Workaround for providing support for types JObject with System.Text.Json

[snjosephms]

We are migrating our code base from Newtonsoft to System.Text.Json. While testing this migration effort, we have encountered errors while working with Newtonsoft.Json.Linq.JObject objects

Issue during serialization:

var c = new DerivedClassA
            {
                F = JObject.FromObject(new
                {
                    Box = "abc",
                    Side = "def"
                })
            };

We observed that while serializing the data using System.Text.Json, the key is present in the serialized data, but the values are not.
"Properties":{"Box":[],"Side":[]}}]}

Issue during de-serialization:
We get the following error on deserialization:
"System.NotSupportedException: The collection type 'Newtonsoft.Json.Linq.JToken' is abstract, an interface, or is read only, and could not be instantiated and populated. Path: $.Message.Attachments[0].Properties.Box | LineNumber: 0 | BytePositionInLine: 934. ---> System.NotSupportedException: The collection type 'Newtonsoft.Json.Linq.JToken' is abstract, an interface, or is read only, and could not be instantiated and populated.   Stack Trace: 
ThrowHelper.ThrowNotSupportedException(ReadStack& state, Utf8JsonReader& reader, NotSupportedException ex)
IEnumerableOfTConverter2.CreateCollection(Utf8JsonReader& reader, ReadStack& state, JsonSerializerOptions options) JsonCollectionConverter2.OnTryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, TCollection& value)
JsonConverter1.TryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value) JsonDictionaryConverter3.OnTryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, TDictionary& value)
JsonConverter1.TryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value) JsonPropertyInfo1.ReadJsonAndSetMember(Object obj, ReadStack& state, Utf8JsonReader& reader)
ObjectDefaultConverter1.OnTryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value) JsonConverter1.TryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value)
JsonCollectionConverter2.OnTryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, TCollection& value) JsonConverter1.TryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value)
JsonPropertyInfo1.ReadJsonAndSetMember(Object obj, ReadStack& state, Utf8JsonReader& reader) ObjectDefaultConverter1.OnTryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value)
JsonConverter1.TryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value) JsonPropertyInfo1.ReadJsonAndSetMember(Object obj, ReadStack& state, Utf8JsonReader& reader)
ObjectWithParameterizedConstructorConverter1.OnTryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value) JsonConverter1.TryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value)
JsonConverter1.ReadCore(Utf8JsonReader& reader, JsonSerializerOptions options, ReadStack& state) JsonSerializer.ReadFromSpan[TValue](ReadOnlySpan1 utf8Json, JsonTypeInfo jsonTypeInfo, Nullable1 actualByteCount) JsonSerializer.ReadFromSpan[TValue](ReadOnlySpan1 json, JsonTypeInfo jsonTypeInfo)
JsonSerializer.Deserialize[TValue](String json, JsonSerializerOptions options)"

Can you please share a workaround for providing support for JObject type with System.Text.Json?

[gregsdennis]

The analog for JToken is JsonNode. You'll need to change your models to this as the Newtonsoft models don't have STJ converters.

There's also JsonObject for JObject, and JsonArray for JArray. Other JSON value deserialize to JsonValue.

[eiriktsarpalis]

Here's an example implementation of a JsonConverter that can be used to serialize and deserialize JToken objects using the System.Text.Json library:

public class JTokenConverter : JsonConverter<JToken>
{
    public override bool CanConvert(Type typeToConvert) => typeof(JToken).IsAssignableFrom(typeToConvert);
    public override JToken Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        // Use JsonDocument to parse the JSON and create a JToken from it
        using JsonDocument document = JsonDocument.ParseValue(ref reader);
        return JToken.Parse(document.RootElement.GetRawText());
    }

    public override void Write(Utf8JsonWriter writer, JToken value, JsonSerializerOptions options)
    {
        // Write the raw JSON from the JToken to the writer
        writer.WriteRawValue(value.ToString());
    }
}

[gregsdennis]

Whole that may help in the immediate term, if they're migrating away from Newtonsoft, I think it makes sense to encourage a transition to JsonNode in the models.

The upside of having this converter is that they can continue to work through the transition instead of having to change everywhere all at once.

[benbenwilde]

value.ToString() makes indented json by default, so something like this is better:
writer.WriteRawValue(value.ToString(options.WriteIndented ? Newtonsoft.Json.Formatting.Indented : Newtonsoft.Json.Formatting.None));

[aalstes]

What worked for me was converting the JObject to a dictionary with this code:

    public static Dictionary<string, object> ToDictionary(JObject jObject)
    {
        var dictionary = new Dictionary<string, object>();

        foreach (var property in jObject)
        {
            dictionary[property.Key] = ToObject(property.Value);
        }

        return dictionary;
    }

    public static object ToObject(JToken token)
    {
        if (token.Type == JTokenType.Object)
        {
            return ToDictionary((JObject)token);
        }
        else if (token.Type == JTokenType.Array)
        {
            return ((JArray)token).ToObject<object[]>();
        }
        else
        {
            return ((JValue)token).Value;
        }
    }