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

Schema-Based vs Reflection-Based

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

Format Recommendations

Formats to Avoid

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,