Posting a string to asp.net core webapi through httpclient fails with a bad request

Question

I am getting Bad Request error from HttpClient while posting a string to a web api. I know this will work if I have a object as paraneter in web api, but I want to have a method that can accept a string and return a string. This api works in POSTMAN. Here is the api and HttpClient codeL

WebAPI Code:

[HttpPost("TestMethod")]
        public ActionResult<string> TestMethod([FromBody] string testString)
        {
            var s = "Hello: " + testString;
            return Content(s.ToString());
        }

Http Client Code:

string apiResponse="";

            string API_URL = configuration["API_URL"].ToString() + "mauser/TestMethod";
            var postParameters = new { testString = "Bob" };
            var jsonString = JsonConvert.SerializeObject(postParameters);
            var stringContent = new StringContent(jsonString, Encoding.UTF8, "application/json");

            using (var httpClient = new HttpClient())
            {
                using (var response = await httpClient.PostAsync(API_URL, stringContent))
                {
                    if (response.IsSuccessStatusCode)
                    {
                        apiResponse = await response.Content.ReadAsStringAsync();
                    }
                    else
                    {
                        //Handle error
                    }
                }
            }
            return apiResponse;

During debugging in Visual Studio, it fails in client code at var response = await httpClient.PostAsync(API_URL, stringContent)

Error I received:

{StatusCode: 400, ReasonPhrase: 'Bad Request', Version: 1.1, Content: System.Net.Http.HttpConnectionResponseContent, Headers:
{
  Transfer-Encoding: chunked
  Server: Kestrel
  X-Powered-By: ASP.NET
  Date: Mon, 23 Nov 2020 01:36:44 GMT
  Content-Type: application/problem+json; charset=utf-8
}}

Answer 1

Since you use a simple string to receive the data, there is no need to set it as an object.

Just Serialize the string.

var postParameters = "Bob";
var jsonString = JsonConvert.SerializeObject(postParameters);
var stringContent = new StringContent(jsonString, Encoding.UTF8, "application/json");

Result:

Answer 2

Because in postman, even though you set JSON as type, you are passing plaintext(string) type. Your controller is suitable to receive only string so it works in postman.

Valid json:

{
  "ActivationCode":"FFFF",
  "Email": "joe@email.com"
}

You can directly pass the jsonString in postAsync to make it work from code behind.