How do I handle polymorphic deserialization in a custom JSON converter for MvvmCross

1. Understanding Polymorphic Deserialization**

Polymorphic deserialization refers to the process of deserializing JSON data into different types based on some type information embedded in the JSON. This is necessary because JSON itself does not contain type information, so additional metadata is needed to guide the deserialization process.

2. Embedding Type Information in JSON**

To enable polymorphic deserialization, you need to embed type information into your JSON. This can be done by adding a discriminator field that indicates the type of object being serialized. For example:

json
{
  "type": "Dog",
  "name": "Buddy",
  "barkVolume": 50
}

Or, if using a more complex structure:

json
{
  "animal": {
    "$type": "Dog",
    "name": "Buddy",
    "barkVolume": 50
  }
}

3. Creating a Custom JSON Converter**

To handle polymorphic deserialization, you will need to create a custom JSON converter. This converter will be responsible for reading the type discriminator from the JSON and instantiating the correct type.

Here's an example of how you might implement such a converter using `NewtonSoft.Json`, which is commonly used in MvvmCross projects:

csharp
using Newtonsoft.Json;
using Newtonsoft.Json.Linq;

public class PolymorphicJsonConverter : JsonConverter
{
    private readonly Dictionary _typeMap = new Dictionary();

    public PolymorphicJsonConverter()
    {
        // Register types here
        _typeMap.Add("Dog", typeof(Dog));
        _typeMap.Add("Cat", typeof(Cat));
    }

    public override bool CanConvert(Type objectType)
    {
        return objectType == typeof(Animal);
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        JObject jObject = JObject.Load(reader);
        string type = jObject["$type"].ToString();

        if (_typeMap.TryGetValue(type, out Type typeToDeserialize))
        {
            return jObject.ToObject(typeToDeserialize);
        }

        throw new JsonSerializationException($"Type {type} not found.");
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        JObject jObject = JObject.FromObject(value);
        jObject["$type"] = value.GetType().Name;

        jObject.WriteTo(writer);
    }
}

public class Animal { }

public class Dog : Animal
{
    public string Name { get; set; }
    public double BarkVolume { get; set; }
}

public class Cat : Animal
{
    public string Name { get; set; }
    public bool LikesCream { get; set; }
}

4. Using the Custom Converter in MvvmCross**

To use this custom converter in MvvmCross, you would typically resolve the `IMvxJsonConverter` instance and use it for serialization and deserialization. However, since you're creating a custom converter, you might need to manually handle serialization and deserialization using `NewtonSoft.Json` directly.

5. Registering the Custom Converter**

If you want to integrate this custom converter with MvvmCross's JSON plugin, you might need to manually configure the serialization settings to use your custom converter. This could involve overriding the default serialization settings in your application's startup code.

Conclusion

Handling polymorphic deserialization in MvvmCross requires creating a custom JSON converter that can interpret type information embedded in the JSON data. By using a type discriminator and a custom converter, you can effectively deserialize JSON into different types based on the embedded metadata. While MvvmCross primarily uses `NewtonSoft.Json`, you can adapt this approach to work with other JSON libraries as well.