C#Web API – 内部服务器错误500

的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;
    }
}

调用WebAPI的MVC控制器：

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型号：

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; }
}

在我的WebAPI中,如果我将返回类型从USER更改为字符串(当然,将返回变量类型更改为某个字符串(userInfo.FIRST_NAME)),我将SuccessStatusCode设置为“OK”,但是从此代码开始,我使用StatusCode获取内部服务器错误：500(无论这意味着什么).我已经尝试在每个可能的点插入断点,我知道api返回结果很好.我根本就不明白为什么以下这一行

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

当我有返回类型USER时,给出InternalServerError错误,并返回整个USER模型而不是只返回一个字符串.

请不要担心我传递给控制器??的userName和domain参数,它们工作正常！