The Problem

Let’s say that you have a simple class structure such as this one:

public class Thing
{
  public string Name { get; set; }
}

public class ThingContainer
{
  public Thing TheThing { get; set; }
}

Here we have a class ThingContainer that has a single reference property of type Thing, and Json.Net will do a great job of serializing and deserializing instances of ThingContainer without any extra help:

static void Main(string[] args)
{
  var container = new ThingContainer
  {
    TheThing = new Thing { Name = "something" }
  };
  var serialized = JsonConvert.SerializeObject(container, Formatting.Indented);
  Console.WriteLine(serialized);
  // {
  //   "TheThing": {
  //      "Name: "something"
  //   }
  // }
  var deserialized = JsonConvert.DeserializeObject<ThingContainer>(serialized);
  Console.WriteLine(deserialized.TheThing.Name);
  // "something"
}

Unfortunately the real-world is rarely that simple and today you are writing a good model so you can’t go about using concrete types. Instead, you want to specify your properties as interfaces:

public interface IThing
{
  string Name { get; set; }
}

public class Thing : IThing
{
  public string Name { get; set; }
}

public class ThingContainer
{
  //notice that the property is now of an interface type...
  public IThing TheThing { get; set; }
}

After making these changes the serialization will still work as before, but when we try to deserialize the model we get the following error:

Could not create an instance of type JsonSerialization.IThing. Type is an interface or abstract class and cannot be instantated

This means that the JSON deserializer has seen that there is a property of type IThing but doesn’t know what type of object it should create to populate it.

Enter JsonConverter

The solution to this is to explicitly tell the deserializer what type it should be instantiating, and we do this using an attribute - specifically the JsonConverterAttribute.

The JsonConverterAttribute is part of Json.Net, and allows you to specify a custom converter to handle serialization and deserialization of an object by extending JsonConverter.

public class ThingContainer
{
  [JsonConverter(typeof(/*CustomConverterType*/))]
  public IThing TheThing { get; set; }
}

In this case we are going to write a custom implementation of JsonConverter that will behave exactly as the non-attributed property would, but using a specific type.

The code below shows the shell of the converter class and the methods we need to override. Notice that we are specifying a generic type parameter TConcrete on the class - this will set the desired concrete type for the property when we actually use the attribute later.

public class ConcreteTypeConverter<TConcrete> : JsonConverter
{
  public override bool CanConvert(Type objectType)
  {
    //determine whether or not this converted can create an instance of
    //the specified object type
  }

  public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
  {
    //deserialize an object from the specified reader
  }

  public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
  {
    //serialize the object value
  }
}

The roles of the missing methods are fairly self explanatory, and seeing as we’re feeling lazy today we’ll pick off the easy ones first.

CanConvert

What we should be doing in CanConvert is determining whether or not the converter can create values of the specified objectType. What I am actually going to do here is just return true (i.e. “yes, we can create anything”). This will get us through the example and leaves the real implementation as an exercise for the reader…

WriteJson

The WriteJson method is responsible for serializing the object, and (as I mentioned above) the serialization of interface properties already works as expected. We can therefore just use the default serialization to fulfil our needs:

public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
  //use the default serialization - it works fine
  serializer.Serialize(writer, value);
}

ReadJson

The ReadJson method is where it starts to get interesting, as this is actually what is causing us the problem.

We need to re-implement this method so that it both instantiates and populates an instance of our concrete type.

Thankfully, Json.Net already knows how to populate the object - and any sub-objects, for that matter - so all we really need to do is get it to use the correct concrete type. We can do this by explicitly calling the overload on the serializer:

public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
  //explicitly specify the concrete type we want to create
  //that was set as a generic parameter on this class
  return serializer.Deserialize<TConcrete>(reader);
}

Usage and Final Code

Now that we have our custom attribute we just need to specify it on the model…

public class ThingContainer
{
  [JsonConverter(typeof(ConcreteTypeConverter<Thing>))]
  public IThing TheThing { get; set; }
}

…and we can deserialize without any errors!

The final code for the converter is below. There are quite a few extensions that could be made to this, but as a quick-fix to a problem that I come across often this will do the job nicely.

public class ConcreteTypeConverter<TConcrete> : JsonConverter
{
  public override bool CanConvert(Type objectType)
  {
    //assume we can convert to anything for now
    return true;
  }

  public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
  {
    //explicitly specify the concrete type we want to create
    return serializer.Deserialize<TConcrete>(reader);
  }

  public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
  {
    //use the default serialization - it works fine
    serializer.Serialize(writer, value);
  }
}