How to send complex object to Web API?

Question

I need to pass a parameter to my Controller in the Web API which is a complex object, this object has some normal properties (the most complex is a Datetime). I’m doing it this way, but I can’t access it:

Webapiconfig Route:

config.Routes.MapHttpRoute("DefaultApiWithActionAndSkip",
                "api/{controller}/{action}/{skip}",
                defaults: new { skip = RouteParameter.Optional });

Place where I make the request:

private CargaInicialHelper()
    {
        _client = new HttpClient();
        _client.DefaultRequestHeaders.Clear();
        _client.DefaultRequestHeaders.Accept.Add(new Windows.Web.Http.Headers.HttpMediaTypeWithQualityHeaderValue("application/xml"));
    }
    _client.DefaultRequestHeaders.Accept.Add(new Windows.Web.Http.Headers.HttpMediaTypeWithQualityHeaderValue("application/json"));
}

Apicontroller:

public async Task<bool> RegistrarTerminal(Terminal terminal)
{
    return await ObterRespostaServicoAsync<bool>("RegistrarTerminal",
                                                    new HttpStringContent(JsonConvert.SerializeObject(terminal),
                                                    Windows.Storage.Streams.UnicodeEncoding.Utf8,
                                                    "application/xml"));
}

Where I get the response sent by the Web API (I know it works because in this place where I do the request I have several other requests being made (but all with normal variables) and they all work):

private async Task<T> ObterRespostaServicoAsync<T>(string relativePath, HttpStringContent content = null)
    {
        try
        {
            var request = new HttpRequestMessage();
            if (content == null)
            {
                request = new HttpRequestMessage(HttpMethod.Get, new Uri(string.Format(URL, relativePath ?? String.Empty)));
            }
            else
            {
                request = new HttpRequestMessage(HttpMethod.Post, new Uri(string.Format(URL, relativePath ?? String.Empty)));
                request.Content = content;
                var teste = await _client.PostAsync(request.RequestUri, content);
            }

            request.Headers.TryAppendWithoutValidation("Content-Type", "application/xml");
            var response = await _client.GetAsync(request.RequestUri);

            response.EnsureSuccessStatusCode();
            string xml = Utils.RemoveAllXmlNamespaces(await response.Content.ReadAsStringAsync());
            reader = new StreamReader(new MemoryStream(Encoding.UTF8.GetBytes(xml)));

            XmlSerializer serializer = new XmlSerializer(typeof(T));
            return (T)serializer.Deserialize(reader);
        }
        catch (Exception e)
        {
            return default(T);
        }
    }

Error:

Bad request (500). "Value cannot be null. Parameter name: Entity"

Answer 1

You are passing an arbitrary value in the request URL - it is possible that there is a character that is not supported, or that you will change the semantics of the URL (e. g.: ., :, /).

This is probably a symptom of a major problem - do not use [HttpGet] (or the GET verb in general) in HTTP operations that are not idempotented, or that need to pass complex parameters, or that will perform some change in the server. RegisterTerminal, by name, should do this, so should be used with a POST (or PUT, depending on the application semantics). These operations accept a body of the request (request body), where you can pass the parameter (terminal) smoothly.

For example, given this controller:

public class ServicoController : ApiController
{
    [HttpPost, Route("RegistrarTerminal/{nome}")]
    public bool RegistrarTerminal(string nome, Terminal terminal)
    {
        return terminal.Nome == terminal.Codigo;
    }
}

public class Terminal
{
    public string Nome { get; set; }
    public string Codigo { get; set; }
    public int Valor { get; set; }
}

You can access it via the following HTTP request:

POST http://localhost:50381/RegistrarTerminal/foo HTTP/1.1
User-Agent: Fiddler
Host: localhost:50381
Content-Length: 82
Content-Type: application/json

{
  "nome":"Nome do terminal",
  "codigo":"TA:12/13&14",
  "valor": 3
}

Or with the following code C#:

class Program
{
    static void Main(string[] args)
    {
        HttpClient c = new HttpClient();
        c.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
        c.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/xml"));

        var req = new HttpRequestMessage(HttpMethod.Post, "http://localhost:50381/RegistrarTerminal/foo");
        var reqBody = @"{
          'nome':'Nome do terminal',
          'codigo':'TA:12/13&14',
          'valor': 3
        }".Replace('\'', '\"');
        req.Content = new StringContent(reqBody, Encoding.UTF8, "application/json");
        var resp = c.SendAsync(req).Result;
        Console.WriteLine(resp);
        Console.WriteLine(resp.Content.ReadAsStringAsync().Result);
    }
}

Note the creation of StringContent (or of HttpStringContent in your case) - you should pass the Content-Type at that point. You are using the TryAddWithoutValidation to set the Content-Type, and I’m pretty sure that this call is returning false (i.e., the operation is not being performed). This causes the request to exit with a Content-Type: text/plain, which is not recognised by Controller, and that causes the answer 415 Unsupported Media Type that you are receiving.

Another Alternative is, after the creation of the HttpStringContent, you set your Content-Type:

HttpStringContent content = ...;
content.Headers.ContentType = new HttpMediaTypeHeaderValue("application/json");