Marc Schier 889572472f Convert DataValue from class to readonly struct for further GC relief (#3796) ...

Convert `DataValue` from a mutable reference type (`class`) to a `readonly struct` to eliminate per-allocation GC pressure on every attribute read, subscription publish, and history sample.

### Key API changes (user-visible)

| Before | After | Pattern |
|---|---|---|
| `class DataValue` | `readonly struct DataValue : INullable` | Foundation |
| `Equals(DataValue?)` / `operator ==(DataValue?, DataValue?)` | `Equals(DataValue)` / `operator ==(DataValue, DataValue)` | Value semantics |
| `IEncoder.WriteDataValue(string?, DataValue?)` | `IEncoder.WriteDataValue(string?, DataValue)` | Non-nullable wire API |
| `IDecoder.ReadDataValue(...) -> DataValue?` | `IDecoder.ReadDataValue(...) -> DataValue` | Same |
| `IDataChangeMonitoredItemQueue.PeekLast/Oldest()` | `TryPeekLastValue/Oldest(out DataValue)` | TryGet pattern |
| `IAggregateCalculator.GetProcessedValue(bool)` | `TryGetProcessedValue(bool, out DataValue)` | Same |
| `IUaPubSubDataStore.ReadPublishedDataItem(...)` | `TryReadPublishedDataItem(NodeId, uint, out DataValue)` | Same |
| `Dictionary<uint, DataValue?>` (NodeCacheContext) | `Dictionary<uint, DataValue>` | Default + IsNull |
| `IHistoryDataSource.FirstRaw/NextRaw` | `TryFirstRaw/TryNextRaw(out DataValue)` | TryGet pattern |
| `NodeState.ReadAttribute(... DataValue)` | `NodeState.ReadAttribute(... ref DataValue)` | Sync mutation via ref |
| `NodeState.ReadAttributeAsync(... DataValue)` | `NodeState.ReadAttributeAsync(...) -> ValueTask<(ServiceResult, DataValue)>` | Async return tuple |
| `obj as DataValue` (heterogeneous Dictionary<string, object>) | `obj is DataValue dv ? dv : default(DataValue)` | Safe struct cast |
| `Assert.That(dv, Is.Not.Null)` (tests) | `Assert.That(dv.IsNull, Is.False)` | Struct test pattern |
| `[Obsolete] Value` setter, all property setters | Removed; use ctor or `With*()` | Immutability |
| `[DataContract]` + `[DataMember]` | Removed; surrogate via `SerializableDataValue` + `DataValueSurrogateProvider` | Serialization |

### Wire format

Wire format is **byte-identical** to the previous class-based implementation across binary / JSON / XML encoders. Default DataValue (`.IsNull == true`) is treated as the equivalent of the old null reference (writes JSON `null`, binary `byte 0`, XML omitted element).

### Why TryGet pattern (instead of leaving `DataValue?`)

For methods that previously returned `DataValue?` to signal "no value available" (queue peek, aggregate result, data store lookup, history navigation), the conversion to `bool TryGet(out DataValue)` matches BCL conventions (`Dictionary.TryGetValue`, `Queue.TryPeek`, `Int32.TryParse`) and gives the caller an explicit "may be absent" affordance without `Nullable<DataValue>` overhead.

### Breaking changes summary

- `DataValue` is now a `readonly struct` - boxing/unboxing semantics change for callers using `object`-typed APIs.
- `IEncoder.WriteDataValue` / `IDecoder.ReadDataValue` / `ReadDataValueArray` interface signatures lose the `Nullable` wrapper.
- `IDataChangeMonitoredItemQueue.PeekLastValue` / `PeekOldestValue` -> `TryPeekLastValue` / `TryPeekOldestValue` (out parameter).
- `IAggregateCalculator.GetProcessedValue` -> `TryGetProcessedValue` (out parameter).
- `IUaPubSubDataStore.ReadPublishedDataItem` -> `TryReadPublishedDataItem` (out parameter).
- `NodeState.ReadAttribute` takes `ref DataValue`; `ReadAttributeAsync` returns `ValueTask<(ServiceResult, DataValue)>`.
- `DataValue` property setters removed (use constructor + `With*` mutators).

2026-05-23 11:38:49 +02:00

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PooledNotificationBenchmarks.md

Convert DataValue from class to readonly struct for further GC relief (#3796)

2026-05-23 11:38:49 +02:00