Json – How to serialize the recursive dictionary in C#?

Well, your implicit operators are just creating empty object/string, whatever data is generated (GUIDs) you discard it with the operators.

To fix that you need to define:

public class HierarchicalRelationshipsMap : Dictionary<HierarchicalRelationshipsMap.EntityID, HierarchicalRelationshipsMap>
{
    public class EntityID
    {
        private readonly string _value;

        public EntityID(string value)
        {
            _value = value;
        }

        public static implicit operator EntityID(string value) => new EntityID(value);
        public static implicit operator string(EntityID entityID) => entityID._value;
    }
}

Then you would need to define custom JsonConverter, as by default it sees EntityID as object and serializes it as such. We need to "instruct it" to do serialization differently:

public class HierarchicalRelationshipsMapConverter : JsonConverter<HierarchicalRelationshipsMap>
{
    public override void WriteJson(JsonWriter writer, HierarchicalRelationshipsMap value, JsonSerializer serializer)
    {
        writer.WriteStartObject();
        foreach (var kvp in value)
        {
            writer.WritePropertyName((string)kvp.Key);
            serializer.Serialize(writer, kvp.Value);
        }
        writer.WriteEndObject();
    }

    public override HierarchicalRelationshipsMap ReadJson(JsonReader reader, Type objectType, HierarchicalRelationshipsMap existingValue, bool hasExistingValue, JsonSerializer serializer)
    {
        var map = new HierarchicalRelationshipsMap();

        if (reader.TokenType == JsonToken.StartObject)
        {
            reader.Read();
            while (reader.TokenType == JsonToken.PropertyName)
            {
                // Read the key as a string and convert it to EntityID
                HierarchicalRelationshipsMap.EntityID entityId = (string)reader.Value;

                // Read the value as a nested HierarchicalRelationshipsMap
                reader.Read();
                var nestedMap = serializer.Deserialize<HierarchicalRelationshipsMap>(reader);

                map[entityId] = nestedMap;
                reader.Read();
            }
        }

        return map;
    }
}

Then following code snippet:

HierarchicalRelationshipsMap hierarchicalRelationshipsMap = new()
{
  {
    Guid.NewGuid().ToString(),
    new HierarchicalRelationshipsMap
    {
      {Guid.NewGuid().ToString(), new HierarchicalRelationshipsMap() },
      {Guid.NewGuid().ToString(), new HierarchicalRelationshipsMap() }
    }
  }
};

// Serialization settings with custom HierarchicalRelationshipsMapConverter
var settings = new JsonSerializerSettings
{
    Formatting = Formatting.Indented,
    Converters = new List<JsonConverter> { new HierarchicalRelationshipsMapConverter() }
};

// Serialize the map to JSON
string json = JsonConvert.SerializeObject(hierarchicalRelationshipsMap, settings);
Console.WriteLine(json);

Produces expected output:

{
  "4cd7a972-29d1-432f-bad3-909de96882b7": {
    "042dfc86-cbfb-4cc7-a9d3-ba0a234e8fc4": {},
    "99c96d01-eafb-4d26-ab7c-e9851b361be3": {}
  }
}