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

The content type is a header of the content, not of the request, which is why this is failing. AddWithoutValidation as suggested by Robert Levy may work, but you can also set the content type when creating the request content itself (note that the code snippet adds application/json in two places-for Accept and Content-Type headers):

HttpClient client = new HttpClient();
client.BaseAddress = new Uri("http://example.com/");
client.DefaultRequestHeaders
      .Accept
      .Add(new MediaTypeWithQualityHeaderValue("application/json"));//ACCEPT header

HttpRequestMessage request = new HttpRequestMessage(HttpMethod.Post, "relativeAddress");
request.Content = new StringContent("{\"name\":\"John Doe\",\"age\":33}",
                                    Encoding.UTF8, 
                                    "application/json");//CONTENT-TYPE header

client.SendAsync(request)
      .ContinueWith(responseTask =>
      {
          Console.WriteLine("Response: {0}", responseTask.Result);
      });

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For those who didn't see Johns comment to carlos solution ...

req.Content.Headers.ContentType = new MediaTypeHeaderValue("application/octet-stream");

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If you don't mind a small library dependency, Flurl.Http [disclosure: I'm the author] makes this uber-simple. Its PostJsonAsync method takes care of both serializing the content and setting the content-type header, and ReceiveJson deserializes the response. If the accept header is required you'll need to set that yourself, but Flurl provides a pretty clean way to do that too:

using Flurl.Http;

var result = await "http://example.com/"
    .WithHeader("Accept", "application/json")
    .PostJsonAsync(new { ... })
    .ReceiveJson<TResult>();

Flurl uses HttpClient and Json.NET under the hood, and it's a PCL so it'll work on a variety of platforms.

PM> Install-Package Flurl.Http

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try to use TryAddWithoutValidation

  var client = new HttpClient();
  client.DefaultRequestHeaders.TryAddWithoutValidation("Content-Type", "application/json; charset=utf-8");

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.Net tries to force you to obey certain standards, namely that the Content-Type header can only be specified on requests that have content (e.g. POST, PUT, etc.). Therefore, as others have indicated, the preferred way to set the Content-Type header is through the HttpContent.Headers.ContentType property.

With that said, certain APIs (such as the LiquidFiles Api, as of 2016-12-19) requires setting the Content-Type header for a GET request. .Net will not allow setting this header on the request itself -- even using TryAddWithoutValidation. Furthermore, you cannot specify a Content for the request -- even if it is of zero-length. The only way I could seem to get around this was to resort to reflection. The code (in case some else needs it) is

var field = typeof(System.Net.Http.Headers.HttpRequestHeaders)
    .GetField("invalidHeaders", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Static) 
  ?? typeof(System.Net.Http.Headers.HttpRequestHeaders) 
    .GetField("s_invalidHeaders", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Static);
if (field != null)
{
  var invalidFields = (HashSet<string>)field.GetValue(null);
  invalidFields.Remove("Content-Type");
}
_client.DefaultRequestHeaders.TryAddWithoutValidation("Content-Type", "text/xml");

Edit:

As noted in the comments, this field has different names in different versions of the dll. In the source code on GitHub, the field is currently named s_invalidHeaders. The example has been modified to account for this per the suggestion of @David Thompson.

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For those who troubled with charset

I had very special case that the service provider didn't accept charset, and they refuse to change the substructure to allow it... Unfortunately HttpClient was setting the header automatically through StringContent, and no matter if you pass null or Encoding.UTF8, it will always set the charset...

Today i was on the edge to change the sub-system; moving from HttpClient to anything else, that something came to my mind..., why not use reflection to empty out the "charset"? ... And before i even try it, i thought of a way, "maybe I can change it after initialization", and that worked.

Here's how you can set the exact "application/json" header without "; charset=utf-8".

var jsonRequest = JsonSerializeObject(req, options); // Custom function that parse object to string
var stringContent = new StringContent(jsonRequest, Encoding.UTF8, "application/json");
stringContent.Headers.ContentType.CharSet = null;
return stringContent;

Note: The null value in following won't work, and append "; charset=utf-8"

return new StringContent(jsonRequest, null, "application/json");

EDIT

@DesertFoxAZ suggests that also the following code can be used and works fine. (didn't test it myself, if it work's rate and credit him in comments)

stringContent.Headers.ContentType = new MediaTypeHeaderValue("application/json");

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Some extra information about .NET Core (after reading erdomke's post about setting a private field to supply the content-type on a request that doesn't have content)...

After debugging my code, I can't see the private field to set via reflection - so I thought I'd try to recreate the problem.

I have tried the following code using .Net 4.6:

HttpRequestMessage httpRequest = new HttpRequestMessage(HttpMethod.Get, @"myUrl");
httpRequest.Content = new StringContent(string.Empty, Encoding.UTF8, "application/json");

HttpClient client = new HttpClient();
Task<HttpResponseMessage> response =  client.SendAsync(httpRequest);  //I know I should have used async/await here!
var result = response.Result;

And, as expected, I get an aggregate exception with the content "Cannot send a content-body with this verb-type."

However, if i do the same thing with .NET Core (1.1) - I don't get an exception. My request was quite happily answered by my server application, and the content-type was picked up.

I was pleasantly surprised about that, and I hope it helps someone!

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Call AddWithoutValidation instead of Add (see this MSDN link).

Alternatively, I'm guessing the API you are using really only requires this for POST or PUT requests (not ordinary GET requests). In that case, when you call HttpClient.PostAsync and pass in an HttpContent, set this on the Headers property of that HttpContent object.

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The trick is that you can just set all kinds of headers like:

HttpRequestMessage request = new HttpRequestMessage();
request.Headers.Add("Accept-Language", "en");

but not any header. For example:

request.Headers.Add("Content-Type", "application/json");//wrong

will raise the run-time exception Misused header name. It may seem that this will work:

request.Headers.Add(
   HttpRequestHeader.ContentType.ToString(), //useless
   "application/json"
);

but this gives a useless header named ContentType, without the hyphen. Header names are not case-sensitive, but are very hyphen-sensitive.

You have to declare the encoding and type of the body when adding the body to the Content part of the http request:

string Body = "...";
request.Content = new StringContent(Body, Encoding.UTF8, "application/json");

Only then the applicable http header is automatically added to the request:

Content-Type: application/json; charset=utf-8

It was hard to find this out, with Fiddler, on a machine without a proxy server. Visual Studio used to have a Network Tool where you could inspect all headers, but only in version 2015, not in newer versions 2017 or 2022. If you use the debugger to inspect request.Headers, you will not find the header added automagically by StringContent().

---

var content = new JsonContent();
content.Headers.ContentType = new MediaTypeHeaderValue("application/json");
content.Headers.ContentType.Parameters.Add(new NameValueHeaderValue("charset", "utf-8"));
content.Headers.ContentType.Parameters.Add(new NameValueHeaderValue("IEEE754Compatible", "true"));

It's all what you need.

With using Newtonsoft.Json, if you need a content as json string.

public class JsonContent : HttpContent
   {
    private readonly MemoryStream _stream = new MemoryStream();
    ~JsonContent()
    {
        _stream.Dispose();
    }

    public JsonContent(object value)
    {
        Headers.ContentType = new MediaTypeHeaderValue("application/json");
        using (var contexStream = new MemoryStream())
        using (var jw = new JsonTextWriter(new StreamWriter(contexStream)) { Formatting = Formatting.Indented })
        {
            var serializer = new JsonSerializer();
            serializer.Serialize(jw, value);
            jw.Flush();
            contexStream.Position = 0;
            contexStream.WriteTo(_stream);
        }
        _stream.Position = 0;

    }

    private JsonContent(string content)
    {
        Headers.ContentType = new MediaTypeHeaderValue("application/json");
        using (var contexStream = new MemoryStream())
        using (var sw = new StreamWriter(contexStream))
        {
            sw.Write(content);
            sw.Flush();
            contexStream.Position = 0;
            contexStream.WriteTo(_stream);
        }
        _stream.Position = 0;
    }

    protected override Task SerializeToStreamAsync(Stream stream, TransportContext context)
    {
        return _stream.CopyToAsync(stream);
    }

    protected override bool TryComputeLength(out long length)
    {
        length = _stream.Length;
        return true;
    }

    public static HttpContent FromFile(string filepath)
    {
        var content = File.ReadAllText(filepath);
        return new JsonContent(content);
    }
    public string ToJsonString()
    {
        return Encoding.ASCII.GetString(_stream.GetBuffer(), 0, _stream.GetBuffer().Length).Trim();
    }
}

---

You can use this it will be work!

HttpRequestMessage msg = new HttpRequestMessage(HttpMethod.Get,"URL");
msg.Content = new StringContent(string.Empty, Encoding.UTF8, "application/json");

HttpResponseMessage response = await _httpClient.SendAsync(msg);
response.EnsureSuccessStatusCode();

string json = await response.Content.ReadAsStringAsync();

---

It appears that Microsoft tries to force the developers to follow their standards, without even giving any options or settings to do otherwise, which is really a shame especially given that this is a client and we are dictated by the server side requirements, especially given that Microsoft server side frameworks themselves require it!

So basically Microsoft tries to force us good habits when connecting to their server technologies that force us non good habits...

If anyone from Microsoft is reading this, then please fix it...

Either way for anyone that needs the content-type header for Get etc., while in an older .Net version it is possible to use the answer of @erdomke at https://stackoverflow.com/a/41231353/640195 this unfortunately no longer works in the newer .Net core versions.

The following code has been tested to work with .Net core 3.1 and from the source code on GitHub it looks like it should work with newer .Net versions as well.

private void FixContentTypeHeaders()
{
    var assembly = typeof(System.Net.Http.Headers.HttpRequestHeaders).Assembly;
    var assemblyTypes = assembly.GetTypes();

    var knownHeaderType = assemblyTypes.FirstOrDefault(n => n.Name == "KnownHeader");
    var headerTypeField = knownHeaderType?
                .GetFields(System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance)
                .FirstOrDefault(n => n.Name.Contains("HeaderType"));
    if (headerTypeField is null) return;

    var headerTypeFieldType = headerTypeField.FieldType;            
    var newValue = Enum.Parse(headerTypeFieldType, "All");

    var knownHeadersType = assemblyTypes.FirstOrDefault(n => n.Name == "KnownHeaders");
    var contentTypeObj = knownHeadersType.GetFields().FirstOrDefault(n => n.Name == "ContentType").GetValue(null);

    if (contentTypeObj is null) return;

    headerTypeField.SetValue(contentTypeObj, newValue);
}

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Ok, it's not HTTPClient but if u can use it, WebClient is quite easy:

using (var client = new System.Net.WebClient())
 {
    client.Headers.Add("Accept", "application/json");
    client.Headers.Add("Content-Type", "application/json; charset=utf-8");
    client.DownloadString(...);
 }

---

You need to do it like this:

    HttpContent httpContent = new StringContent(@"{ the json string }");
    httpContent.Headers.ContentType = new MediaTypeHeaderValue("application/json");
    client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));                
    HttpResponseMessage message = client.PostAsync(@"{url}", httpContent).Result;

---

try to use HttpClientFactory

services.AddSingleton<WebRequestXXX>()
        .AddHttpClient<WebRequestXXX>("ClientX", config =>
        {
           config.BaseAddress = new System.Uri("https://jsonplaceholder.typicode.com");
           config.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
           config.DefaultRequestHeaders.TryAddWithoutValidation("Content-Type", "application/json; charset=utf-8");
        });

======================

public class WebRequestXXXX
{
    private readonly IHttpClientFactory _httpClientFactory;

    public WebRequestXXXX(IHttpClientFactory httpClientFactory)
    {
        _httpClientFactory = httpClientFactory;
    }

    public List<Posts> GetAllPosts()
    {
        using (var _client = _httpClientFactory.CreateClient("ClientX"))
        {
            var response = _client.GetAsync("/posts").Result;

            if (response.IsSuccessStatusCode)
            {
                var itemString = response.Content.ReadAsStringAsync().Result;
                var itemJson = System.Text.Json.JsonSerializer.Deserialize<List<Posts>>(itemString, 
                    new System.Text.Json.JsonSerializerOptions 
                    {
                        PropertyNameCaseInsensitive = true
                    });

                return itemJson;
            }
            else
            {
                return new List<Posts>();
            }
        }
    }
}

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I got the answer whith RestSharp:

        private async Task<string> GetAccessTokenAsync()
        {
            var client = new RestClient(_baseURL);

            var request = new RestRequest("auth/v1/login", Method.POST, DataFormat.Json);

            request.AddHeader("Content-Type", "application/json");
            request.AddHeader("x-api-key", _apiKey);
            request.AddHeader("Accept-Language", "br");
            request.AddHeader("x-client-tenant", "1");
        
            ...
        }

It worked for me.

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For those wanting to set the Content-Type to Json specifically, you can use the extension method PostAsJsonAsync.

using System.Net.Http.Json; //this is needed for PostAsJsonAsync to work
//....
HttpClient client = new HttpClient();
HttpResponseMessage response = await
    client.PostAsJsonAsync("http://example.com/" + "relativeAddress",
                new
                {
                    name = "John Doe",
                    age = 33
                });
//Do what you need to do with your response

The advantage here is cleaner code and you get to avoid stringified json. More details can be found at: https://docs.microsoft.com/en-us/previous-versions/aspnet/hh944339(v=vs.118)

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I find it most simple and easy to understand in the following way:

async Task SendPostRequest()
{
    HttpClient client = new HttpClient();
    var requestContent = new StringContent(<content>);
    requestContent.Headers.ContentType = new MediaTypeHeaderValue("application/json");
    var response = await client.PostAsync(<url>, requestContent);
    var responseString = await response.Content.ReadAsStringAsync();
}
...

SendPostRequest().Wait();

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I end up having similar issue. So I discovered that the Software PostMan can generate code when clicking the "Code" button at upper/left corner. From that we can see what going on "under the hood" and the HTTP call is generated in many code language; curl command, C# RestShart, java, nodeJs, ...

That helped me a lot and instead of using .Net base HttpClient I ended up using RestSharp nuget package.

Hope that can help someone else!

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Api returned

"Unsupported Media Type","status":415

Adding ContentType to the jsonstring did the magic and this is my script working 100% as of today

    using (var client = new HttpClient())
            {
                   var endpoint = "api/endpoint;
                    var userName = "xxxxxxxxxx";
                    var passwd = "xxxxxxxxxx";
                    var content = new StringContent(jsonString, Encoding.UTF8, "application/json");

                    var authToken = Encoding.ASCII.GetBytes($"{userName}:{passwd}");

                client.BaseAddress = new Uri("https://example.com/");

                client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Basic", Convert.ToBase64String(authToken));
            
                HttpResponseMessage response = await client.PostAsync(endpoint, content);

                if (response.IsSuccessStatusCode)
                {
                    // Get the URI of the created resource.
                    Uri returnUrl = response.Headers.Location;
                    Console.WriteLine(returnUrl);
                }
                string responseBody = await response.Content.ReadAsStringAsync();
                return responseBody;
        }

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stringContent.Headers.ContentType = new MediaTypeHeaderValue(contentType);

And 🎉 YES! 🎉 ... that cleared up the problem with ATS REST API: SharedKey works now! 😄 👍 🍻

Source: https://github.com/dotnet/runtime/issues/17036#issuecomment-212046628

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