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How to use Post to get data from API?

I wrote a function which consumes Web API. This function works great for GET requests. However, I need to get some other resources from API which, according to API provider's docs, requires POST method. So I've simply changed HttpMethod from Get to HttpMethod.Post. When I call the API then I get Error 400 Bad Request.
CallAPI
private static async Task<T> CallAPI<T>(string endpoint, string accessToken)
{
var request = new HttpRequestMessage(HttpMethod.Post,
new Uri(ApiUri, endpoint));
request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", accessToken);
var _httpClient = new HttpClient();
var response = await _httpClient.SendAsync(request);
response.EnsureSuccessStatusCode();
var responseStream = await response.Content.ReadAsStreamAsync();
var responseObject = await JsonSerializer.DeserializeAsync<T>(responseStream);
return responseObject;
}
I call this API:
var result = await CallAPI<SomeDataModel>("cars/locations", accessToken);
Don't know where is a problem. It works great for GET as I said. Moreover, I don't understand why there's a need to use POST instead of GETto get data. I think it's against REST best-practices. According to provider's API docs, I don't need to attach any parameters to POST request, it's just raw endpoint. Anyways, I need to use POST here to get data.

C# - Set header within HTTP POST request [duplicate]

I'm trying to add a custom header to the request header of my web application. In my web application im retrieving data from a web api, in this request i want to add a custom header which contains the string sessionID. I'm looking for a general solution so that I dont have to add the same code before every call I make.
My Controller looks like this:
[HttpGet]
public async Task<ActionResult> getCall()
{
string url = "http://localhost:51080/";
string customerApi = "customer/1";
using (var client = new HttpClient())
{
//get logged in userID
HttpContext context = System.Web.HttpContext.Current;
string sessionID = context.Session["userID"].ToString();
//Create request and add headers
client.BaseAddress = new Uri(url);
client.DefaultRequestHeaders.Accept.Clear();
client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
//Custom header
//Response
HttpResponseMessage response = await client.GetAsync(customerApi);
if (response.IsSuccessStatusCode)
{
string jsondata = await response.Content.ReadAsStringAsync();
return Content(jsondata, "application/json");
}
return Json(1, JsonRequestBehavior.AllowGet);
}
}
Hope anybody can help!
Thanks in advance!

Try this:
client.DefaultRequestHeaders.Add("X-Version","1");

Collection behind DefaultRequestHeaders has Add method which allows you to add whatever header you need:
client.DefaultRequestHeaders.Add("headerName", sesssionID);

Is there a way to parameterize the HTTP method when using System.Net.Http.HttpClient?

I'm using HttpClient in .Net core 3.1. Most of my requests follow a similar pattern regardless of the HTTP method used:
build URL
build (optional) JSON payload
send request
await response
check status code
parse (optional) JSON response
so I've built a wrapper function that does all these things, and it takes the HTTP method as a parameter. However, when it comes to the "send request" step, I need to use a switch statement to invoke the appropriate method on HttpClient to invoke.
I'm sure that under the skin, get GetAsync() PostAsync() etc. are calling the same underlying function and passing the Http method as a parameter. but I can't see any way of calling it like this from the outside. It seems a strange omission as in my experience most HTTP libraries work that way.

Hope this will help you.
// For JsonConvert use Newtonsoft.Json
string url = "YourURL";
string body = JsonConvert.SerializeObject(BodyModel);
string headerParameter = "ASD123456789";
HttpClient client = new HttpClient();
client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json")); // Content-Type of request, it can be application/xml to other
client.DefaultRequestHeaders.Add("Device", headerParameter ); // first is name, second one is value
HttpRequestMessage request = new HttpRequestMessage(HttpMethod.Post, url) // there you can have Get, Post, Put, Delete and etc. And every request needs to be configured by its settings
{
Content = new StringContent(body, Encoding.UTF8, "application/json")
};
HttpResponseMessage response = await client.SendAsync(request);
if ((int)response.StatusCode == 200)
{
string responseString = await response.Content.ReadAsStringAsync();
ResponseModel responseModel = JsonConvert.DeserializeObject<ResponseModel>(responseString);
}

C# how to not serialize an already serialized string with Content

So I have built an REST API that in itself also consumes another API. Now I could just call the other api, create objects from that call and then make a new request and send it on it's way but that would use up a bit of performance.
I tried just sending the second request again but the problem is that Content serializes it again so I get alot of backslashes. This is my code that does this:
[Route("")]
public IHttpActionResult GetAllDevices()
{
var request = new RestRequest();
request = new RestRequest("devices", Method.GET);
IRestResponse response = client.Execute(request);
return Content(HttpStatusCode.OK, response.Content);//response.Content get's serialized again.
}
As I said, I could deserialized the first call and then just put that in Content, but it feels unnecessary.

Here is one way of doing it, remember to set the content-type explicitly to application/json if needed:
[HttpGet]
[Route("test")]
public HttpResponseMessage Test()
{
const string json = "{ \"test\": 123 }"; // from RestClient
var res = Request.CreateResponse();
res.Content = new StringContent(json);
res.Content.Headers.ContentType = MediaTypeHeaderValue.Parse("application/json");
return res;
}

.NET HttpClient Request Content-Type

I'm not sure, but it appears to me that the default implementation of .NET HttpClient library is flawed. It looks like it sets the Content-Type request value to "text/html" on a PostAsJsonAsync call. I've tried to reset the request value, but not sure if I'm doing this correctly. Any suggestions.
public async Task<string> SendPost(Model model)
{
var client = new HttpClient();
client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
var response = await client.PostAsJsonAsync(Url + "api/foo/", model);
response.EnsureSuccessStatusCode();
return await response.Content.ReadAsStringAsync();
}

You should set the content type. With the Accept you define what you want as response.
http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html
The Accept request-header field can be used to specify certain media types which are acceptable for the response. Accept headers can be used to indicate that the request is specifically limited to a small set of desired types, as in the case of a request for an in-line image.
public async Task<string> SendPost(Model model)
{
var client = new HttpClient(); //You should extract this and reuse the same instance multiple times.
var request = new HttpRequestMessage(HttpMethod.Post, Url + "api/foo");
using(var content = new StringContent(Serialize(model), Encoding.UTF8, "application/json"))
{
request.Content = content;
var response = await client.SendAsync(request).ConfigureAwait(false);
response.EnsureSuccessStatusCode();
return await response.Content.ReadAsStringAsync().ConfigureAwait(false);
}
}