Dotnet Serialization

dotnet-serialization

AOT-friendly serialization patterns for .NET applications. Covers System.Text.Json source generators for compile-time serialization, Protocol Buffers (Protobuf) for efficient binary serialization, and MessagePack for high-performance compact binary format. Includes performance tradeoff guidance for choosing the right serializer and warnings about reflection-based serialization in AOT scenarios.

Scope

• System.Text.Json source generators for compile-time serialization
• Protocol Buffers (Protobuf) for binary serialization
• MessagePack for high-performance compact format
• Performance tradeoff guidance for serializer selection
• AOT-safe serialization patterns and anti-patterns

Out of scope

• Source generator authoring patterns -- see [skill:dotnet-csharp-source-generators]
• HTTP client factory and resilience pipelines -- see [skill:dotnet-http-client] and [skill:dotnet-resilience]
• Native AOT architecture and trimming -- see [skill:dotnet-native-aot] and [skill:dotnet-trimming]

Cross-references: [skill:dotnet-csharp-source-generators] for understanding how STJ source generators work under the hood. See [skill:dotnet-integration-testing] for testing serialization round-trip correctness.

Serialization Format Comparison

When to Choose What

• System.Text.Json with source generators: Default choice for APIs, configuration, and any scenario where human-readable output or web client consumption matters. AOT-safe when using source generators.
• Protobuf: Default wire format for gRPC. Best throughput and smallest payload size for service-to-service communication. Schema-first development with .proto files.
• MessagePack: When you need binary compactness without .proto schema management. Good for caching layers, real-time messaging, and high-throughput scenarios where schema evolution is managed via attributes.

System.Text.Json Source Generators

System.Text.Json source generators produce compile-time serialization code, eliminating runtime reflection. This is required for Native AOT and strongly recommended for all new projects. See [skill:dotnet-csharp-source-generators] for the underlying incremental generator mechanics.

Basic Setup

Define a JsonSerializerContext with [JsonSerializable] attributes for each type you serialize:

using System.Text.Json.Serialization;

[JsonSerializable(typeof(Order))]
[JsonSerializable(typeof(List<Order>))]
[JsonSerializable(typeof(OrderStatus))]
public partial class AppJsonContext : JsonSerializerContext
{
}

Using the Generated Context

// Serialize
string json = JsonSerializer.Serialize(order, AppJsonContext.Default.Order);

// Deserialize
Order? result = JsonSerializer.Deserialize(json, AppJsonContext.Default.Order);

// With options (created once, reused)
var options = new JsonSerializerOptions
{
    PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
    TypeInfoResolver = AppJsonContext.Default
};

string json = JsonSerializer.Serialize(order, options);

ASP.NET Core Integration

Register the source-generated context so Minimal APIs use it automatically. Note that ConfigureHttpJsonOptions applies to Minimal APIs only -- MVC controllers require separate configuration via AddJsonOptions:

var builder = WebApplication.CreateBuilder(args);

// Minimal APIs: ConfigureHttpJsonOptions
builder.Services.ConfigureHttpJsonOptions(options =>
{
    options.SerializerOptions.TypeInfoResolverChain.Insert(0, AppJsonContext.Default);
});

// MVC Controllers: AddJsonOptions (if using controllers)
builder.Services.AddControllers()
    .AddJsonOptions(options =>
    {
        options.JsonSerializerOptions.TypeInfoResolverChain.Insert(0, AppJsonContext.Default);
    });

var app = builder.Build();

// Minimal API endpoints automatically use the registered context
app.MapGet("/orders/{id}", async (int id, OrderService service) =>
{
    var order = await service.GetAsync(id);
    return order is not null ? Results.Ok(order) : Results.NotFound();
});

app.MapPost("/orders", async (Order order, OrderService service) =>
{
    await service.CreateAsync(order);
    return Results.Created($"/orders/{order.Id}", order);
});

Combining Multiple Contexts

When your application has multiple serialization contexts (e.g., different bounded contexts or libraries):

builder.Services.ConfigureHttpJsonOptions(options =>
{
    options.SerializerOptions.TypeInfoResolver = JsonTypeInfoResolver.Combine(
        AppJsonContext.Default,
        CatalogJsonContext.Default,
        InventoryJsonContext.Default
    );
});

Common Configuration

[JsonSourceGenerationOptions(
    PropertyNamingPolicy = JsonKnownNamingPolicy.CamelCase,
    DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
    WriteIndented = false)]
[JsonSerializable(typeof(Order))]
[JsonSerializable(typeof(List<Order>))]
public partial class AppJsonContext : JsonSerializerContext
{
}

Handling Polymorphism

[JsonDerivedType(typeof(CreditCardPayment), "credit_card")]
[JsonDerivedType(typeof(BankTransferPayment), "bank_transfer")]
[JsonDerivedType(typeof(WalletPayment), "wallet")]
public abstract class Payment
{
    public decimal Amount { get; init; }
    public string Currency { get; init; } = "USD";
}

public class CreditCardPayment : Payment
{
    public string Last4Digits { get; init; } = "";
}

// Register the base type -- derived types are discovered via attributes
[JsonSerializable(typeof(Payment))]
public partial class AppJsonContext : JsonSerializerContext
{
}

Protobuf Serialization

Protocol Buffers provide schema-first binary serialization. Protobuf is the default wire format for gRPC and is AOT-safe.

Package

<PackageReference Include="Google.Protobuf" Version="3.*" />
<PackageReference Include="Grpc.Tools" Version="2.*" PrivateAssets="All" />

Proto File

syntax = "proto3";

import "google/protobuf/timestamp.proto";

option csharp_namespace = "MyApp.Contracts";

message OrderMessage {
  int32 id = 1;
  string customer_id = 2;
  repeated OrderItemMessage items = 3;
  google.protobuf.Timestamp created_at = 4;
}

message OrderItemMessage {
  string product_id = 1;
  int32 quantity = 2;
  double unit_price = 3;
}

Standalone Protobuf (Without gRPC)

Use Protobuf for binary serialization without gRPC when you need compact payloads for caching, messaging, or file storage:

using Google.Protobuf;

// Serialize to bytes
byte[] bytes = order.ToByteArray();

// Deserialize from bytes
var restored = OrderMessage.Parser.ParseFrom(bytes);

// Serialize to stream
using var stream = File.OpenWrite("order.bin");
order.WriteTo(stream);

Proto File Registration in .csproj

<ItemGroup>
  <Protobuf Include="Protos\*.proto" GrpcServices="Both" />
</ItemGroup>

MessagePack Serialization

MessagePack-CSharp provides high-performance binary serialization with smaller payloads than JSON and good .NET integration.

Package

<PackageReference Include="MessagePack" Version="3.*" />
<!-- For AOT support -->
<PackageReference Include="MessagePack.SourceGenerator" Version="3.*" />

Basic Usage with Source Generator (AOT-Safe)

using MessagePack;

[MessagePackObject]
public partial class Order
{
    [Key(0)]
    public int Id { get; init; }

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

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

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

Serialization

// Serialize
byte[] bytes = MessagePackSerializer.Serialize(order);

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

// With compression (LZ4)
var lz4Options = MessagePackSerializerOptions.Standard.WithCompression(
    MessagePackCompression.Lz4BlockArray);
byte[] compressed = MessagePackSerializer.Serialize(order, lz4Options);

AOT Resolver Setup

For Native AOT compatibility, use the MessagePack source generator to produce a resolver:

// In your project, the source generator automatically produces a resolver
// from types annotated with [MessagePackObject].
// Register the generated resolver at startup:
MessagePackSerializer.DefaultOptions = MessagePackSerializerOptions.Standard
    .WithResolver(GeneratedResolver.Instance);

Anti-Patterns: Reflection-Based Serialization

Do not use reflection-based serializers in Native AOT or trimming scenarios. Reflection-based serialization fails at runtime when the linker removes type metadata.

Newtonsoft.Json (JsonConvert)

Newtonsoft.Json (JsonConvert.SerializeObject / JsonConvert.DeserializeObject) relies heavily on runtime reflection. It is incompatible with Native AOT and trimming:

// BAD: Reflection-based -- fails under AOT/trimming
var json = JsonConvert.SerializeObject(order);
var order = JsonConvert.DeserializeObject<Order>(json);

// GOOD: Source-generated -- AOT-safe
var json = JsonSerializer.Serialize(order, AppJsonContext.Default.Order);
var order = JsonSerializer.Deserialize(json, AppJsonContext.Default.Order);

System.Text.Json Without Source Generators

Even System.Text.Json falls back to reflection without a source-generated context:

// BAD: No context -- uses runtime reflection
var json = JsonSerializer.Serialize(order);

// GOOD: Explicit context -- uses source-generated code
var json = JsonSerializer.Serialize(order, AppJsonContext.Default.Order);

Migration Path from Newtonsoft.Json

1. Replace JsonConvert.SerializeObject / DeserializeObject with JsonSerializer.Serialize / Deserialize
2. Replace [JsonProperty] with [JsonPropertyName]
3. Replace JsonConverter base class with JsonConverter<T> from System.Text.Json
4. Create a JsonSerializerContext with [JsonSerializable] for all serialized types
5. Replace JObject / JToken dynamic access with JsonDocument / JsonElement or strongly-typed models
6. Test serialization round-trips -- attribute semantics differ between libraries

Performance Guidance

Throughput Benchmarks (Approximate)

Optimization Tips

• Reuse JsonSerializerOptions -- creating options is expensive; create once and reuse
• Use JsonSerializerContext -- eliminates warm-up cost and reduces allocation
• Use Utf8JsonWriter / Utf8JsonReader for streaming scenarios where you process JSON without full materialization
• Use Protobuf ByteString for binary data instead of base64-encoded strings in JSON
• Enable MessagePack LZ4 compression for large payloads over the wire

Key Principles

• Default to System.Text.Json with source generators for all JSON serialization -- it is AOT-safe, fast, and built into the framework
• Use Protobuf for service-to-service binary serialization -- especially as the wire format for gRPC
• Use MessagePack for high-throughput caching and real-time -- when binary compactness matters but .proto schema management is unwanted
• Never use Newtonsoft.Json for new AOT-targeted projects -- it is reflection-based and incompatible with trimming
• Always register JsonSerializerContext in ASP.NET Core -- use ConfigureHttpJsonOptions for Minimal APIs and AddJsonOptions for MVC controllers (they are separate registrations)
• Annotate all serialized types -- STJ source generators only generate code for types listed in [JsonSerializable]; MessagePack requires [MessagePackObject]

See [skill:dotnet-native-aot] for comprehensive AOT compilation pipeline, [skill:dotnet-aot-architecture] for AOT-first design patterns, and [skill:dotnet-trimming] for trimming strategies and ILLink descriptor configuration.

Agent Gotchas

1. Do not use JsonSerializer.Serialize(obj) without a context in AOT projects -- it falls back to reflection and fails at runtime. Always pass the source-generated TypeInfo.
2. Do not forget to list collection types in [JsonSerializable] -- [JsonSerializable(typeof(Order))] does not cover List<Order>. Add [JsonSerializable(typeof(List<Order>))] separately.
3. Do not use Newtonsoft.Json [JsonProperty] attributes with System.Text.Json -- they are silently ignored. Use [JsonPropertyName] instead.
4. Do not mix MessagePack [Key] integer keys with [Key] string keys in the same type hierarchy -- pick one strategy and stay consistent.
5. Do not omit GrpcServices attribute on <Protobuf> items -- without it, both client and server stubs are generated, which may cause build errors if you only need one.

References

• System.Text.Json source generation
• Migrate from Newtonsoft.Json to System.Text.Json
• Protocol Buffers for .NET
• MessagePack-CSharp
• Native AOT deployment