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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 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0 Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.

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.

[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.

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.

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.

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(...).

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