无法设置Content-Type标头

我在HttpClient上设置Content-Type时遇到问题.我按照这个问题:如何为HttpClient请求设置Content-Type标头？ 但仍然没有运气.

String rcString = JsonConvert.SerializeObject(new RoadsmartChecks() { userguid = user_guid, coords = coordinates, radius = (radius * 100) + "" }, ROADSMART_JSON_FORMAT, JSONNET_SETTINGS);
HttpClient c = new HttpClient();
c.BaseAddress = new Uri(BASE_URL);
c.DefaultRequestHeaders.TryAddWithoutValidation("Content-Type", "application/json"); //Keeps returning false
c.DefaultRequestHeaders.TryAddWithoutValidation("Authorization", hash_aes);
c.DefaultRequestHeaders.TryAddWithoutValidation("Roadsmart-app", Constant.APP_ID);
c.DefaultRequestHeaders.TryAddWithoutValidation("Roadsmart-user", user_guid);
c.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
HttpRequestMessage req = new HttpRequestMessage(HttpMethod.Post, BASE_URL + URL_CHECKS + "/fetch");
req.COntent= new StringContent(rcString);
await c.SendAsync(req).ContinueWith(respTask =>
{
    Debug.WriteLine("Response: {0}", respTask.Result);
});

我也试过使用Flurl库,但在尝试添加'Content-Type'时它崩溃了.

misused header name content-type

那么我该如何强制它以便真正添加呢？提前致谢.

我想你应该试试这个

req.COntent= new StringContent(rcString, Encoding.UTF8, "application/json");

结帐此链接:

如何为HttpClient请求设置Content-Type标头？

编辑

删除此行c.DefaultRequestHeaders.TryAddWithoutValidation("Content-Type", "application/json");并检查