Serialization

dotnet add package Google.Protobuf
dotnet add package Grpc.Tools

// orders.proto
syntax = "proto3";

message Order {
    string id = 1;
    string customer_id = 2;
    repeated OrderItem items = 3;
    int64 created_at_ticks = 4;

    // Adding new fields is always safe
    string notes = 5;  // Added in v2 - old readers ignore it
}

message OrderItem {
    string product_id = 1;
    int32 quantity = 2;
    int64 price_cents = 3;
}

// SAFE: Add new fields with new numbers
message Order {
    string id = 1;
    string customer_id = 2;
    string shipping_address = 5;  // NEW - safe
}

// SAFE: Remove fields (old readers ignore unknown, new readers use default)
// Just stop using the field, keep the number reserved
message Order {
    string id = 1;
    // customer_id removed, but field 2 is reserved
    reserved 2;
}

// UNSAFE: Change field types
message Order {
    int32 id = 1;  // Was: string - BREAKS!
}

// UNSAFE: Reuse field numbers
message Order {
    reserved 2;
    string new_field = 2;  // Reusing 2 - BREAKS!
}

dotnet add package MessagePack
dotnet add package MessagePack.Annotations

[MessagePackObject]
public sealed class Order
{
    [Key(0)]
    public required string Id { get; init; }

    [Key(1)]
    public required string CustomerId { get; init; }

    [Key(2)]
    public required IReadOnlyList<OrderItem> Items { get; init; }

    [Key(3)]
    public required DateTimeOffset CreatedAt { get; init; }

    // New field - old readers skip unknown keys
    [Key(4)]
    public string? Notes { get; init; }
}

// Serialize
var bytes = MessagePackSerializer.Serialize(order);

// Deserialize
var order = MessagePackSerializer.Deserialize<Order>(bytes);

// Use source generator for AOT
[MessagePackObject]
public partial class Order { }  // partial enables source gen

// Configure resolver
var options = MessagePackSerializerOptions.Standard
    .WithResolver(CompositeResolver.Create(
        GeneratedResolver.Instance,  // Generated
        StandardResolver.Instance));

// Newtonsoft (reflection-based)
public class Order
{
    [JsonProperty("order_id")]
    public string Id { get; set; }

    [JsonProperty(NullValueHandling = NullValueHandling.Ignore)]
    public string? Notes { get; set; }
}

// System.Text.Json (source-gen compatible)
public sealed record Order(
    [property: JsonPropertyName("order_id")]
    string Id,

    string? Notes  // Null handling via JsonSerializerOptions
);

[JsonSerializable(typeof(Order))]
[JsonSourceGenerationOptions(
    PropertyNamingPolicy = JsonKnownNamingPolicy.SnakeCaseLower,
    DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull)]
public partial class OrderJsonContext : JsonSerializerContext { }

// .NET 7+ polymorphism
[JsonDerivedType(typeof(CreditCardPayment), "credit_card")]
[JsonDerivedType(typeof(BankTransferPayment), "bank_transfer")]
public abstract record Payment(decimal Amount);

public sealed record CreditCardPayment(decimal Amount, string Last4) : Payment(Amount);
public sealed record BankTransferPayment(decimal Amount, string AccountNumber) : Payment(Amount);

// Serializes as:
// { "$type": "credit_card", "amount": 100, "last4": "1234" }

// Protobuf/MessagePack: Automatic - unknown fields skipped
// System.Text.Json: Configure to allow
var options = new JsonSerializerOptions
{
    UnmappedMemberHandling = JsonUnmappedMemberHandling.Skip
};

// Phase 1: Add deserializer (deployed everywhere)
public Order Deserialize(byte[] data, string manifest) => manifest switch
{
    "Order.V1" => DeserializeV1(data),
    "Order.V2" => DeserializeV2(data),  // NEW - can read V2
    _ => throw new NotSupportedException()
};

// Phase 2: Enable serializer (next release, after V1 deployed everywhere)
public (byte[] data, string manifest) Serialize(Order order) =>
    _useV2Format
        ? (SerializeV2(order), "Order.V2")
        : (SerializeV1(order), "Order.V1");

// BAD: Type name in payload - renaming class breaks wire format
{
    "$type": "MyApp.Order, MyApp.Core",
    "id": "123"
}

// GOOD: Explicit discriminator - refactoring safe
{
    "type": "order",
    "id": "123"
}

akka {
  actor {
    serializers {
      messagepack = "Akka.Serialization.MessagePackSerializer, Akka.Serialization.MessagePack"
    }
    serialization-bindings {
      "MyApp.Messages.IMessage, MyApp" = messagepack
    }
  }
}

// Use source generators for System.Text.Json
[JsonSerializable(typeof(Order))]
public partial class AppJsonContext : JsonSerializerContext { }

// Use explicit field numbers/keys
[MessagePackObject]
public class Order
{
    [Key(0)] public string Id { get; init; }
}

// Use records for immutable message types
public sealed record OrderCreated(OrderId Id, CustomerId CustomerId);

// Don't use BinaryFormatter (ever)
var formatter = new BinaryFormatter();  // Security risk!

// Don't embed type names in wire format
settings.TypeNameHandling = TypeNameHandling.All;  // Breaks on rename!

// Don't use reflection serialization for hot paths
JsonConvert.SerializeObject(order);  // Slow, not AOT-compatible