Newtonsoft.Json don't support FrozenDictionary. Can I write a converter for it

sybil
November 11, 2024
185 views
1 vote
2 Answers

Newtonsoft.Json don’t support FrozenDictionary. I know I can write a JsonConverter starting from scratch, but it is difficult for me, and FrozenDictionary is very similar to a Dictionary when serialize/deserialize. Can I tell Newtonsoft.Json to serialize/deserialize it as Dictionary in some simple way?

Or is there another question on here with a FrozenDictionaryJsonConverter already written by someone else?

Tags: .net c#json json.net

Answers

The problem is FrozenDictionary doesn’t have a new() – personally I’d deal with it by having two properties and dealing with it in your class constructor or something. If you want JSON though

Foo myFoo = new Foo()
{
    FrozenDict = new Dictionary<string, string>() { { "Foo", "Bar" } }.ToFrozenDictionary()
};
var fooData = JsonConvert.SerializeObject(myFoo);

var settings = new JsonSerializerSettings
{
    Converters = { new FrozenDictionaryConverter<string, string>() }
};
var newFoo = JsonConvert.DeserializeObject<Foo>(fooData, settings);

public class Foo
{
    public FrozenDictionary<string, string> FrozenDict;
}

public class FrozenDictionaryConverter<TKey, TValue> : JsonConverter<FrozenDictionary<TKey, TValue>>
{
    public override void WriteJson(JsonWriter writer, FrozenDictionary<TKey, TValue>? value, Newtonsoft.Json.JsonSerializer serializer)
    {
        serializer.Serialize(writer, value);
    }

    public override FrozenDictionary<TKey, TValue>? ReadJson(JsonReader reader, Type objectType, FrozenDictionary<TKey, TValue>? existingValue, bool hasExistingValue, Newtonsoft.Json.JsonSerializer serializer)
    {
        var dictionary = serializer.Deserialize<Dictionary<TKey, TValue>>(reader);
        return dictionary?.ToFrozenDictionary();
    }
}

The above isn’t generic, you need to set the types on your json converter settings, but it will work.

Personally, if I had that use case – I would do something like this instead (but with better accessors ofc)

public class Foo
{
    [JsonIgnore]
    public FrozenDictionary<string, string> MyFrozenDict;
    public Dictionary<string, string> _jsonMyFrozenDict;
    public Foo() {
        MyFrozenDict = _jsonMyFrozenDict?.ToFrozenDictionary();
    }
}

If you’re building a ASP.NET API and want a more generic way to do this, you could use a converter like this:

public class FrozenDictionaryConverter : JsonConverter
{
    private static readonly MethodInfo _deserializeMethod = typeof(FrozenDictionaryConverter)
        .GetMethod(nameof(DeserializeFrozenDictionary), BindingFlags.Static | BindingFlags.NonPublic | BindingFlags.InvokeMethod);

    public override bool CanConvert(Type objectType)
    {
        return objectType.IsConstructedGenericType && objectType.GetGenericTypeDefinition() == typeof(FrozenDictionary<,>);
    }

    public override bool CanWrite => false;

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        var typedDeserializeMethod = _deserializeMethod.MakeGenericMethod(objectType.GetGenericArguments());
        return typedDeserializeMethod.Invoke(null, new object[] { reader, serializer });
    }

    private static object DeserializeFrozenDictionary<TKey, TValue>(JsonReader reader, JsonSerializer serializer)
    {
        var dataAsDictionary = serializer.Deserialize<Dictionary<TKey, TValue>>(reader);
        return dataAsDictionary?.ToFrozenDictionary();
    }

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

This uses some reflection to invoke a typed generic method to do the deserialization. It could probably be made faster by caching the constructed method.

While this is likely going to be marginally slower than a converter that you have to supply the key/value types to, it has the benefit that you can register this as a converter on your JsonSerializerSettings and it will handle any and all FrozenDictionary types within your models, without explicitly having to register a container for each key/value type pairing.

I want to add that this doesn’t implement the WriteJson method because that is already supported by JSON.NET (presumably through the IDictionary<TKey, TValue> interface), so we don’t have to create our own.

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