Posting Jsonobject with Httpclient from Web Api

The easiest way is to use a StringContent, with the JSON representation of your JSON object.

httpClient.Post(
    "",
    new StringContent(
        myObject.ToString(),
        Encoding.UTF8,
        "application/json"));

Depending on your .NET version you could also use HttpClientExtensions.PostAsJsonAsync method.

If using Newtonsoft.Json:

using Newtonsoft.Json;
using System.Net.Http;
using System.Text;

public static class Extensions
{
    public static StringContent AsJson(this object o)
        => new StringContent(JsonConvert.SerializeObject(o), Encoding.UTF8, "application/json");
}

Example:

var httpClient = new HttpClient();
var url = "";
var data = new { identifier = "username", password = "password" };
var result = await httpClient.PostAsync(url, data.AsJson())

I don't have enough reputation to add a comment on the answer from pomber so I'm posting another answer. Using pomber's approach I kept receiving a "400 Bad Request" response from an API I was POSTing my JSON request to (Visual Studio 2017, .NET 4.6.2). Eventually the problem was traced to the "Content-Type" header produced by StringContent() being incorrect (see ).

tl;dr

Use pomber's answer with an extra line to correctly set the header on the request:

var content = new StringContent(jsonObject.ToString(), Encoding.UTF8, "application/json");
content.Headers.ContentType = new MediaTypeHeaderValue("application/json");
var result = client.PostAsync(url, content).Result;

the code over it in vbnet:

dim FeToSend as new (object--> define class)

Dim client As New HttpClient
Dim content = New StringContent(FeToSend.ToString(), Encoding.UTF8,"application/json")
content.Headers.ContentType = New MediaTypeHeaderValue( "application/json" )
Dim risp = client.PostAsync(Chiamata, content).Result

msgbox(risp.tostring)

Hope this help

Thank you pomber but for

var result = client.PostAsync(url, content).Result;

I used

var result = await client.PostAsync(url, content);

because Result makes app lock for high request

I want to answer all in one response when doing this job as a note for all and myself:

According to Serez's answer HttpContent derived classes list as below

HttpClient PostAsync has some background depending on the context you working on!

• You can post data by the type that you want to send to server in cases Server context waits it as bellow

    [HttpPost]
    public async Task<IActionResult> Submit(MyModel model)
    [HttpPost]
    public async Task<IActionResult> Submit([FromForm] MyModel model)
    [HttpPost]
    public async Task<IActionResult> Submit([FromBody] MyModel model)

When writing FromForm or Body it has working as FromForm. FromBody needs json content otherwise it requires KeyValuePairs as rows. There is some implementations for both of them such as below:

For FromForm: I have used an extension

public static class HelperExtensions
    {
        public static FormUrlEncodedContent ToFormData(this object obj)
        {
            var formData = obj.ToKeyValue();

            return new FormUrlEncodedContent(formData);
        }

        public static IDictionary<string, string> ToKeyValue(this object metaToken)
        {
            if (metaToken == null)
            {
                return null;
            }

            // Added by me: avoid cyclic references
            var serializer = new JsonSerializer { ReferenceLoopHandling = ReferenceLoopHandling.Ignore };
            if (metaToken is not JToken token)
            {
                // Modified by me: use serializer defined above
                return ToKeyValue(JObject.FromObject(metaToken, serializer));
            }

            if (token.HasValues)
            {
                var contentData = new Dictionary<string, string>();
                foreach (var child in token.Children().ToList())
                {
                    var childContent = child.ToKeyValue();
                    if (childContent != null)
                    {
                        contentData = contentData.Concat(childContent)
                                                 .ToDictionary(k => k.Key, v => v.Value);
                    }
                }

                return contentData;
            }

            var jValue = token as JValue;
            if (jValue?.Value == null)
            {
                return null;
            }

            var value = jValue?.Type == JTokenType.Date ?
                            jValue?.ToString("o", CultureInfo.InvariantCulture) :
                            jValue?.ToString(CultureInfo.InvariantCulture);

            return new Dictionary<string, string> { { token.Path, value } };
        }
    }

For FromBody: Use any json converter library Newtonsoft or microsoft

using Newtonsoft.Json;

var jsonString = JsonConvert.SerializeObject(obj);

In both of them, content type should be defined according the requirement, for example for json (Write to header)

request.Headers.Accept.Clear();
request.Headers.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

or another usage

        using (var content = new StringContent(JsonConvert.SerializeObject(answer), System.Text.Encoding.UTF8, "application/json"))
        {
            var answerResponse = await client.PostAsync(url, content);
            //use await it has moved in some context on .core 6.0
        }

If you should use authorization on the context also you can provide authorization as below:

httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", "Your Oauth token");

I spent hours trying to solve this. But @anthls anwser saved my skin.

var data = new StringContent(JsonConvert.SerializeObject(new
                    {
                        abc = "jsjs",
                        xyz = "hhhh"
                    }));
data.Headers.ContentType = new MediaTypeHeaderValue("application/json"); // <-
var response = client.PostAsync(url, data).Result;

I Faced same issue i.e var content = new StringContent(jsonObject.ToString(), Encoding.UTF8, "application/json"); gave "400 Bad Request" Serilizing jsonObject seperately and passing the string in StringContent() solved issue for me, no need to set Encoding.UTF8 seperately.