Web API Abstract Parameters

Question

Please Please Please could somebody help me finish implementing the code below. I followed the example at http://dotnetbyexample.blogspot.com/2012/02/json-deserialization-with-jsonnet-class.html This example is posted all over on lots of blogs.

I'm trying to find a somewhat reusable way to handle abstract parameters in Web API actions. It works perfectly in Fiddler, but the EntityNote returned is null when I try to call Get via HttpClient. I find that the ReadJson method is never called, so Create is also never called. I guess I do not understand the flow of Web API enough to understand where ReadJson is supposed to be called.

Global.asax.cs

config.IncludeErrorDetailPolicy = IncludeErrorDetailPolicy.LocalOnly;
config.Formatters.JsonFormatter.SerializerSettings.TypeNameHandling = TypeNameHandling.All;
config.Formatters.JsonFormatter.SerializerSettings.ReferenceLoopHandling = ReferenceLoopHandling.Ignore;
config.Formatters.JsonFormatter.SerializerSettings.Converters.Add(new JsonDeviceConverter());

The converter from the link (Nobody shows how to implement WriteJson)

public abstract class JsonCreationConverter<T> : JsonConverter
{
    protected abstract T Create(Type objectType, JObject jsonObject);

    public override bool CanConvert(Type objectType)
    {
        return objectType.IsSubclassOf(typeof(T));
    }

    //This method never gets called
    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        var jsonObject = JObject.Load(reader);
        var target = Create(objectType, jsonObject);
        serializer.Populate(jsonObject.CreateReader(), target);
        return target;
    }

    //I just kinda guessed at this code
    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        if (this.CanConvert(value.GetType()))
        {
            serializer.Serialize(writer, value);
        }
    }
}

EntityNote is an abstract class with PortalProviderRefundNote as an implementation.

public class JsonDeviceConverter : JsonCreationConverter<EntityNote>
{
    //This method never gets called, because ReadJson is never called
    protected override EntityNote Create(Type objectType, JObject jsonObject)
    {
        var typeName = jsonObject["EntityNote"].ToString();
        switch (typeName)
        {
            case "PortalProviderRefundNote":
                return new PortalProviderRefundNote() { EntityId = 1 };
            default: return null;
        }
    }
}

HttpClient - the tutorial uses the JsonConvert.DeserializeObject, but I can't figure out how that fits in here...ReadAsAsync does this task

//simplified **Suggested Edit**
this.jsonFormatter = new JsonMediaTypeFormatter();
jsonFormatter.SerializerSettings.TypeNameHandling = TypeNameHandling.All;
jsonFormatter.SerializerSettings.ReferenceLoopHandling = ReferenceLoopHandling.Ignore;
jsonFormatter.SerializerSettings.Converters.Add(new JsonDeviceConverter());

new HttpClient().GetAsync("http://localhost/api/EntityNotesController/").Result.Content.ReadAsAsync<EntityType>(new List<MediaTypeFormatter>(){ jsonFormatter }).Result;

Service Controller

public EntityNote Get(int id)
{
    return new PortalProviderRefundNote() { EntityId = 1 };
}

so your converter's ReadJson method never gets called on the client side? — Youssef Moussaoui
– Youssef Moussaoui, Commented Dec 10, 2012 at 21:28
I put a breakpoint on it and it never gets called. Like I said, I'm not sure when it's supposed to be called, but it never is. — quitstalin
– quitstalin, Commented Dec 10, 2012 at 21:38

Maggie Ying · Accepted Answer · 2012-12-10 19:48:13Z

1

You'll need make the same serializer settings changing on the JsonMediaTypeFormatter on the client side as well:

var jsonFormatter = new JsonMediaTypeFormatter();
jsonFormatter.SerializerSettings.TypeNameHandling = TypeNameHandling.All;
jsonFormatter.SerializerSettings.ReferenceLoopHandling = ReferenceLoopHandling.Ignore;
jsonFormatter.SerializerSettings.Converters.Add(new JsonDeviceConverter());

response.Content.ReadAsAsync<EntityNote>(new MediaTypeFormatter() { jsonFormatter }).Result;

answered Dec 10, 2012 at 19:48

Maggie Ying

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1 Comment

quitstalin Over a year ago

That makes perfect sense, but this addition (as shown in the original question) did not resolve the issue

quitstalin · Accepted Answer · 2012-12-17 18:20:47Z

0

I switched to XML and used the KnownType attribute. This works out of the box. I'd be interested to know if there is a solution with JSON, but this works good enough.

answered Dec 17, 2012 at 18:20

quitstalin

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