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!
There is no polymorphic deserialization (equivalent to Newtonsoft. Json's TypeNameHandling ) support built-in to System.
You can use this schema when defining XML Type hierarchies by using only the base XML Types. The XML schema defines XML Types that inherit from each other. In the JSON, an object carries no additional information about the type.
A common way to deserialize JSON is to first create a class with properties and fields that represent one or more of the JSON properties. Then, to deserialize from a string or a file, call the JsonSerializer. Deserialize method.
JSON is a format that encodes objects in a string. Serialization means to convert an object into that string, and deserialization is its inverse operation (convert string -> object).
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
Use this JsonKnownTypes, similar way to do that:
[JsonConverter(typeof(JsonKnownTypesConverter<BaseClass>))] [JsonKnownType(typeof(Base), "base")] [JsonKnownType(typeof(Derived), "derived")] public class Base { public string Name; } public class Derived : Base { public string Something; }
Now when you serialize object in json will be add "$type"
with "base"
and "derived"
value and it will be use for deserialize
Serialized list example:
[ {"Name":"some name", "$type":"base"}, {"Name":"some name", "Something":"something", "$type":"derived"} ]
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