C# Setting custom headers to httpclient gives error as misused header name

Question

I am trying to set custom headers like these

x-my-header:somecustomval
content-type:application/vnd.status+json
.
.
.
other headers

There are two approaches I tried, using a dictionary of headers

#1

_httpClient.DefaultRequestHeaders.Clear();

foreach (var h in headers)
{
  _httpClient.DefaultRequestHeaders.Add(h.Key.ToString(), h.Value.ToString());
}

#2

HttpRequestMessage req = new HttpRequestMessage();

foreach (var h in headers)
{
  req.Headers.Add(h.Key.ToString(), h.Value.ToString());
}

The error I am getting in both approaches is this:

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

How do you set the headers for a POST in the HttpClient in C# net core 3.1? Can you not set custom headers?

Nkosi · Accepted Answer · 2021-12-09 00:01:12Z

10

Use the HttpHeaders.TryAddWithoutValidation method in either of your attempts.

Internally the default HttpHeaders.Add will try to validate that you are adding a known HTTP header and will fail if not valid.

Reference HttpHeaders

edited Dec 9, 2021 at 0:01

answered Dec 8, 2021 at 23:56

Nkosi

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1 Comment

user20358 Over a year ago

thanks. tried that with approach #1 and it worked .

Matt Borja · Accepted Answer · 2024-04-03 18:11:49Z

Apparently this is a SO shortcut: but I came across this first, trying to figure this out myself before eventually finding the other 2 SO posts referred to below (2021 posts > 2013 posts, I guess).

Unfortunately, while .TryAddWithoutValidation in the originally accepted answer does seem to circumvent the issue on headers like Accept-Encoding, it will not work for others like Content-Encoding. For example, APIs like Salesforce Bulk API 2.0 will reject your request if you don't specify this header when Using Compression for uploading data.

Usage in the original post was the same as mine today, so I'm posting my workaround which was derived from https://stackoverflow.com/a/10679340 and https://stackoverflow.com/a/16959464/901156 indicating HttpRequestMessage.Content.Headers as the intended collection, not HttpRequestMessage.Headers.

For future reference, the correct implementation for using .Headers.Add(...) to add both of these headers would be req.Content.Headers.Add(...) not req.Headers.Add(...) for content-based headers, as demonstrated below (where I'm actually using their alias collections .ContentType and .ContentEncoding where they do exist; and not on their parent request object).

public async Task<AuthHttpResponse> AuthHttpRequest(HttpMethod method, string requestUri, byte[] data = null, string contentType = "text/csv; charset=UTF-8", bool enableCompression = true)
{
    using (var req = new HttpRequestMessage(method, requestUri))
    {
        req.Headers.Authorization = new AuthenticationHeaderValue(DefaultAuthenticationScheme, _authToken);

        if (enableCompression)
        {
            req.Headers.Add("Accept-Encoding", "gzip");

            if (data != null)
            {
                req.Content = Compress(data);
                req.Content.Headers.ContentType = new MediaTypeHeaderValue(contentType);
                req.Content.Headers.ContentEncoding.Add("gzip");
            }
        }
        else
        {
            if (data != null)
            {
                req.Content = new ByteArrayContent(data);
                req.Content.Headers.ContentType = new MediaTypeHeaderValue(contentType);
            }
        }

        var res = await _httpClient.SendAsync(req);
        res.EnsureSuccessStatusCode();

        string json;

        // Only decompress if server acknowledged
        if (res.Content.Headers.ContentEncoding.FirstOrDefault() == "gzip")
        {
            var buffer = await res.Content.ReadAsByteArrayAsync();
            using (var stream = new MemoryStream(buffer))
                json = await Decompress(stream);
        }
        else
        {
            json = await res.Content.ReadAsStringAsync();
        }

        return new AuthHttpResponse { Content = json, Response = res };
    }
}

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