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Add content-type header to httpclient GET request

I am trying to add Content-Type header to HttpClient GET request, here my code:
HttpClient client=new ....
bool added = client.DefaultRequestHeaders.TryAddWithoutValidation("Content-Type", "application/x-www-form-urlencoded");
var response = await client.GetAsync(...
but the added variable is false, i.e it failed to add the header.
How can I add this header?
NOTE:
This post deals with POST request, I asked about GET

If you look at the Http/1.1 specification:
A sender that generates a message containing a payload body SHOULD
generate a Content-Type header field in that message unless the
intended media type of the enclosed representation is unknown to the
sender. If a Content-Type header field is not present, the recipient
MAY either assume a media type of "application/octet-stream"
([RFC2046], Section 4.5.1) or examine the data to determine its type.
Check also the MDN on get requests
The HTTP GET method requests a representation of the specified resource. Requests using GET should only retrieve data.
Sending body/payload in a GET request may cause some existing implementations to reject the request — while not prohibited by the specification, the semantics are undefined. It is better to just avoid sending payloads in GET requests.
Effectively, that means that wether you send or not the header, it's going to be ignored and/or rejected.
When setting the content type, it's better to set it from the content itself: How do you set the Content-Type header for an HttpClient request?

Im currently working on a project, where I call an api using a POST request.
This might help in your case. Its how its done in an official Microsoft Documentation.
using (var content = new ByteArrayContent(byteData))
{
// This example uses the "application/octet-stream" content type.
// The other content types you can use are "application/json"
// and "multipart/form-data".
content.Headers.ContentType = new mediaTypeHeaderValue("application/octet-stream");
response = await client.PostAsync(uriBase, content);
}

How do I not exclude charset in Content-Type when using HttpClient?

I am attempting to use HttpClient in a .net core project to make a GET request to a REST service that accepts/returns JSON. I don't control the external service.
No matter how I try, I can't figure out to set the Content-Type header to application/json only.
When I use
client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
it sends in the HTTP GET request:
Content-Type: application/json; charset=utf-8
However, this particular service does not work with this. It will only work if the header is:
Content-Type: application/json
I've tried setting headers without validation, and all the approaches I've found on the web/SO doesn't apply to .net core. All the other the approaches to sending HTTP requests aren't available in .net core, so I need to figure this out. How can I exclude the charset in content-type?
EDIT with workaround
As mentioned in the answers, the service should be using the Accept header. The workaround (as Shaun Luttin has in his answer) is to add an empty content to the GET (what? GETs don't have content! yeah...). It's not pretty, but it does work.

You're setting the Accept header. You need to set the ContentType header instead, which is only canonical for a POST.
var client = new HttpClient();
var content = new StringContent("myJson");
content.Headers.ContentType = MediaTypeHeaderValue.Parse("application/json");
var result = client.PostAsync("http://bigfont.ca", content).Result;
If you really want to set it for a GET, you can do this:
var client = new HttpClient();
var message = new HttpRequestMessage(HttpMethod.Get, "http://www.bigfont.ca");
message.Content = new StringContent(string.Empty);
message.Content.Headers.Clear();
message.Content.Headers.Add("Content-Type", "application/json");
var result = client.SendAsync(message).Result;

If you are the client and you perform a GET request how can you specify the Content-Type? Isn't it supposed to say what your are able to Accept ? According to this 7.2.1 Type you can only set Content-Type when there is Body.

Modify request headers per request C# HttpClient PCL

I'm currently using the System.Net.Http.HttpClient for cross platform support.
I read that it is not a good practice to instantiate a HttpClient object for each request and that you should reuse it whenever possible.
Now I have a problem while writing a client library for a service. Some API calls need to have a specific header, some MUST not include this specific header.
It seems that I can only manipulate the "DefaultRequestHeaders" which will be send with each request.
Is there an option when actually making the request with e.g. "client.PostAsync()" to modify the headers only for the specific request?
(Info: Requests can be multi threaded).
Thanks in advance!

Yes, you can create a new HttpRequestMessage, set all the properties you need to, and then pass it to SendAsync.
var request = new HttpRequestMessage() {
RequestUri = new Uri("http://example.org"),
Method = HttpMethod.Post,
Content = new StringContent("Here is my content")
}
request.Headers.Accept.Add(...); // Set whatever headers you need to
var response = await client.SendAsync(request);

Use HttpContent.Headers. Simply create HttpContent instance with required headers and pass it to PostAsync method.

Resend HTTP header

I have application. It send request to my proxy class. Proxy must to parse http header string (I done this) and resend it request to server to get a video.
At first, mediacomponent connect to proxy:
var uri = new Uri("http://127.0.0.1:2233/files/1.mp4");
videoPlayer.Source = uri;
Play();
Proxy get http header string
"GET /files/1.mp4 HTTP/1.1\r\nCache-Control: no-cache\r\nConnection: Keep-Alive\r\nPragma: getIfoFileURI.dlna.org\r\nAccept: */*\r\nUser-Agent: NSPlayer/12.00.7601.17514 WMFSDK/12.00.7601.17514\r\nGetContentFeatures.DLNA.ORG: 1\r\nHost: 127.0.0.1:2233\r\n\r\n"
I replase host:
"GET /files/1.mp4 HTTP/1.1\r\nCache-Control: no-cache\r\nConnection: Keep-Alive\r\nPragma: getIfoFileURI.dlna.org\r\nAccept: */*\r\nUser-Agent: NSPlayer/12.00.7601.17514 WMFSDK/12.00.7601.17514\r\nGetContentFeatures.DLNA.ORG: 1\r\nHost: myserver.ru\r\n\r\n"
Now proxy must get video from server. What must I do?

When using .NET, you don't have to manually create the HTTP message itself. Instead, use the classes in the System.Net.Http namespace to form and send an HTTP message and process the response.
For example, sending an HTTP GET message to a URL can be as simple as:
var uri = new Uri("http://www.foobar.com/");
var client = new HttpClient();
string body = await client.GetStringAsync(uri);
Note that this general approach will download the entire contents of the resource at the given URI. In your case, you may not want to wait for the whole video to download before you start playing/processing/storing it. In which case, you might want to use the HttpClient.ReadAsStream() method which will return a stream from which you can read until the stream closes.

Why I cannot set 'Allow' in HTTP response header?

I've written a RESTful API using ASP.NET Web Api. Now I'm trying to make it returns the allowed verbs for a controller. I'm trying to do it with the following code:
[AcceptVerbs("OPTIONS")]
public HttpResponseMessage Options()
{
var response = new HttpResponseMessage(HttpStatusCode.OK);
response.Headers.Add("Access-Control-Allow-Origin", "*");
response.Headers.Add("Access-Control-Allow-Methods", "POST");
response.Headers.Add("Allow", "POST");
return response;
}
But instead of getting a Allow Header on my response, I'm getting a 500 Internal Server Error. While debugging I receive the following error:
{"Misused header name. Make sure request headers are used with HttpRequestMessage, response headers with HttpResponseMessage, and content headers with HttpContent objects."}
Is that possible to set that header?

As the error message says, you must use content headers with HttpContent objects.
response.Content.Headers.Add("Allow", "POST");
Must admit this is kinda weird API...

Allow is a content header.
response.Content.Headers.Allow.Add("POST");