I have a custom JsonConverter that interprets a json object that is a polymorphic class, and generates the correct subclass based on a child property.

public class Metadata // base class
public class MetadataText : Metadata
public class MetadataImage : Metadata
... etc

Each class with a child Metadata property has it marked as nullable:

public class Author
{
   [CanBeNull] public Metadata metadata { get; set; }
}

This works fine for many variations of “metadata” in the incoming JSON. However… I have no control over the incoming json – huge files from a third party – and annoyingly they sometimes return “{}” where they should have returned “null”.

i.e.

...
"author":{
 "metadata":
  {}
 }

…which SHOULD BE:

"author":{
 "metadata": null
 }

This seems to be generating a huge number of fake errors on deserialization (currently captured and logged to console using a custom error-handler)

var jsonSettings = new JsonSerializerSettings()
        {
            NullValueHandling = NullValueHandling.Ignore,
            MissingMemberHandling = MissingMemberHandling.Ignore,
            Error = HandleDeserializationError // just outputs the error
 };

I can’t seem to find a way to get rid of those fake errors:

Value cannot be null.
Parameter name: value, path = [0].[OMITTED].author.metadata

class MetadataConverter : JsonConverter
{
    public override bool CanConvert(Type objectType)
    {
        return typeof(Metadata).IsAssignableFrom(objectType);
    }

    // has no effect: [CanBeNull]
    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        JObject item = JObject.Load(reader);

        if( item.Count == 0 )
        {
            Debug.Log( "Found a Metadata with count = 0; "+reader.TokenType+", "+reader.Path+", "+reader.Value+" -- item: "+item.Type+", "+item.Parent?.Parent+", "+item.Path+", "+item.Properties().Count() );
            return (Metadata)null; // should be OK!
        }
        
        switch (item["command"].Value<string>())
        {
            case "open_url":    return item.ToObject<Metadata_OpenURL>();
            case "search":  return item.ToObject<Metadata_Search>();
            case "mclick":  return item.ToObject<Metadata_MClick>();
            case "msearch": return item.ToObject<Metadata_MSearch>();
            default: return item.ToObject<Metadata>();
        }
    }

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