WebAPI Custom Model binding of complex abstract object

Question

This is a tough one. I have an issue with binding a model from JSON. I am attempting to resolve polymorphic-ally the record supplied with the type of record that it will res

Answer 1

The TypeNameSerializationBinder class is not necessary anymore as well as the WebApiConfig configuration.

First, you need to create enum for record type:

public enum ResourceRecordTypeEnum
{
    a,
    b
}

Then, change your "Type" field in ResourceRecord to be the enum we just created:

public abstract class ResourceRecord
{
    public abstract ResourceRecordTypeEnum Type { get; }
}

Now you should create these 2 classes:

Model Binder

public class ResourceRecordModelBinder : IModelBinder
{
    public bool BindModel(HttpActionContext actionContext, ModelBindingContext bindingContext)
    {
        if (bindingContext.ModelType != typeof(T))
            return false;

        try
        {
            var json = ExtractRequestJson(actionContext);
            bindingContext.Model = DeserializeObjectFromJson(json);
            return true;
        }
        catch (JsonException exception)
        {
            bindingContext.ModelState.AddModelError("JsonDeserializationException", exception);
            return false;
        }
    }

    private static T DeserializeObjectFromJson(string json)
    {
        // This is the main part of the conversion
        var obj = JsonConvert.DeserializeObject(json, new ResourceRecordConverter());
        return obj;
    }

    private string ExtractRequestJson(HttpActionContext actionContext)
    {
        var content = actionContext.Request.Content;
        string json = content.ReadAsStringAsync().Result;
        return json;
    }
}

Converter class

public class ResourceRecordConverter : CustomCreationConverter
{
    private ResourceRecordTypeEnum _currentObjectType;

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        var jobj = JObject.ReadFrom(reader);
        // jobj is the serialized json of the reuquest
        // It pulls from each record the "type" field as it is in requested json,
        // in order to identify which object to create in "Create" method
        _currentObjectType = jobj["type"].ToObject();
        return base.ReadJson(jobj.CreateReader(), objectType, existingValue, serializer);
    }

    public override ResourceRecord Create(Type objectType)
    {
        switch (_currentObjectType)
        {
            case ResourceRecordTypeEnum.a:
                return new ARecord();
            case ResourceRecordTypeEnum.b:
                return new BRecord();
            default:
                throw new NotImplementedException();
        }
    }
}

Controller

[HttpPost]
public void Post([ModelBinder(BinderType = typeof(ResourceRecordModelBinder))] RecordCollection recordCollection)
{ 
}