Newtonsoft Json Deserialize Dictionary as Key/Value list from DataContractJsonSerializer

Extending Andrew Whitaker’s answer, here’s a completely generic version that works on any type of writable dictionary:

public class JsonGenericDictionaryOrArrayConverter: JsonConverter
{
    public override bool CanConvert(Type objectType)
    {
        return objectType.GetDictionaryKeyValueTypes().Count() == 1;
    }

    public override bool CanWrite { get { return false; } }

    object ReadJsonGeneric<TKey, TValue>(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        var tokenType = reader.TokenType;

        var dict = existingValue as IDictionary<TKey, TValue>;
        if (dict == null)
        {
            var contract = serializer.ContractResolver.ResolveContract(objectType);
            dict = (IDictionary<TKey, TValue>)contract.DefaultCreator();
        }

        if (tokenType == JsonToken.StartArray)
        {
            var pairs = new JsonSerializer().Deserialize<KeyValuePair<TKey, TValue>[]>(reader);
            if (pairs == null)
                return existingValue;
            foreach (var pair in pairs)
                dict.Add(pair);
        }
        else if (tokenType == JsonToken.StartObject)
        {
            // Using "Populate()" avoids infinite recursion.
            // https://github.com/JamesNK/Newtonsoft.Json/blob/ee170dc5510bb3ffd35fc1b0d986f34e33c51ab9/Src/Newtonsoft.Json/Converters/CustomCreationConverter.cs
            serializer.Populate(reader, dict);
        }
        return dict;
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        var keyValueTypes = objectType.GetDictionaryKeyValueTypes().Single(); // Throws an exception if not exactly one.

        var method = GetType().GetMethod("ReadJsonGeneric", BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.Public);
        var genericMethod = method.MakeGenericMethod(new[] { keyValueTypes.Key, keyValueTypes.Value });
        return genericMethod.Invoke(this, new object [] { reader, objectType, existingValue, serializer } );
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        throw new NotImplementedException();
    }
}

public static class TypeExtensions
{
    /// <summary>
    /// Return all interfaces implemented by the incoming type as well as the type itself if it is an interface.
    /// </summary>
    /// <param name="type"></param>
    /// <returns></returns>
    public static IEnumerable<Type> GetInterfacesAndSelf(this Type type)
    {
        if (type == null)
            throw new ArgumentNullException();
        if (type.IsInterface)
            return new[] { type }.Concat(type.GetInterfaces());
        else
            return type.GetInterfaces();
    }

    public static IEnumerable<KeyValuePair<Type, Type>> GetDictionaryKeyValueTypes(this Type type)
    {
        foreach (Type intType in type.GetInterfacesAndSelf())
        {
            if (intType.IsGenericType
                && intType.GetGenericTypeDefinition() == typeof(IDictionary<,>))
            {
                var args = intType.GetGenericArguments();
                if (args.Length == 2)
                    yield return new KeyValuePair<Type, Type>(args[0], args[1]);
            }
        }
    }
}

Then use it like

        var settings = new JsonSerializerSettings { Converters = new JsonConverter[] {new JsonGenericDictionaryOrArrayConverter() } };

        var d2 = JsonConvert.DeserializeObject<Data>(newJson, settings);
        var d3 = JsonConvert.DeserializeObject<Data>(oldJson, settings);