docs/guide/rust/custom-serializers.md

---
title: Custom Serializers
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id: rust_custom_serializers
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  this work for additional information regarding copyright ownership.
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For types that don't support `#[derive(ForyObject)]`, implement the `Serializer` trait manually.

## When to Use Custom Serializers

- External types from other crates
- Types with special serialization requirements
- Legacy data format compatibility
- Performance-critical custom encoding

## Implementing the Serializer Trait

```rust
use fory::{Fory, ReadContext, WriteContext, Serializer, ForyDefault, Error};
use std::any::Any;

#[derive(Debug, PartialEq, Default)]
struct CustomType {
    value: i32,
    name: String,
}

impl Serializer for CustomType {
    fn fory_write_data(&self, context: &mut WriteContext, is_field: bool) {
        context.writer.write_i32(self.value);
        context.writer.write_varuint32(self.name.len() as u32);
        context.writer.write_utf8_string(&self.name);
    }

    fn fory_read_data(context: &mut ReadContext, is_field: bool) -> Result<Self, Error> {
        let value = context.reader.read_i32();
        let len = context.reader.read_varuint32() as usize;
        let name = context.reader.read_utf8_string(len);
        Ok(Self { value, name })
    }

    fn fory_type_id_dyn(&self, type_resolver: &TypeResolver) -> u32 {
        Self::fory_get_type_id(type_resolver)
    }

    fn as_any(&self) -> &dyn Any {
        self
    }
}

// ForyDefault delegates to Default
impl ForyDefault for CustomType {
    fn fory_default() -> Self {
        Self::default()
    }
}
```

> **Note**: When implementing `ForyDefault` manually, ensure your type also implements `Default` if you use `Self::default()`.
> Alternatively, you can construct a default instance directly in `fory_default()`.
>
> **Tip**: If your type supports `#[derive(ForyObject)]`, you can use `#[fory(generate_default)]` to automatically generate both `ForyDefault` and `Default` implementations.

## Registering Custom Serializers

```rust
let mut fory = Fory::default();
fory.register_serializer::<CustomType>(100);

let custom = CustomType {
    value: 42,
    name: "test".to_string(),
};
let bytes = fory.serialize(&custom);
let decoded: CustomType = fory.deserialize(&bytes)?;
assert_eq!(custom, decoded);
```

## WriteContext and ReadContext

The `WriteContext` and `ReadContext` provide access to:

- **writer/reader**: Binary buffer operations
- **type_resolver**: Type registration information
- **ref_resolver**: Reference tracking (for shared/circular references)

### Common Writer Methods

```rust
// Primitive types
context.writer.write_i8(value);
context.writer.write_i16(value);
context.writer.write_i32(value);
context.writer.write_i64(value);
context.writer.write_f32(value);
context.writer.write_f64(value);
context.writer.write_bool(value);

// Variable-length integers
context.writer.write_varint32(value);
context.writer.write_varuint32(value);

// Strings
context.writer.write_utf8_string(&string);
```

### Common Reader Methods

```rust
// Primitive types
let value = context.reader.read_i8();
let value = context.reader.read_i16();
let value = context.reader.read_i32();
let value = context.reader.read_i64();
let value = context.reader.read_f32();
let value = context.reader.read_f64();
let value = context.reader.read_bool();

// Variable-length integers
let value = context.reader.read_varint32();
let value = context.reader.read_varuint32();

// Strings
let string = context.reader.read_utf8_string(len);
```

## Best Practices

1. **Use variable-length encoding** for integers that may be small
2. **Write length first** for variable-length data
3. **Handle errors properly** in read methods
4. **Implement ForyDefault** for schema evolution support

## Related Topics

- [Type Registration](type-registration.md) - Registering serializers
- [Basic Serialization](basic-serialization.md) - Using ForyObject derive
- [Schema Evolution](schema-evolution.md) - Compatible mode