How to inject the JSON $schema value during Newtonsoft.JsonConvert.SerializeObject

Question

I created a JSON schema for my C# code using:

// Create JSON schema
var generator = new JSchemaGenerator();
var schema = generator.Generate(typeof(ConfigFileJsonSchema));
schema.Title = "PlexCleaner Schema";
schema.Description = "PlexCleaner config file JSON schema";
schema.SchemaVersion = new Uri("http://json-schema.org/draft-06/schema");
schema.Id = new Uri("https://raw.githubusercontent.com/ptr727/PlexCleaner/main/PlexCleaner.schema.json");
Console.WriteLine(schema);

I want to add a reference to this scheme whenever I create JSON output from my code:

    private static string ToJson(ConfigFileJsonSchema settings)
    {
        return JsonConvert.SerializeObject(settings, Settings);
    }

    private static readonly JsonSerializerSettings Settings = new()
    {
        Formatting = Formatting.Indented,
        StringEscapeHandling = StringEscapeHandling.EscapeNonAscii,
        NullValueHandling = NullValueHandling.Ignore,
        // We expect containers to be cleared before deserializing
        // Make sure that collections are not read-only (get; set;) else deserialized values will be appended
        // https://stackoverflow.com/questions/35482896/clear-collections-before-adding-items-when-populating-existing-objects
        ObjectCreationHandling = ObjectCreationHandling.Replace
        // TODO: Add TraceWriter to log to Serilog
    };

How can I programmatically add the $schema URI to the created JSON, not meaning creating schema on the fly, but something like this:

public class ConfigFileJsonSchemaBase
{
    // Schema reference
    [JsonProperty(PropertyName = "$schema", Order = -2)]
    public string Schema { get; } = "https://raw.githubusercontent.com/ptr727/PlexCleaner/main/PlexCleaner.schema.json";

    // Default to 0 if no value specified, and always write the version first
    [DefaultValue(0)]
    [JsonProperty(DefaultValueHandling = DefaultValueHandling.Populate, Order = -2)]
    public int SchemaVersion { get; set; } = ConfigFileJsonSchema.Version;
}

Without needing to add a $schema entry to the class. E.g. equivalent of:

schema.SchemaVersion = new Uri("http://json-schema.org/draft-06/schema");

There is a similar unanswered question: json serialization to refer schema

Answer 1

You can use a JsonConverter:

public class SchemaJsonConverter : JsonConverter
{
    private readonly string _schemaUrl;

    private readonly Type[] _types;

    public SchemaJsonConverter(string schemaUrl, params Type[] types)
    {
        this._schemaUrl = schemaUrl;
        this._types = types;
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        JToken t = JToken.FromObject(value);

        if (t.Type != JTokenType.Object)
        {
            t.WriteTo(writer);
        }
        else
        {
            var o = (JObject)t;
            o.AddFirst(new JProperty("$Schema", this._schemaUrl));
            o.WriteTo(writer);
        }
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        throw new NotImplementedException("Unnecessary because CanRead is false. The type will skip the converter.");
    }

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

    public override bool CanConvert(Type objectType)
    {
        return this._types.Any(t => t == objectType);
    }
}

You need the type to check the types affected by the converter and the schema url to inject it in your JSON. The converter allow you a fine control about the process of serialization.

I use a simple class to test the converter:

public class Something
{
    public int Integer { get; set; }
    public string Text { get; set; }
}

And a method to run the sample:

public static void Test()
{
    var something = new Something
    {
        Integer = 37, 
        Text = "A text"
    };

    var settings = new JsonSerializerSettings
    {
        Formatting = Formatting.Indented,
        StringEscapeHandling = StringEscapeHandling.EscapeNonAscii,
        NullValueHandling = NullValueHandling.Ignore,
        // We expect containers to be cleared before deserializing
        // Make sure that collections are not read-only (get; set;) else deserialized values will be appended
        // https://stackoverflow.com/questions/35482896/clear-collections-before-adding-items-when-populating-existing-objects
        ObjectCreationHandling = ObjectCreationHandling.Replace
        // TODO: Add TraceWriter to log to Serilog
    };

    var schemaUrl = "http://json-schema.org/draft-06/schema";
    settings.Converters.Add(new SchemaJsonConverter(schemaUrl, something.GetType()));

    var json = JsonConvert.SerializeObject(something, settings);
    Console.WriteLine(json);
}

Output:

{
  "$Schema": "http://json-schema.org/draft-06/schema",
  "Integer": 37,
  "Text": "A text"
}

UPDATE

A static method for serialization:

public static string SerializeJson(object obj, JsonSerializerSettings settings, string schemaUrl = null)
{
    if (!string.IsNullOrEmpty(schemaUrl))
    {
        settings.Converters.Add(new SchemaJsonConverter(schemaUrl, obj.GetType()));
    }

    return JsonConvert.SerializeObject(obj, settings);
}

Usage:

var json = SerializeJson(something, settings, schemaUrl);