers.ContentType.Parameters.Add(new NameValueHeaderValue("boundry", boundary));
    ...
}

No "Content-Type" is present in the request. And also ignores:

string boundary = "--" + GenerateRandomString();
using (var content = new MultipartFormDataContent(boundary))
{
    content.Headers.Remove("Content-Type");
    content.Headers.TryAddWithoutValidation("Content-Type", "multipart/form-data; boundary=" + boundary);
    ...
}

Attemting to set it at on the HttpClient

client.DefaultRequestHeaders.TryAddWithoutValidation("Content-Type", "multipart/form-data; boundary=" + boundary);

throws the following error:

System.InvalidOperationException: 'Misused header name. Make sure request headers are used with HttpRequestMessage, response headers with HttpResponseMessage, and content headers with HttpContent objects.'

I can find plenty of examples of how to do this using StringContent but none with MultipartFormDataContent. MultipartFormDataContent allows setting the Content-Type and Content-Disposition with each field, I need this more at the client level. I need a header that looks something like this:

accept: application/json
Content-Type: multipart/form-data; boundary=----WebKitFormBoundary7MA4YWxkTrZu0gW
Authorization: Basic ZXlKbGRDSTZJakUxTWpZNU9UTXpOTnpkMjl5WkNJNklqa3haRGxtWkdKa1lUazRaVEJqWmpsalpUaGxNV1V3TXpOalxuWmpCbE1tVXhJaXdpZFhObGNpSTZJbUZrYldsdUluMD1cbjo=
Cache-Control: no-cache

Many non-Microsoft APIs require the "boundary" tag so it can distinguish the individual fields of data being sent. The validation here on the request seems a little over the top. Even TryAddWithoutValidation doesn't work (maybe a bug?). I realize that it may be possible to interpret RFC7578 in a way that says it shouldn't be required but flat out not allowing it doesn't seem right to me either. Anyone else ever run into this issue and solve it.

But MultipartFormDataContent already sets Content-Type header correctly by itself (with boundary), so seems no additional efforts required? I just tested with HttpClient in .NET 4.6 and it sends Content-Type just fine — Evk
Hmm, it doesn't do that for me. I am using latest .Net Standard, so maybe it actually is a bug? I thought that it should do it automatically and I was surprised not to see it in the request header. I recorded the request with Fiddler and also used HttpLoggingHandler to dump the request out to file. — user2033791
@user2033791 what runtime did you target? .NET Standard isnt' a runtime. HttpClient itself is available through the System.Net.Http package. Which version did you use? — Panagiotis Kanavos
I can't reproduce any issue using MultipartFormDataContent even when the call to HttpClient.PostAsync is in a .NET Standard 2.0 library. The boundary is there and so is the content type. No header manipulations were needed — Panagiotis Kanavos
How do you know there is a problem? Did you inspect the HTTP request with a proxy like Fiddler ir did you get a server error and assumed it's the boundary? — Panagiotis Kanavos

user2033791 user2033791 · Accepted Answer · 2018-05-24T12:01:18

Initially I thought this was an HttpClient bug. I added logging to capture the request and the response. That logging was missing headers which lead me to believe that the missing "multi-part/form-data" content header was the issue and the reason the API I'm using kept telling me it couldn't find a required field. It turns out to be an issue with how the API handles the data it's sent when it's multi-part/form-data. After comparing both my HttpWebRequest and the HttpClient request in fiddler I discovered the following difference in the data being sent:

HttpWebRequest

----vekhftkcthxr
Content-Disposition: form-data; name="name";

d30-20180524

HttpClient

----bcgifxyjkmkw
Content-Type: text/plain; charset=utf-8
Content-Disposition: form-data; name=name

d30-20180524

I build the HttpWebRequest manually so I included the quotes and ending semi-colons. The HttpClient request is built for me and does not include the extra quotes and semi-colon. So the API I am using does not play well with request being generated by the HttpClient even though the request is technically correct.

Thanks to Panagiotis Kanavos for showing me my error.

How do you set the Content-Type header for an HttpClient request with MultipartFormDataContent?

1 Answers