How to serialize a JObject the same way as an object with Json.NET?

Question

How do I control the serialization of a JObject to string?

I have some APIs that return a JObject and I usually apply some changes and persist or return them. I want to avoid persisting null properties and apply some additional formatting, but JsonConvert seems to completely ignore my settings.

Here is the sample of the problem:

// startup.cs has the following
services.AddMvc().AddJsonOptions(o =>
{
    o.SerializerSettings.NullValueHandling = NullValueHandling.Ignore;
});

public class SampleController : Controller
{
    JsonSerializerSettings _settings = new JsonSerializerSettings
    {
        NullValueHandling = NullValueHandling.Ignore
    };

    [HttpPost]
    [Route("object")]
    public object PostObject([FromBody] SomeObject data)
    {
        return JsonConvert.SerializeObject(data, _settings);
    }

    [HttpPost]
    [Route("jobject")]
    public object PostJObject([FromBody] JObject data)
    {
        return JsonConvert.SerializeObject(data, _settings);
    }

    public class SomeObject
    {
        public string Foo { get; set; }
        public string Bar { get; set; }
    }
}

Posting { "Foo": "Foo", "Bar": null }:

• /object returns {"Foo":"Foo"}
• /jobject returns {"Foo":"Foo","Bar":null}

I want the JObject method to return the same output json as if I were using an object. How do I achieve this without creating helpers? Is there a way to serialize the JObject using the same settings?

Answer 1

A solution that well integrates with NewtonSoft framework is to provide a custom JObject converter that honours the NamingStrategy.

JObject Custom Converter

public class JObjectNamingStrategyConverter : JsonConverter<JObject> {

private NamingStrategy NamingStrategy { get; }

public JObjectNamingStrategyConverter (NamingStrategy strategy) {
    if (strategy == null) {
        throw new ArgumentNullException (nameof (strategy));
    }
    NamingStrategy = strategy;
}

public override void WriteJson (JsonWriter writer, JObject value, JsonSerializer serializer) {
    writer.WriteStartObject ();
    foreach (JProperty property in value.Properties ()) {
        var name = NamingStrategy.GetPropertyName (property.Name, false);
        writer.WritePropertyName (name);
        serializer.Serialize (writer, property.Value);
    }
    writer.WriteEndObject ();
}

public override JObject ReadJson (JsonReader reader, Type objectType, JObject existingValue, bool hasExistingValue, JsonSerializer serializer) {
    throw new NotImplementedException ();
}
}

Usage

var snakeNameStrategy = new SnakeCaseNamingStrategy ();
var jsonSnakeSettings = new JsonSerializerSettings {
Formatting = Formatting.Indented,
Converters = new [] { new JObjectNamingStrategyConverter (snakeNameStrategy) },
   ContractResolver = new DefaultContractResolver {
       NamingStrategy = snakeNameStrategy
   },
};

var json = JsonConvert.SerializeObject (obj, jsonSnakeSettings);

You can find a working PoC on GitHub.

Answer 2

The only way I was able to do this is by first converting the JObject to a string, then deserializing that string into an ExpandoObject (don't deserialize to object because you'll get back a JObject). The ExpandoObject is like a dictionary, which will cause JsonConvert to actually invoke the configured name case strategy. I'm not sure why the author of Newtonsoft.Json didn't handle JObject types the same way as they seem to be doing for dictionary types, but at least this work around works.

Example:

// Construct a JObject.
var jObject = JObject.Parse("{ SomeName: \"Some value\" }");

// Deserialize the object into an ExpandoObject (don't use object, because you will get a JObject).
var payload = JsonConvert.DeserializeObject<ExpandoObject>(jObject.ToString());

// Now you can serialize the object using any serializer settings you like.
var json = JsonConvert.SerializeObject(payload, new JsonSerializerSettings
{
    ContractResolver = new DefaultContractResolver
    {
        NamingStrategy = new CamelCaseNamingStrategy
        {
            // Important! Make sure to set this to true, since an ExpandoObject is like a dictionary.
            ProcessDictionaryKeys = true,
        }
    }
}
);

Console.WriteLine(json); // Outputs: {"someName":"Some value"}

I picked-up the trick with the ExpandoObject here: JObject & CamelCase conversion with JSON.Net

Answer 3

When a JObject is serialized its raw JSON is written. JsonSerializerSettings do not effect its written JSON.