docs/guide/csharp/custom-serializers.md

---
title: Custom Serializers
sidebar_position: 11
id: custom_serializers
license: |
  Licensed to the Apache Software Foundation (ASF) under one or more
  contributor license agreements.  See the NOTICE file distributed with
  this work for additional information regarding copyright ownership.
  The ASF licenses this file to You under the Apache License, Version 2.0
  (the "License"); you may not use this file except in compliance with
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---

Use a custom serializer when a type is not generated with `[ForyStruct]` or
requires specialized encoding.

[External-type serialization](external-types.md) generates serializers for
mutable third-party targets, including exact private or renamed fields. Use a
custom serializer for immutable, constructor-only, factory-only, readonly,
init-only, converted, or custom-wire targets. On .NET 8, a private external
wire field whose declaring type or accessor signature is generic also requires
a custom serializer. Exact storage-only mappings do not emit private accessors.

## Implement `Serializer<T>`

```csharp
using Apache.Fory;

public sealed class Point
{
    public int X { get; set; }
    public int Y { get; set; }
}

public sealed class PointSerializer : Serializer<Point>
{
    public override Point DefaultValue => new();

    public override void WriteData(WriteContext context, in Point value, bool hasGenerics)
    {
        context.Writer.WriteVarInt32(value.X);
        context.Writer.WriteVarInt32(value.Y);
    }

    public override Point ReadData(ReadContext context)
    {
        return new Point
        {
            X = context.Reader.ReadVarInt32(),
            Y = context.Reader.ReadVarInt32(),
        };
    }
}
```

## Register the Serializer

```csharp
Fory fory = Fory.Builder().Build();
fory.Register<Point, PointSerializer>(200);

Point value = new() { X = 10, Y = 20 };
byte[] payload = fory.Serialize(value);
Point decoded = fory.Deserialize<Point>(payload);
```

Use the named overload when peers identify the type by name instead of a numeric ID:

```csharp
fory.Register<Point, PointSerializer>("com.example.Point");
```

## Serializer Behavior Notes

- `WriteData` / `ReadData` only handle payload content.
- Ref flags and type info are handled by base `Serializer<T>.Write` / `Read` unless overridden.
- `DefaultValue` is used for null/default fallback paths.

## Best Practices

1. Keep serializers deterministic and symmetric.
2. Use varint/fixed/tagged encoding intentionally for integer-heavy payloads.
3. Register custom serializers on all reader/writer peers.
4. Prefer generated `[ForyStruct]` serializers for normal domain models.

## Related Topics

- [Type Registration](type-registration.md)
- [Schema Metadata](schema-metadata.md)
- [Troubleshooting](troubleshooting.md)