C# Web API - Internal Server Error 500

Question

When I have a return type of 'string' in my WebAPI controller, the SuccessStatusCode returns 'OK' in my MVC Controller, but when the return type is of a model named 'USER', I get this Internal Server Error. Here's my code:

WebAPI:

public class UserController : ApiController
{
    OnlineCenterEntities db = new OnlineCenterEntities();

    public USER GetUserInfo(string userName, string domain)
    {
        USER userInfo = (from u in db.USERs
                         where u.USER_NAME.ToUpper() == userName.ToUpper() && u.LDAP_NAME.ToUpper() == domain.ToUpper()
                         select u).FirstOrDefault();

        return userInfo;
    }
}

MVC Controller that calls the WebAPI:

 public class HomeController : Controller
{
    HttpClient client;
    string url = "http://localhost:61566/api/user/";

    public HomeController()
    {
        client = new HttpClient();
        client.BaseAddress = new Uri(url);
        client.DefaultRequestHeaders.Accept.Clear();
        client.DefaultRequestHeaders.Accept.Add(new System.Net.Http.Headers.MediaTypeWithQualityHeaderValue("application/json"));
    }

    public async Task<ActionResult> Index(string userName, string domain)
    {
        string GetUserInfoURL = String.Format("GetUserInfo?userName={0}&domain={1}", userName, domain);
        HttpResponseMessage responseMessage = await client.GetAsync(url+GetUserInfoURL);

        if (responseMessage.IsSuccessStatusCode)
        {
            var responseData = responseMessage.Content.ReadAsStringAsync().Result;
            var userInfor = JsonConvert.DeserializeObject<USER>(responseData);
        }

        return View();
    }

USER model:

public partial class USER
{

    public int USER_ID { get; set; }
    public string USER_NAME { get; set; }
    public string FIRST_NAME { get; set; }
    public string LAST_NAME { get; set; }
    public string LDAP_NAME { get; set; }
    public string EMAIL { get; set; }
}

In my WebAPI, if I change the return type from USER to string (and of course, change the return variable type to some string (userInfo.FIRST_NAME)), I get the SuccessStatusCode as 'OK', but as of this code, I get Internal Server Error with StatusCode: 500 (whatever that means). I have tried inserting breakpoint at every possible points, and I know that the api is returning the result fine. I simply don't understand why the following line

HttpResponseMessage responseMessage = await client.GetAsync(url+GetUserInfoURL);

gives InternalServerError error when I have the return type of USER, and return the whole USER model instead of just one string.

Please don't worry about the userName and domain parameters that I'm passing to the controllers, they are working fine!

Answer 1

Typically when this happens, it means it is failing to serialize the response. Once your controller returns a USER instance, somewhere WebAPI has to serialize that into the format requested by the client.

In this case the client requested "application/json". The default JsonMediaTypeFormatter uses JSON.Net to turn your C# object into json for the client. Apparently that serialization step is failing, but the response code doesn't tell you exactly why.

The easiest way to see exactly what is happening is to use a custom MessageHandler which forces the body to buffer earlier so you can see the actual exception. Take a look at this blog post for an example to force it to show you the real failure.