Web Api JSON中丢失了EF TPH继承

Question

I've successfully set up some classes that use TPH EF inheritance , MyBaseClass , MySubClass1 , MySubClass2 etc.

When querying using Linq context.MyBaseClasses.Where(...) , the objects returned all correctly use the subclass specified by the Discriminator field in the database. (So I might end up with a list containing a mix of objects of MySubClass1 , or MySubClass2 .)

However, when I pass these objects to a WPF application, via a JSON Web Api call, the objects received are all of MyBaseClass , rather than the correct sub class they started off at.

The object property they are returned via is of type public virtual List<MyBaseClass> MyThings , so I guess it makes sense that they all end up as this type, but I want to retain the correct sub class type for each object.

How do I achieve this? Do I need to force the EF Discriminator field to be sent along with all the other data somehow?

Edit

1.

At the client end, I'm now attempting to deserialize the JSON (including $type data) like so (with no luck, the items are still converted back to their base class)

HttpResponseMessage response = GetClient().GetAsync(url).Result;

if (response.IsSuccessStatusCode)
{
    string jsonMessage;
    using (Stream responseStream = response.Content.ReadAsStreamAsync().Result)
    {
        jsonMessage = new StreamReader(responseStream).ReadToEnd();
    }


    List<My.Domain.Models.MyBaseClass> thingsToReturn;

    //new method    
    thingsToReturn = JsonConvert.DeserializeObject<List<My.Domain.Models.MyBaseClass>>(jsonMessage);

    //previous method
    //thingsToReturn = response.Content.ReadAsAsync<List<My.Domain.Models.MyBaseClass>>().Result;

    return thingsToReturn;
}

2.

Following Anish's advice re SerializerSettings.TypeNameHandling , I've now got $type information appearing in my JSON, however this is of type System.Data.Entity.DynamicProxies.SubClass1_5E07A4CE2F037430DC7BFA00593.... is this OK for the client end to deserialize back into SubClass1 successfully?

Answer 1

This is a serialization concern.

You can customize Json.Net's serializer settings to include type information with your json objects.

Add this to your HttpConfiguration :

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

Where config is the HttpConfiguration instance that you use to configure and initialize Asp.Net WebApi .

This will tell Json.Net to add some type information to each json object that has type ambiguity. Such an object would look like this:

{
    "$type":"MyProjectContainingMyTypes.MySubClass1, MyProjectContainingMyTypes",
    "Name": "Tyrion Lannister",
    "DisplayName": "The Imp",
    "Traits": ["funny", "awesome", "clever"]
}

Json.Net will know how to deal with this when you deserialize this on the WPF side.

This should work, in your WPF app:

var things = JsonConvert.DeserializeObject<List<MyBaseClass>>(jsonString);

Then you can cast the objects in the things list to their respective derived types.

Of course, your WPF application will need to have a reference to the project where you define MyBaseClass and MySubClass1 .

Edit

Thanks Anish, that's almost sorted it. I can see the correct $type data in the JSON, I'm just being a dunce, and I'm not sure how to get the WebApi response as a jsonString? At the minute I'm doing response.Content.ReadAsAsync>().Result; to auto deserialize the data.

In response to your comment, you can read the content as a string like this:

var jsonString = response.Content.ReadAsStringAsync().Result;

Edit 2

Anish, thanks so much for your input. As you can see, I've managed to get the JSON as a string now, but even using JsonConvert.DeserializeObject I'm still getting the same issue. Do you think it is (as I mention in Edit 2) to do with the $type being returned from the service incorrectly (as an EF proxy type)?

In response to your comment, yes you will not be able to deserialize the EF proxy type into the type you want. This issue needs to be resolved on the WebApi side.

The proxy classes are created to allow you to lazy load entities. Proxies are generally used represent referenced/nested entities. This allows the fetching of referenced entities from the database to be deferred until required, if required at all.

Here is a link to some documentation around lazy and eager loading entities with EF .

Solution

You want to hydrate the list of objects in your WebApi controller action and return it, this will tell EF to load the entities from the database and new up instances of your classes.

You have a few options here:

Option 1

Call ToList() on the query to hydrate the collection:

var result = (from t in dbContext.Things select t).ToList();

or

var result = dbContext.Things.ToList();

Naturally, you don't want to return an unbounded result set so add a range:

var result = (from t in dbContext.Things select t).Skip(0).Take(10).ToList();

or

var result = dbContext.Things.Skip(0).Take(10).ToList();

Bear in mind that with method you will have to explicitly hydrate nested objects like this:

var result = dbContext
             .Things
             .Include(t => t.SomePropertyThatRepresentsSomeNestedObject) 
             .Skip(0)
             .Take(10)
             .ToList();

Option 2

Turn off lazy loading for your DbContext.

Personally, I'd go with Option 1 , I think it is better to know your entities and have control over when and what you hydrate.

Answer 2

Many, many thanks to Anish , who's answer set me off on the right track, along with this excellent article .

The steps I had to take to get the types of the inherited objects to pass through the web api service are as follows:

Server Side

The key things were to set the JsonFormatter Serializer TypeNameHandling setting to TypeNameHandling.Auto . This can be done in WebApiConfig.Register() , but it will then obviously add a $type property to all JSON objects returned by your web service calls, or, you can simply decorate the property of the object you need the $type for.

WebApiConfig Method

GlobalConfiguration.Configuration.Formatters.JsonFormatter.SerializerSettings.TypeNameHandling = Newtonsoft.Json.TypeNameHandling.Auto;        

Property Decoration Method

[Newtonsoft.Json.JsonProperty(ItemTypeNameHandling = Newtonsoft.Json.TypeNameHandling.Auto)]
public virtual List<MyBaseClass> Things { get; set; }

To get the correct value in the $type property in the JSON, and not the EF proxy class name, I turned off the ProxyCreationEnabled property before performing the Linq query that returned the objects based on MyBaseClass.

dbContext.Configuration.ProxyCreationEnabled = false;
List<MyBaseClass> things = dbContext.MyBaseClasses.Include("This").Include("That").ToList();

Client Side

I had to add a JsonMediaTypeFormatter with SerializerSettings = { TypeNameHandling = TypeNameHandling.Auto } to the ReadAsAsync() call and then the (correctly typed) objects mapped to their sub class happily.

HttpResponseMessage response = GetClient().GetAsync(url).Result;

if (response.IsSuccessStatusCode)
{
    //this formatter responds to the $type parameter passed in the JSON to allow us to correctly map object types
    //https://kirmir.wordpress.com/2014/05/16/polymorphic-serialization-using-newton-json-net-in-httpcontent/
    var formatter = new JsonMediaTypeFormatter
    {
        SerializerSettings = { TypeNameHandling = TypeNameHandling.Auto }
    };

    List<MyBaseClass> thingsToReturn;
    thingsToReturn = response.Content.ReadAsAsync<List<MyBaseClass>>(new List<MediaTypeFormatter> { formatter }).Result;
    return productTestsToReturn;
}