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How to add the header Xamarin Forms using System.Net.Http; [duplicate]

I'm trying to set the Content-Type header of an HttpClient object as required by an API I am calling.
I tried setting the Content-Type like below:
using (var httpClient = new HttpClient())
{
httpClient.BaseAddress = new Uri("http://example.com/");
httpClient.DefaultRequestHeaders.Add("Accept", "application/json");
httpClient.DefaultRequestHeaders.Add("Content-Type", "application/json");
// ...
}
It allows me to add the Accept header but when I try to add Content-Type it throws the following exception:
Misused header name. Make sure request headers are used with
HttpRequestMessage, response headers with HttpResponseMessage, and
content headers with HttpContent objects.
How can I set the Content-Type header in a 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);
});

For those who didn't see Johns comment to carlos solution ...
req.Content.Headers.ContentType = new MediaTypeHeaderValue("application/octet-stream");

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

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

.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.

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");

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!

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.

The trick is that you can just set all kinds of headers like:
HttpRequestMessage request = new HttpRequestMessage();
request.Headers.Add("Accept-Language", "en"); //works OK
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.
The solution is 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();
}
}

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);
}

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();

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(...);
}

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>();
}
}
}
}

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.

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;

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://learn.microsoft.com/en-us/previous-versions/aspnet/hh944339(v=vs.118)

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();

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!

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;
}

For my scenario, a third-party API was creating the HttpRequestMessage, so I was not able to use the top-voted answers to resolve the issue. And I didn't like the idea of messing with reflection so the other answers didn't work either.
Instead, I extended from AndroidMessageHandler and used this new class as a parameter to HttpClient. AndroidMessageHandler contains the method SendAsync which can be overridden in order to make changes to the HttpRequestMessage object before it is sent. If you don't have access to the Android Xamarin libaries, you may be able to figure something out with HttpMessageHandler.
public class XamarinHttpMessageHandler : global::Xamarin.Android.Net.AndroidMessageHandler
{
protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
{
// Here I make check that I'm only modifying a specific request
// and not all of them.
if (request.RequestUri != null && request.RequestUri.AbsolutePath.EndsWith("download") && request.Content != null)
{
request.Content.Headers.Add("Content-Type", "text/plain");
}
return base.SendAsync(request, cancellationToken);
}
}
Then to use:
var client = new HttpClient(new XamarinHttpMessageHandler());

So if you're trying to do a /$batch OData request like this Microsoft article demonstrates where you're supposed to have a Content-Type header like:
Content-Type: multipart/mixed;boundary=batch_d3bcb804-ee77-4921-9a45-761f98d32029
string headerValue = "multipart/mixed;boundary=batch_d3bcb804-ee77-4921-9a45-761f98d32029";
//You need to set it like thus:
request.Content.Headers.ContentType = MediaTypeHeaderValue.Parse(headerValue);
Again, the magic you need is: MediaTypeHeaderValue.Parse(...)

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

.NET Core Httpclient works but .Net Framework 4.7.2 httpclient doesn't

Sorry for the awful title, I'm not really sure how to phrase my issue in a short title format.
I'm trying to communicate with an external API. I make a basic authentication request to that API and get an x-csrf-token and a session token from the api.
I then make another request to that API, now using the x-csrf-token as a header and attach the session token to the header as "cookie".
The team that maintains the API sent me an example project that handles all of the above, and it looks like this:
public static async Task<string> Send(string apiname, string value)
{
// Fetch the authorization tokens from SAP
HttpClient client = new HttpClient();
client.BaseAddress = new Uri(basePath);
client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Basic", System.Convert.ToBase64String(System.Text.Encoding.UTF8.GetBytes(user + ":" + password)));
client.DefaultRequestHeaders.Add("x-csrf-token", "Fetch");
string csrfToken = "";
string sessionCookie = "";
HttpResponseMessage response = await client.GetAsync(string.Empty);
IEnumerable<string> values;
if (response.Headers.TryGetValues("x-csrf-token", out values))
{
csrfToken = values.FirstOrDefault();
}
if (response.Headers.TryGetValues("set-cookie", out values))
{
sessionCookie = values.Where(s => s.StartsWith("SAP_SESSION")).FirstOrDefault();
}
// Reinstantiate the HttpClient, adding the tokens we just got from SAP
client = new HttpClient();
client.DefaultRequestHeaders.Add("x-csrf-token", csrfToken);
client.DefaultRequestHeaders.Add("cookie", sessionCookie);
client.BaseAddress = new Uri(basePath);
client.DefaultRequestHeaders.Accept.Clear();
client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
// Have to parse the string this way otherwise it'll break the dates
JToken token;
using (var sr = new StringReader(value))
using (var jr = new JsonTextReader(sr) { DateParseHandling = DateParseHandling.None })
{
token = JToken.ReadFrom(jr);
}
HttpResponseMessage response2 = await client.PostAsJsonAsync(apiname, token);
string responseBody = await response2.Content.ReadAsStringAsync();
return responseBody;
}
This all works great as a .NET Core webAPI (and also as a .netcore console app).
Interestingly enough (in my opinion anyway), when I use the exact same code in a .net 4.7.2 project, it doesn't append the "cookie" header properly, and so I'm getting an unauthorized redirect back from the API.
To be absolutely sure that I didn't change any code, I started from scratch with a brand new .netcore 2.0 console app and a brand new .net 4.7.2 console app and copy-pasted the exact same code and installed the same nuget packages (Newtonsoft.JSON and Microsoft.WebApi.Client). I inspected my web traffic with fiddler (seen below) and you can see that in .netcore, the cookie appends properly and everything works, but in .net 4.7.2, the API returns a redirect to authenticate.

HttpClient will eat the custom cookie if you do not setUseCookies to false,
using (var handler = new HttpClientHandler { UseCookies = false })
using (client = new HttpClient(handler) { BaseAddress = new Uri(Path) }){
client.DefaultRequestHeaders.Add("cookie", cookieValue);
}
It will try to use the cookie container and at the same time ignore any custom cookie headers, very frustrating behavior if you ask me.

.Net Framework uses Cookie Container.
Also core, perhaps its a better implementation then what you are doing now and more supported.
Please see cookie container docs
Small example:
var cookieContainer = new CookieContainer();
this.handler = new HttpClientHandler
{
CookieContainer = cookieContainer,
UseCookies = true
};
client = new HttpClient(handler);

Post rest web api working from Postman but not returning any response from code in C# project

There is a POST rest api which used to work from code before. But recently it is broken and is not returning any response. However if I try to call the api from the Postman, then it works fine.
In what way can I debug this to find the root cause of the issue ?
Following is the C# code which I am using to call this post rest api
public async Task SaveToServerAsync()
{
string filePath = #"<filePath>";
ServicePointManager.ServerCertificateValidationCallback = delegate { return true; };
// tried this line of code from another SO answer, but this didn't work either
ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;
using (var client = new HttpClient())
{
client.BaseAddress = new Uri("https://<server name>/");
client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", "d2ebf9aefbaa416adcd0");
client.DefaultRequestHeaders.Accept.Clear();
client.DefaultRequestHeaders.Add("Accept", "*/*");
client.DefaultRequestHeaders.Add("Connection", "keep-alive");
using (var fileStream = new FileStream(filePath, FileMode.Open))
{
var content = new MultipartFormDataContent();
content.Add(new StreamContent(fileStream), "file", filePath);
content.Add(new StringContent("e8d002f9-f381-44c2-bce0-13416929f14d"), "Id");
try
{
var response = await client.PostAsync("<rest api end point>", content).ConfigureAwait(false);
if (response.IsSuccessStatusCode)
{
Debug.Write("Response received");
}
}
catch (Exception ex)
{
Debug.Write("Exception occured");
Debug.Write(ex.Message);
}
finally
{
}
}
}
}
It always goes to the exception block with exception as "The task was cancelled"
Not sure how can I debug it when it anyway works from the Postman.

So the problem was related to ExpectContinue header which goes as true by default. Somehow server was not handling it properly and client was waiting for continue (100) message for indefinite time.
For the time being manually setting this header to be false worked for us:
httpClient.DefaultRequestHeaders.ExpectContinue = false;

Concur V3 Api not working for Users

I configured a Concur Sandbox and played around with the api. Since the User api is not supported by the .Net SDK I wrote following code.
Issue is that code for Expenses work (returns 200 with valida result), but code for users returns 401 Unauthorized.
Expenses
using (var httpClient = new HttpClient())
{
httpClient.DefaultRequestHeaders.Authorization = new System.Net.Http.Headers.AuthenticationHeaderValue("OAuth", oauthAccessToken);
httpClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/xml"));
var requestUri = "https://www.concursolutions.com/api/v3.0/expense/receiptimages";
var respone = await httpClient.GetAsync(requestUri);
if (respone.IsSuccessStatusCode)
{
var result = respone.Content.ReadAsStringAsync();
//throw new InvalidUriException(string.Format("Invalid uri: {0}", requestUri));
}
}
Ideally this should also work as the previous code works,
Users
using (var httpClient = new HttpClient())
{
httpClient.DefaultRequestHeaders.Authorization = new System.Net.Http.Headers.AuthenticationHeaderValue("OAuth", oauthAccessToken);
httpClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/xml"));
var requestUri = "http://www.concursolutions.com/api/v3.0/common/users?user=user1%40company.net";
var respone = await httpClient.GetAsync(requestUri);
if (!respone.IsSuccessStatusCode)
{
//throw new InvalidUriException(string.Format("Invalid uri: {0}", requestUri));
}
}
}
I thought it's a permission issue, but I given allowed enough permission.
Here is the Administration->Webservice app configuration,

Issue is that the User Api call needs to be https.

401 Unauthorized on SECOND HttpClient/HttpWebRequest call

I have a application that uses the SharePoint 2010 REST API.
In the process of creating an Item there are multiple request done after each other:
1 Call: Getting Items from List: Succes
2 Call: Create Item: 401 Unauthorized
This is the same if I do it like this:
1 Call: Create Item: Succes
2 Call: Delete Item: 401 Unauthorized
What I know is that my functions work separately they DON'T work when they are called after each other.
When I close the application (Windows Phone 8.1 app) after creating a item and when restarted try to delete the item it works.
First I thought it had to do with the way I handle my fields so I changed them to NULL in a finally statement but that didn't work.
public async Task<bool> CreateNewItem(NewItem myNewItem)
{
try
{
StatusBar statusBar = await MyStatusBar.ShowStatusBar("Creating new List Item.");
//Retrieving Settings from Saved file
mySettings = await MyCredentials.GetMySettings();
myCred = new NetworkCredential(mySettings.UserName, mySettings.Password, mySettings.Domain);
using (var handler = new HttpClientHandler { Credentials = myCred })
{
HttpClient client = new HttpClient(handler);
client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
NewItem newItem = myNewItem;
var jsonObject = JsonConvert.SerializeObject(newItem);
HttpResponseMessage response = await client.PostAsync(new Uri(baseUrl + listNameHourRegistration), new StringContent(jsonObject.ToString(), Encoding.Unicode, "application/json"));
response.Content.Headers.ContentType = new MediaTypeHeaderValue("application/json");
response.EnsureSuccessStatusCode();
string responseMessage = await response.Content.ReadAsStringAsync();
client.Dispose();
if (responseMessage.Length > 0)
return true;
}
}
catch (Exception ex)
{
Debug.WriteLine(ex.Message);
return false;
}
finally
{
request = null;
response = null;
myCred = null;
mySettings = null;
}
return false;
}

Just run into the same problem.
Anyway, the 2nd request does not follow the same authentication procedure. Even if you initialize a new HttpClient object. I sniffed the HTTP traffic.
After the 1st request I am doing another with different credentials. This is also ending in a 401. I am really confused...
Seems the NTLM Handshake stucks at the 2nd of 6 steps
http://www.innovation.ch/personal/ronald/ntlm.html
Edit:
You may want to use the CSOM.
http://social.msdn.microsoft.com/Forums/office/en-US/efd12f11-cdb3-4b28-a9e0-32bfab71a419/windows-phone-81-sdk-for-sharepoint-csom?forum=sharepointdevelopment

While I still don't know what the actual problem is, at least I found a workaround: Use the WebRequest class instead of HttpClient.

I was running into this same error when I realized I was adding the headers each time I was calling the endpoint. Hopefully this will help someone.
Instead I initialized the HttpClient instance in my class constructor and set the headers there. Also I learned it is better practice to only use 1 instance instead of recreating with "using" (See this article https://www.aspnetmonsters.com/2016/08/2016-08-27-httpclientwrong/)
I'm invoking CallApiAsync from another class in a loop.
Here's my final solution:
class ApiShared
{
private HttpClient client;
public ApiShared() {
client = new HttpClient();
client.DefaultRequestHeaders.Add("x-api-key", ConfigurationManager.AppSettings["ApiKey"]);
client.DefaultRequestHeaders.Accept.Clear();
client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
}
public async Task<ApiResponse_Root> CallApiAsync(string endpoint)
{
// Make API call
Uri endpointUri = new Uri(endpoint);
var stringTask = client.GetStringAsync(endpointUri);
var data = JsonConvert.DeserializeObject<ApiResponse_Root>(await stringTask);
return data;
}
}

On a windows machine you can resolve this with this registry setting change:
Go to the following Registry entry:
Computer\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa
Now add a new DWORD to the Lsa folder called: DisableLoopBackCheck and set this to 1

I see that this question has been posted long back. But I don't see a correctly working solution posted yet to this thread.
I faced exactly the same issue where the next requests kept on failing returning me 401 UnAuthorized.
I figured out using fiddler that from SECOND request onwards, there was a Cookie added to the request which was possibly a result of Set-Cookie response sent by the server along with first response.
So here's how I tackled the situation - Make UseCookies false:
new HttpClientHandler { Credentials = myCred, UseCookies = false }
This should resolve your issue. Hope this helps someone who's looking for a solution to a similar issue.