serialization

name	serialization
description	Choose the right serialization format for .NET applications. Prefer schema-based formats (Protobuf, MessagePack) over reflection-based (Newtonsoft.Json). Use System.Text.Json with AOT source generators for JSON scenarios. Use when this capability is needed.
metadata	{"author":"foxminchan"}

Serialization in .NET

When to Use This Skill

Use this skill when:

• Choosing a serialization format for APIs, messaging, or persistence
• Migrating from Newtonsoft.Json to System.Text.Json
• Implementing AOT-compatible serialization
• Designing wire formats for distributed systems
• Optimizing serialization performance

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Schema-Based vs Reflection-Based

Aspect	Schema-Based	Reflection-Based
Examples	Protobuf, MessagePack, System.Text.Json (source gen)	Newtonsoft.Json, BinaryFormatter
Type info in payload	No (external schema)	Yes (type names embedded)
Versioning	Explicit field numbers/names	Implicit (type structure)
Performance	Fast (no reflection)	Slower (runtime reflection)
AOT compatible	Yes	No
Wire compatibility	Excellent	Poor

Recommendation: Use schema-based serialization for anything that crosses process boundaries.

---

Format Recommendations

Use Case	Recommended Format	Why
REST APIs	System.Text.Json (source gen)	Standard, AOT-compatible
gRPC	Protocol Buffers	Native format, excellent versioning
Actor messaging	MessagePack or Protobuf	Compact, fast, version-safe
Event sourcing	Protobuf or MessagePack	Must handle old events forever
Caching	MessagePack	Compact, fast
Configuration	JSON (System.Text.Json)	Human-readable
Logging	JSON (System.Text.Json)	Structured, parseable

Formats to Avoid

Format	Problem
BinaryFormatter	Security vulnerabilities, deprecated, never use
Newtonsoft.Json default	Type names in payload break on rename
DataContractSerializer	Complex, poor versioning
XML	Verbose, slow, complex

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System.Text.Json with Source Generators

For JSON serialization, use System.Text.Json with source generators for AOT compatibility and performance.

Setup

// Define a JsonSerializerContext with all your types
[JsonSerializable(typeof(Order))]
[JsonSerializable(typeof(OrderItem))]
[JsonSerializable(typeof(Customer))]
[JsonSerializable(typeof(List<Order>))]
[JsonSourceGenerationOptions(
    PropertyNamingPolicy = JsonKnownNamingPolicy.CamelCase,
    DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull)]
public partial class AppJsonContext : JsonSerializerContext { }

Usage

// Serialize with context
var json = JsonSerializer.Serialize(order, AppJsonContext.Default.Order);

// Deserialize with context
var order = JsonSerializer.Deserialize(json, AppJsonContext.Default.Order);

// Configure in ASP.NET Core
builder.Services.ConfigureHttpJsonOptions(options =>
{
    options.SerializerOptions.TypeInfoResolverChain.Insert(0, AppJsonContext.Default);
});

Benefits

• No reflection at runtime - All type info generated at compile time
• AOT compatible - Works with Native AOT publishing
• Faster - No runtime type analysis
• Trim-safe - Linker knows exactly what's needed

---

Protocol Buffers (Protobuf)

Best for: Actor systems, gRPC, event sourcing, any long-lived wire format.

Setup

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

Define Schema

// 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;
}

Versioning Rules

// 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!
}

---

MessagePack

Best for: High-performance scenarios, compact payloads, actor messaging.

Setup

dotnet add package MessagePack
dotnet add package MessagePack.Annotations

Usage with Contracts

[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);

AOT-Compatible Setup

// 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));

---

Migrating from Newtonsoft.Json

Common Issues

Newtonsoft	System.Text.Json	Fix
$type in JSON	Not supported by default	Use discriminators or custom converters
JsonProperty	JsonPropertyName	Different attribute
DefaultValueHandling	DefaultIgnoreCondition	Different API
NullValueHandling	DefaultIgnoreCondition	Different API
Private setters	Requires [JsonInclude]	Explicit opt-in
Polymorphism	[JsonDerivedType] (.NET 7+)	Explicit discriminators

Migration Pattern

// 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 { }

Polymorphism with Discriminators

// .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" }

---

Wire Compatibility Patterns

Tolerant Reader

Old code must safely ignore unknown fields:

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

Introduce Read Before Write

Deploy deserializers before serializers for new formats:

// 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");

Never Embed Type Names

// 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"
}

---

Performance Comparison

Approximate throughput (higher is better):

Format	Serialize	Deserialize	Size
MessagePack	★★★★★	★★★★★	★★★★★
Protobuf	★★★★★	★★★★★	★★★★★
System.Text.Json (source gen)	★★★★☆	★★★★☆	★★★☆☆
System.Text.Json (reflection)	★★★☆☆	★★★☆☆	★★★☆☆
Newtonsoft.Json	★★☆☆☆	★★☆☆☆	★★★☆☆

For hot paths, prefer MessagePack or Protobuf.

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Best Practices

DO

// 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

// 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

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Resources

• System.Text.Json Source Generation: https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json/source-generation
• Protocol Buffers: https://protobuf.dev/
• MessagePack-CSharp: https://github.com/MessagePack-CSharp/MessagePack-CSharp

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