POST to Web API action with IEnumerable<Interface> type parameter

Question

I am trying to post to a Web API method from a client, as follows:

// Create list of messages that will be sent
IEnumerable<IMessageApiEntity> messages = new List<IMessageApiEntity>();
// Add messages to the list here. 
// They are all different types that implement the IMessageApiEntity interface.

// Create http client
HttpClient client = new HttpClient {BaseAddress = new Uri(ConfigurationManager.AppSettings["WebApiUrl"])};
client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

// Post to web api
HttpResponseMessage response = client.PostAsJsonAsync("Communications/Messages", messages).Result;

// Read results
IEnumerable<ApiResponse<IMessageApiEntity>> results = response.Content.ReadAsAsync<IEnumerable<ApiResponse<IMessageApiEntity>>>().Result;

My Web API controller action looks like this:

public HttpResponseMessage Post([FromBody]IEnumerable<IMessageApiEntity> messages)
{
    // Do stuff
}

The problem I am having is that messages is always empty (but not null) when coming into the web API controller action. I have verified in the debugger that the messages object on the client side does have items in it right before being posted.

I suspect it might have something to do with the interface type not being converted to a concrete type when trying to pass the objects, but I don't know how to go about making it work. How can I achieve this?

Answer 1

I figured out how to do it without a custom model binder. Posting the answer in case anyone else has this issue...

Client:

// Create list of messages that will be sent
IEnumerable<IMessageApiEntity> messages = new List<IMessageApiEntity>();
// Add messages to the list here. 
// They are all different types that implement the IMessageApiEntity interface.

// Create http client
HttpClient client = new HttpClient {BaseAddress = new Uri(ConfigurationManager.AppSettings["WebApiUrl"])};
client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

// Post to web api (this is the part that changed)
JsonMediaTypeFormatter json = new JsonMediaTypeFormatter
{
    SerializerSettings =
    {
        TypeNameHandling = TypeNameHandling.All
    }
};
HttpResponseMessage response = client.PostAsync("Communications/Messages", messages, json).Result;

// Read results
IEnumerable<ApiResponse<IMessageApiEntity>> results = response.Content.ReadAsAsync<IEnumerable<ApiResponse<IMessageApiEntity>>>().Result;

Add to Register method in WebApiConfig.cs:

config.Formatters.JsonFormatter.SerializerSettings.TypeNameHandling = TypeNameHandling.Auto;

The key is to send the type as part of the json and turn on automatic type name handling, so that web API can figure out what type it is.