Draft for EntityCosmosJsonConverter that avoids poluting domain objects with Cosmos specific properties

Entity.cs

using System;

public abstract class Entity
{
    public Guid Id { get; set; }
}

EntityCosmosJsonConverter.cs

using System;
using System.Reflection;
using System.Text.Json;
using System.Text.Json.Serialization;

public class EntityCosmosJsonConverter : JsonConverter<Entity>
{
    private readonly JsonSerializerOptions converterOptions;

    public override bool CanConvert(Type typeToConvert)
        => typeof(Entity).IsAssignableFrom(typeToConvert);

    public EntityCosmosJsonConverter(JsonSerializerOptions options)
    {
        converterOptions = CopyOptionsExceptSelf(options);
    }

    public override Entity Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        var result = JsonSerializer.Deserialize(ref reader, typeToConvert, converterOptions) as Entity;

        return result ?? throw new JsonException("TAAAADA");
    }

    public override void Write(Utf8JsonWriter writer, Entity entity, JsonSerializerOptions options)
    {
        var entityType = entity.GetType();
        writer.WriteStartObject();

        // Add cosmos specific fields
        writer.WriteString("id", entity.Id);
        writer.WriteString("pk", entityType.Name);

        foreach (var propertyInfo in entityType.GetProperties(BindingFlags.Public | BindingFlags.Instance))
        {
            if (propertyInfo.Name.Equals("id", StringComparison.OrdinalIgnoreCase))
                continue;

            if (!propertyInfo.CanWrite && options.IgnoreReadOnlyProperties)
                continue;

            var value = propertyInfo.GetValue(entity);
            if (value is { } || !options.IgnoreNullValues)
            {
                var propertyName = options.PropertyNamingPolicy?.ConvertName(propertyInfo.Name) ?? propertyInfo.Name;
                writer.WritePropertyName(propertyName);
                JsonSerializer.Serialize(writer, value, propertyInfo.PropertyType, converterOptions);
            }
        }

        writer.WriteEndObject();
    }

    private static JsonSerializerOptions CopyOptionsExceptSelf(JsonSerializerOptions options)
    {
        var result = new JsonSerializerOptions
        {
            AllowTrailingCommas = options.AllowTrailingCommas,
            DefaultBufferSize = options.DefaultBufferSize,
            DictionaryKeyPolicy = options.DictionaryKeyPolicy,
            Encoder = options.Encoder,
            IgnoreNullValues = options.IgnoreNullValues,
            IgnoreReadOnlyProperties = options.IgnoreReadOnlyProperties,
            MaxDepth = options.MaxDepth,
            PropertyNameCaseInsensitive = options.PropertyNameCaseInsensitive,
            PropertyNamingPolicy = options.PropertyNamingPolicy,
            ReadCommentHandling = options.ReadCommentHandling,
            WriteIndented = options.WriteIndented
        };

        // Copy options but skip the adding this converter.
        foreach (var converter in options.Converters)
        {
            if (converter is EntityCosmosJsonConverter)
                continue;

            result.Converters.Add(converter);
        }

        return result;
    }
}

Todo-in-cosmos.json

{
    "id": "3353613d-a173-4e25-b99c-092f35145d1d",
    "pk": "Todo",
    "name": "Remove Cosmos from entities",
    "_rid": "EFNFAIGlGTwYAAAAAAAAAA==",
    "_self": "dbs/EFNFAA==/colls/EFNFAIGlGTw=/docs/EFNFAIGlGTwYAAAAAAAAAA==/",
    "_etag": "\"00000000-0000-0000-b8ed-658a050c01d6\"",
    "_attachments": "attachments/",
    "_ts": 1605183280
}