How to serialize top level objects with Json.NET as objects and nested objects as references - serialization

Question

I want to be able to serialize an object fully if it is at the top level of the serialization context but serialize objects lower in the context by reference.
I've searched and tried tests with Custom Contract Resolvers, Custom Json Converters and custom IReferenceResolver but I can't find a way to do this.
For example, imagine an IdType class that at the top level I want to serialize with all its properties but where I come across a reference to such an object in a property or list or dictionary I want to produce a reference.
For this type and test
public class IdType
{
public IdType(string id)
{
Id = id;
}
public string Id {get;}
public string Name {get;set;}
public int Number {get; set;}
public IdType OtherType { get; set; }
public IEnumerable<IdType> Types { get; set;}
public IDictionary<IdType, string> { get; set; }
public IDictionary<string, IdType> {get; set; }
}
[TestMethod]
public void SerializeTest()
{
var t1 = new IdType(1) { Name = 'Alice', Number = 42 };
var t2 = new IdType(2) { Name = 'Bob', Number = 21, OtherType = t1 };
var t3 = new IdType(2) { Name = 'Charlie', Number = 84, OtherType = t2, Types = new[] {t1, t2} };
var testTypes = new[]
{
t1,
t3
};
var serializer = new JsonSerializer
{
Formatting = Formatting.Indented,
};
StringWriter writer;
using (writer = new StringWriter())
{
serializer.Serialize(writer, myObject);
}
Console.WriteLine(writer.ToString());
}
I want output like this
[
{
"Id": "1",
"Name": "Alice"
"Number": 42,
},
{
"Id": "3",
"Name": "Charlie"
"Number": 84,
"OtherType": 2
"Types": [
"Id" : 1, 2
]
}
]
A JsonConverter has no context so it'll either always convert one way or another.
A custom resolver (derived from DefaultContractResolver) will work for a property of type IdType but I can't work out how to make it work with lists and dictionaries.
Latterly I've tried using PreserveReferenceHandling and a custom IReferenceResolver that has the IDs of the top level elements. But this doesn't work because the serialization is depth first.
Any suggestions to achieve this would be gratefully received

Answer 1

I think I've answered my own question. If I use a combination of a custom contract resolver and a custom converter and conditionally add the converter to the properties I want to serialize to IDs then it seems to work.
I've not implemented dictionaries yet but this works for basic properties and lists:
public class CustomResolver : DefaultContractResolver
{
readonly CamelCasePropertyNamesContractResolver innerResolver = new CamelCasePropertyNamesContractResolver();
protected override JsonProperty CreateProperty(MemberInfo member, MemberSerialization memberSerialization)
{
var jsonProperty = base.CreateProperty(member, memberSerialization);
if (!jsonProperty.PropertyType.IsPrimitive && jsonProperty.PropertyType != typeof(string) && jsonProperty.Readable)
{
var innerContract = innerResolver.ResolveContract(jsonProperty.PropertyType);
if (typeof(IdType).IsAssignableFrom(innerContract.UnderlyingType) && innerContract is JsonObjectContract objectContract)
{
jsonProperty.Converter = new IdConverter();
}
else if (typeof(IEnumerable<IdType>).IsAssignableFrom(innerContract.UnderlyingType) && innerContract is JsonArrayContract arrayContract)
{
jsonProperty.Converter = new IdConverter();
}
}
return jsonProperty;
}
}
internal class IdConverter : JsonConverter
{
public override bool CanConvert(Type objectType)
{
return (typeof(IdType).IsAssignableFrom(objectType) ||
typeof(IEnumerable<IdType>).IsAssignableFrom(objectType));
}
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
if (value is IdType item)
{
JToken token = JToken.FromObject(item.Id);
token.WriteTo(writer);
}
else if (value is IEnumerable<IdType> itemCollection)
{
JArray array = new JArray();
foreach (var i in itemCollection)
{
JToken token = JToken.FromObject(i.Id);
array.Add(token);
}
array.WriteTo(writer);
}
}
public override bool CanRead
{
get { return false; }
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
throw new NotImplementedException();
}
}
To serialize and making use of the custom resolver you would:
var serializer = new JsonSerializer
{
Formatting = Formatting.Indented,
ContractResolver = new CustomResolver(),
};