ASP.NET Core API with WPF front-end „Newtonsoft.Json.JsonSerializationException” w mscorlib.dll Error converting value "Value cannot be null

Question

„Newtonsoft.Json.JsonSerializationException” w mscorlib.dll Error converting value "Value cannot be null. (Parameter 'userName')" to type 'Desktop.Model.UserAccess'.

I'm trying to make an request to my API with log in credentials such as Login and Password, and for unknown reason my API gets only NULLs instead of credentials i've put in the app.

My request sends correct values but API's model gets NULLs instead. Is it an issue with my backend or my front-end?

HTTP Request to my API:

string api = ConfigurationManager.AppSettings["api"];

 apiClient = new HttpClient(); apiClient.BaseAddress = new Uri(api); apiClient.DefaultRequestHeaders.Accept.Clear(); apiClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json")); } public async Task<UserAccess> Authenticate(string userName, string password) { var data = new FormUrlEncodedContent(new[] { new KeyValuePair<string, string>("grant_type", password), new KeyValuePair<string, string>("userName", userName), new KeyValuePair<string, string>("password", password), }); try { using (HttpResponseMessage responseMessage = await apiClient.PutAsJsonAsync("api/Authentication/login", data)) { if (responseMessage.IsSuccessStatusCode) { var result = await responseMessage.Content.ReadAsAsync<UserAccess>(); result.GetType(); return result; } else { throw new Exception(responseMessage.ReasonPhrase); } } }catch(Exception e) { Console.WriteLine(e.Message); } return null; }

Auth Controller:

[HttpPost]
        [Route("login")]
        public async Task<IActionResult> Login([FromBody] LoginModel model)
        {
            try
            {
                var user = await _userManager.FindByNameAsync(model.UserName);
                if (user != null && await _userManager.CheckPasswordAsync(user, model.Password))
                {
                    var userRoles = await _userManager.GetRolesAsync(user);

                    var authClaims = new List<Claim>
                {
                    new Claim(ClaimTypes.Name, user.UserName),
                    new Claim(JwtRegisteredClaimNames.Jti, Guid.NewGuid().ToString()),
                };

                    foreach (var userRole in userRoles)
                    {
                        authClaims.Add(new Claim(ClaimTypes.Role, userRole));
                    }

                    var authSigningKey = new SymmetricSecurityKey(Encoding.UTF8.GetBytes(_configuration["JWT:Secret"]));

                    var token = new JwtSecurityToken(
                        issuer: _configuration["JWT:ValidIssuer"],
                        audience: _configuration["JWT:ValidAudience"],
                        expires: DateTime.Now.AddHours(3),
                        claims: authClaims,
                        signingCredentials: new SigningCredentials(authSigningKey, SecurityAlgorithms.HmacSha256)
                        );

                    return Ok(new
                    {
                        token = new JwtSecurityTokenHandler().WriteToken(token),
                        expiration = token.ValidTo,
                        userRoles
                    });
                }
                return Unauthorized();
            }
            catch(Exception e)
            {
                var exception = e.Message;
                return Ok(exception);
            }
    

Answer 1

You have an exception which you return as OK

catch(Exception e)
{
    var exception = e.Message;
    return Ok(exception);
}

Login method returns a string Value cannot be null. (Parameter 'userName') Value cannot be null. (Parameter 'userName') and another part of your application is trying to deserialise this and fails.

I'm pretty sure the exception is thrown by var user = await _userManager.FindByNameAsync(model.UserName); becuase model.UserName is null. You need to fix that part first.