docs/guide/csharp/schema-metadata.md

---
title: Schema Metadata
sidebar_position: 4
id: schema_metadata
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
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This page covers schema metadata for C# generated serializers.

## `[ForyStruct]` and `[ForyField]`

Use `[ForyStruct]` to enable source-generated serializers. Use `[ForyField]` to assign an optional stable non-negative field id or to override the Fory schema type used for a field.

External-type serialization puts `Target` on a local abstract serializer
declaration. Its properties own the field names, IDs, schema descriptors,
and nullability. A standalone declaration also owns its `Evolving` setting.
An external `BaseOnly` declaration cannot set `Evolving`; each concrete
descendant owns that setting. The target supplies the runtime values and
directly accessed members.

```csharp
[ForyStruct(Target = typeof(ThirdParty.User))]
internal abstract class UserSerializer
{
    [ForyField(
        1,
        TargetDeclaringType = typeof(ThirdParty.User),
        TargetMemberName = "<Name>k__BackingField")]
    public abstract string Name { get; }
}
```

## Inherited Fields

The concrete class serializer has one field list containing its own fields and
the selected fields from every annotated base class. That same flattened list
drives field ordering, schema hashes, and compatible metadata.

Property overrides contribute one logical field. The nearest override carrying
`[ForyField]` supplies the ID and schema descriptor. Fields and members hidden
with `new` remain distinct and must use unique IDs or unique normalized names.
Duplicate IDs and duplicate name-based identities are generation errors.

Changing a wire member on an annotated base changes every derived schema.
Rebuild assemblies containing derived serializers after updating the base
package.

```csharp
using Apache.Fory;
using S = Apache.Fory.Schema.Types;

[ForyStruct]
public sealed class Metrics
{
    [ForyField(Type = typeof(S.UInt32))]
    public uint Count { get; set; }

    [ForyField(Type = typeof(S.Tagged<S.UInt64>))]
    public ulong TraceId { get; set; }

    public long LatencyMicros { get; set; }
}
```

`Id` is optional. When it is omitted, compatible mode still matches the field by name.

```csharp
using Apache.Fory;
using S = Apache.Fory.Schema.Types;

[ForyStruct]
public sealed class NestedMetrics
{
    [ForyField(Type = typeof(S.Map<S.Fixed<S.UInt32>, S.List<S.Tagged<S.UInt64>>>))]
    public Dictionary<uint, List<ulong?>?> Values { get; set; } = [];

    [ForyField(3, Type = typeof(S.UInt64))]
    public ulong StableCount { get; set; }
}
```

## Schema Descriptor Types

Schema descriptors live under `Apache.Fory.Schema.Types` and are metadata only. They do not replace normal C# carrier types.

Common scalar descriptors include:

- `S.Int32`, `S.UInt32`
- `S.Int64`, `S.UInt64`
- `S.Float16`, `S.BFloat16`, `S.Float32`, `S.Float64`

Container descriptors are composable:

- `S.Fixed<TScalar>` and `S.Tagged<TScalar>` for scalar integer encodings
- `S.List<TElement>`
- `S.Set<TElement>`
- `S.Map<TKey, TValue>`
- `S.Array<TElement>`

Dense array fields use `S.Array<TElement>`, for example `S.Array<S.Int32>` or `S.Array<S.BFloat16>`.

Nullability comes from the C# carrier type. Use `List<ulong?>` for nullable list elements and `NullableKeyDictionary<TKey, TValue>` when a map needs nullable keys.

## `[ForyUnion]` and `[ForyCase]`

Generated union cases use `[ForyCase]` for both the stable case ID and optional
case payload schema type. Do not put `[ForyField]` on union case payload
members. Known case record names use PascalCase FDL case names; payload types
use qualified references when needed to avoid name conflicts. A typed union must
declare at least one non-`Unknown` case; `Unknown(UnknownCase)` is only the
Fory-owned forward-compatibility carrier. The marker only selects the carrier and
does not add an entry to the schema case table.

```csharp
using Apache.Fory;
using S = Apache.Fory.Schema.Types;

[ForyUnion]
public abstract partial record Shape
{
    private Shape() {}

    [ForyUnknownCase]
    public sealed partial record Unknown(UnknownCase Value) : Shape;

    [ForyCase(0)]
    public sealed partial record Circle(global::example.Circle Value) : Shape;

    [ForyCase(1, Type = typeof(S.Fixed<S.Int32>))]
    public sealed partial record Code(int Value) : Shape;
}
```

## Nullability and Reference Tracking

- Field nullability comes from C# type nullability (`string?`, nullable value types, etc.).
- Reference tracking is configured with `ForyBuilder.TrackRef(...)`.

## Related Topics

- [Configuration](configuration.md)
- [External Types](external-types.md)
- [Schema Evolution](schema-evolution.md)
- [Supported Types](supported-types.md)