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Getting Invalid Content Type doing a Delete httpclient call. What am I doing wrong?

When I try to do the code below, it just results in Invalid Content Type (with error number 612).
I'm trying to delete a lead id from a static list. I can add lead ids or get the static list leads fine.
The post and get calls I make are working fine, although the post calls I make seem to require the data right on the url string (as in $"{endpointURL}/rest/v1/lists/{listID}/leads.json?id={leadID}"; If I include the id as a json object, it fails too. This might be a clue to what I'm doing wrong with the delete call.
string url = $"{endpointURL}/rest/v1/lists/{listID}/leads.json?id={leadID}";
HttpClient client = new HttpClient();
client.BaseAddress = new Uri(url);
client.DefaultRequestHeaders.Authorization = new
AuthenticationHeaderValue("Bearer", _access_token);
HttpResponseMessage response = await client.DeleteAsync(url);
The response here always results in Invalid Content Type.
If I add this line before I do the deleteasync call, it gives me a different error before it even hits the deleteAsync call.
client.DefaultRequestHeaders.Add("Content-Type", "application/json");
Error is "Misused header name. Make sure request headers are used with HttpRequestMessage, response headers with HttpResponseMessage, and content headers with HttpContent objects."

Try using HttpRequestMessage in your code like this
string url = $"{endpointURL}/rest/";
HttpClient client = new HttpClient
{
BaseAddress = new Uri(url)
};
//I'm assuming you have leadID as an int parameter in the method signature
Dictionary<string, int> jsonValues = new Dictionary<string, int>();
jsonValues.Add("id", leadID);
//create an instance of an HttpRequestMessage() and pass in the api end route and HttpMethod
//along with the headers
HttpRequestMessage request = new HttpRequestMessage
(HttpMethod.Delete, $"v1/lists/{listID}") //<--I had to remove the leads.json part of the route... instead I'm going to take a leap of faith and hit this end point with the HttpMethod Delete and pass in a Id key value pair and encode it as application/json
{
Content = new StringContent(new JavaScriptSerializer().Serialize(jsonValues), Encoding.UTF8, "application/json")
};
request.Headers.Add("Bearer", _access_token);
//since we've already told the request what type of httpmethod we're using
//(in this case: HttpDelete)
//we could just use SendAsync and pass in the request as the argument
HttpResponseMessage response = await client.SendAsync(request);

The solution turned out to be a combination of a couple of suggestions.
HttpClient client = new HttpClient();
client.BaseAddress = new Uri(url);
HttpRequestMessage request = new HttpRequestMessage(HttpMethod.Delete, data);
// The key part was the line below
request.Content = new StringContent(string.Empty, Encoding.UTF8, "application/json");
if (!string.IsNullOrEmpty(_access_token))
{
client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", _access_token);
}
HttpResponseMessage response = await client.SendAsync(request);
This worked for me.

How to get stringContent from server side?

I have a client Call my API service as this:
var paramDiction = new Dictionary<string, string>{{"datefROM", "2018/1/1"}};
string content = JsonConvert.SerializeObject(paramDiction);
var stringContent = new StringContent(content, Encoding.UTF8, "application/json");
// call the API service
var x = await _httpClient.PostAsync(url, stringContent);
I tried many ways to get the stringContent from Server Side, but still can't get it. I'm I on the wrong way?
[HttpPost]
[Route("GetStringContent")]
public IActionResult GetStringContent()
{
var stringContent = Request.HttpContext.ToString();
return stringContent;
}
Don't know why the Request here is a httpRequest, only have the HTTPContext and this httpContent can't read the content out like
Request.Content.ReadAsStringAsync();

First of all for API Methods you normally return a HttpResponseMessage that you can create with Request.CreateResponse(HttpStatusCode, Message).
Now to your question, in Asp.Net an API Method has parameters according to what you're expecting to be sent. For Example in your case your Method signatrue would look like this public HttpResponseMessage GetStringContent([FromBody] Dictionary<string, string> stringContent). The [FromBody] Attribute is used in Post Methods to signal that the Content is coming from the request body

Posting JSON Array of Integers

I am attempting to POST some JSON data to an API to add accounts.
The instructions specify the ids parameter can be: a string (comma-separated), or array of integers
I realize I could put comma delimited ids into the query string however I would like to POST this data as JSON as I may have a large number of these.
Here is what I have tried:
public static HttpClient GetHttpClient()
{
var property = Properties.Settings.Default;
HttpClient client = new HttpClient();
client.BaseAddress = new Uri(property.apiUrl);
client.DefaultRequestHeaders.Accept.Clear();
client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
client.DefaultRequestHeaders.Add("X-OrgSync-API-Key", property.apiKey);
return client;
}
HttpClient client = Api.GetHttpClient();
string json = "{\"ids\":[10545801,10731939]}";
var httpContent = new StringContent(json, Encoding.UTF8, "application/json");
var response = await client.PostAsync($"{client.BaseAddress}/classifications/{classification.id}/accounts/add", httpContent);
It runs "successfully" but nothing actually gets set on the API server side.
Any ideas as to what I might be doing wrong here?
Additionally, any kind of tools/techniques etc., particularly in Visual Studio that would give me better visibility of the request/response traffic?
I know that this is possible as it correctly adds the account ids when I use a tool like Postman:

Regarding the Tools/Techniques, you can use Fiddler to capture the request and response on the fly to check if the Raw request is correct.
If you haven't used it before, have a look here for instructions on how to capture the requests and responses.

I was able to get the json string method working by changing the StringContent encoding type from Encoding.UTF8 to null OR Encoding.Default.
string json = "{\"ids\":[10545801,10731939]}";
var httpContent = new StringContent(json, Encoding.Default, "application/json");
var response = await client.PostAsync($"{client.BaseAddress}/classifications/{classification.id}/accounts/add", httpContent);
I also figured out a way to use an object containing an int array of ids with the Encoding.UTF8;
HttpClient client = Api.GetHttpClient();
var postData = new PostData {ids = new[] {10545801,10731939}};
var json = JsonConvert.SerializeObject(postData);
var httpContent = new StringContent(json, Encoding.UTF8, "application/json");
var response = await client.PostAsync($"{client.BaseAddress}/classifications/{classification.id}/accounts/add", httpContent);
If you don't want to go to the trouble of creating a class just to store post data you can use an anonymous type:
var postData = new { ids = new[] {10545801,10731939}};
var json = JsonConvert.SerializeObject(postData);
var httpContent = new StringContent(json, Encoding.UTF8, "application/json");
var response = await client.PostAsync($"{client.BaseAddress}/classifications/{classification.id}/accounts/add", httpContent);

Try below code it will work
using (var client= new HttpClient()) {
string json = "{\"ids\":[10545801,10731939]}";
var httpContent = new StringContent(json, Encoding.UTF8, "application/json");
var response = await client.PostAsync($"{client.BaseAddress}/classifications/{classification.id}/accounts/add", httpContent);
// If the response contains content we want to read it!
if (response .Content != null) {
var responseContent = await response.Content.ReadAsStringAsync();
//you will get your response in responseContent
}

Getting response header

Used the Flurl to Get response from API.
var response = await url.WithClient(fc)
.WithHeader("Authorization", requestDto.ApiKey)
.GetJsonAsync<T>();
dynamic httpResponse = response.Result;
But I cant able to access httpResponse.Headers
How to access response headers while using GetJsonAsync .

You can't get a header from GetJsonAsync<T> because it returns Task<T> instead of raw response. You can call GetAsync and deserialize your payload at next step:
HttpResponseMessage response = await url.GetAsync();
HttpResponseHeaders headers = response.Headers;
FooPayload payload = await response.ReadFromJsonAsync<FooPayload>();
ReadFromJsonAsync is an extention method:
public static async Task<TBody> ReadFromJsonAsync<TBody>(this HttpResponseMessage response)
{
if (response.Content == null) return default(TBody);
string content = await response.Content.ReadAsStringAsync();
return JsonConvert.DeserializeObject<TBody>(content);
}
P.S. This is why I prefer and recommend to use raw HttpClient instead of any third-party high-level client like RestSharp or Flurl.

You could also await for the HttpResponseMessage, pick off the .Headers object, then send the completed task to ReceiveJson<T> for deserialization. Here's how to do it without an extension method:
var task = url.GetAsync();
HttpResponseMessage response = await task;
HttpResponseHeaders headers = response.Headers;
//Get what I need now from headers, .ReceiveJson<T>() will dispose
//response object above.
T obj = await task.ReceiveJson<T>();

Why is ResponseMessage.Content consistently empty after an await PostAsJsonAsync request?

We are posting a "maintenanceEvent" to an API which consistently returns [] in ResponseMessage.Content. I'd need some expert guidance in case the code I wrote here is faulty.
private async Task SendMaintenanceEvent(object maintenanceEvent, MaintenanceEventType maintenanceEventType)
{
string endpointAddress = "TheEndpointURI";
string credentials = "OurCredentials";
string credentialsBase64 = Convert.ToBase64String(System.Text.ASCIIEncoding.ASCII.GetBytes(credentials));
// Convert the maintenanceEvent object to consumable JSON, then encode it to a StringContent object.
this.responseInfo.MaintenanceEventAsJSON = System.Web.Helpers.Json.Encode(maintenanceEvent);
StringContent stringContent = new StringContent(this.responseInfo.MaintenanceEventAsJSON, Encoding.UTF8, "application/json");
using (HttpClient httpClient = new HttpClient())
{
httpClient.BaseAddress = new Uri(endpointAddress);
httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Basic", credentialsBase64);
this.responseInfo.AuthorizationHeader = httpClient.DefaultRequestHeaders.Authorization.ToString();
this.responseInfo.EndpointUri = httpClient.BaseAddress.AbsoluteUri;
// The async post.
this.responseInfo.ResponseMessage = await httpClient.PostAsJsonAsync(access.EndpointDirectory, stringContent).ConfigureAwait(false);
this.responseInfo.ResponseStatusCode = (int)this.responseInfo.ResponseMessage.StatusCode;
// Consistently returns true so long as my credentials are valid.
// When the auth credentials are invalid, this returns false.
if (this.responseInfo.ResponseMessage.IsSuccessStatusCode)
{
// I expect to see some data from the service.
this.responseInfo.ResponseContent = this.responseInfo.ResponseMessage.Content.ReadAsStringAsync();
}
}
}
Try/Catch blocks and some company specific info is omitted. The responseInfo object above is just a model with some properties to collect information from this method, so we can record the event.
Where I suspect a problem may be, is in the code below the PostAsJsonAsync command. But I'm at a loss for what to do there. Thanks for your help.

This (slightly adjusted) is what you want to do (substitute your own variables as needed):
using (HttpClient httpClient = new HttpClient())
{
// ...
HttpResponseMessage responseMessage = await httpClient.PostAsJsonAsync(access.EndpointDirectory, stringContent).ConfigureAwait(false);
// ...
string responseBody = await responseMessage.Content.ReadAsStringAsync().ConfigureAwait(false);
// ...
}
I.e. you must await ReadAsStringAsync() to get the actual content.
For completeness, note that HttpResponseMessage and HttpResponseMessage.Content are IDisposable.