Web API complex parameter properties are all null

Maybe it will help, I was having the same problem.

Everything was working well, and suddently, every properties was defaulted.

After some quick test, I found that it was the [Serializable] that was causing the problem :

public IHttpActionResult Post(MyComplexClass myTaskObject)
{
    //MyTaskObject is not null, but every member are (the constructor get called).
}

and here was a snippet of my class :

[Serializable]  <-- have to remove that [if it was added for any reason..]
public class MyComplexClass()
{
     public MyComplexClass ()
     {
        ..initiate my variables..
     }

     public string blabla {get;set;}
     public int intTest {get;set;
}

So there are 3 possible issues I'm aware of where the value does not bind:

1. no public parameterless constructor
2. properties are not public settable
3. there's a binding error, which results in a ModelState.Valid == false - typical issues are: non compatible value types (json object to string, non-guid, etc.)

So I'm considering if API calls should have a filter applied that would return an error if the binding results in an error!

This seems to be a common issue in regards to Asp.Net WebAPI.
Generally the cause of null objects is the deserialization of the json object into the C# object. Unfortunately, it is very difficult to debug - and hence find where your issue is.
I prefer just to send the full json as an object, and then deserialize manually. At least this way you get real errors instead of nulls.
If you change your method signature to accept an object, then use JsonConvert:

public HttpResponseMessage Post(Object model)
        {
            var jsonString = model.ToString();
            PreferenceRequest result = JsonConvert.DeserializeObject<PreferenceRequest>(jsonString);
        }