JSON serializer creates an empty object on failed deserialization instead of null

By default Web.API used the MissingMemberHandling.Ignore setting in the JsonMediaTypeFormatter.

You need to set it to MissingMemberHandling.Error with:

GlobalConfiguration.Configuration
   .Formatters.JsonFormatter
   .SerializerSettings.MissingMemberHandling = MissingMemberHandling.Error;

and you should get null when sending JSON like:

{
   "somenotexistingprop": ""
}

However if you send a completely empty JSON object: {} then you will still get an object with empty properties and not null. Because JsonConvert.DeserializeObject returns a empty object if it deserializes an empty JSON (see this unit test of github).

After one of my team members helped me understand why this is the case, following are some notes about this scenario:

1. We by default do not have "MissingMemberHandling.Error" on Json formatter because this can help in scenarios where you a newer version of client sending data with extra members to an older version of service. The older version of service should still be able to accept this request and ignore the extra properties. This behavior is consistent with how Xml formatter also behaves.

2. @Sinaesthetic: if you are setting "MissingMemberHandling.Error" and do not want to rely on "ModelState.IsValid" check only, then you could alway check for ModelState's Keys property to figure out if your request is indeed invalid because of a missing member or something else.

Example below:

public class Customer
{
    public int Id { get; set; }

    [MaxLength(5)]
    public string Name { get; set; }

    public Address Address { get; set; }
}

public class Address
{
    public string Line1 { get;set;}
}

//action
public void Post([FromBody]Customer customer)
    {
        if (!ModelState.IsValid)
        {
            ModelStateDictionary msd = new ModelStateDictionary();

            foreach (string key in ModelState.Keys)
            {
                if (ModelState[key].Errors.Count > 0)
                {
                    foreach (ModelError error in ModelState[key].Errors)
                    {
                        Exception ex = error.Exception;

                        if (ex != null 
                            && typeof(JsonSerializationException).IsAssignableFrom(ex.GetType()) 
                            && ex.Message.StartsWith("Could not find member"))
                        {
                            msd.AddModelError(key, ex);
                        }
                    }
                }
            }

            if (msd.Count > 0)
            {
                throw new HttpResponseException(Request.CreateErrorResponse(HttpStatusCode.BadRequest, msd));
            }
        }

        //process the supplied customer
    }