JSON Deserialization with an array of polymorphic objects

I’m having a problem with JSON Deserialization involving an array of polymorphic objects. I’ve tried the solutions for serialization documented here and here which work great for serialization, but both blow up on deserialization.

My class structure is as follows:

IDable

[DataContract(IsReference=true)]
public abstract class IDable<T> {

    [DataMember]
    public T ID { get; set; }
}

Observation Group

[DataContract(IsReference=true)]
[KnownType(typeof(DescriptiveObservation))]
[KnownType(typeof(NoteObservation))]
[KnownType(typeof(NumericObservation))]
[KnownType(typeof(ScoredObservation))]
public class ObservationGroup : IDable<int> {

    [DataMember]
    public string Title { get; set; }

    [DataMember]
    public List<Observation> Observations { get; set; }

    [OnDeserializing]
    void OnDeserializing(StreamingContext context)
    {
        init();
    }

    public ObservationGroup()  {
        init();
    }

    private void init()
    {
        Observations = new List<Observation>();
        ObservationRecords = new List<ObservationRecord>();
    }

}

DescriptiveObservation

[DataContract(IsReference = true)]
public class DescriptiveObservation : Observation
{

    protected override ObservationType GetObservationType()
    {
        return ObservationType.Descriptive;
    }
}

NoteObservation

[DataContract(IsReference = true)]
public class NoteObservation : Observation
{
    protected override ObservationType GetObservationType()
    {
        return ObservationType.Note;
    }
}

NumericObservation

[DataContract(IsReference = true)]
public class NumericObservation : Observation
{
    [DataMember]
    public double ConstraintMaximum { get; set; }
    [DataMember]
    public double ConstraintMinimum { get; set; }
    [DataMember]
    public int PrecisionMaximum { get; set; }
    [DataMember]
    public int PrecisionMinimum { get; set; }
    [DataMember]
    public string UnitType { get; set; }

    protected override ObservationType GetObservationType()
    {
        return ObservationType.Numeric;
    }
}

ScoredObservation

[DataContract(IsReference = true)]
public class ScoredObservation : Observation {
    [DataMember]
    public int Value { get; set; }

    protected override ObservationType GetObservationType() {
        return ObservationType.Scored;
    }
}

I’m impartial to using either the built in JavaScriptSerializer or the Newtonsoft JSON library.

Serialization Code

var settings = new Newtonsoft.Json.JsonSerializerSettings();
settings.TypeNameHandling = Newtonsoft.Json.TypeNameHandling.Objects;

Newtonsoft.Json.JsonConvert.SerializeObject(AnInitializedScoresheet, Newtonsoft.Json.Formatting.None, settings);

Deserialization Code

return Newtonsoft.Json.JsonConvert.DeserializeObject(returnedStringFromClient, typeof(Scoresheet));
//Scoresheet contains a list of observationgroups

The error that I get is

“Could not create an instance of type ProjectXCommon.DataStores.Observation. Type is an interface or abstract class and cannot be instantated.”

Any help would be much appreciated!

Answer

You have not added any settings upon deserialization. You need to apply settings with TypeNameHandling set to Object or All.

Like this:

JsonConvert.DeserializeObject(
    returnedStringFromClient, 
    typeof(Scoresheet), 
    new JsonSerializerSettings 
    { 
        TypeNameHandling = TypeNameHandling.Objects 
    });

Documentation: TypeNameHandling setting