Fix nullable build errors in Fluent server API + FileSystemClient + BaseVariableState

After merging master with the new Fluent server API (PR #3765), FileSystemClient
(PR #3760), and certificate group support (PR #3585), several new code paths
needed nullable annotations to align with the nullable4 branch:

- Stack/Opc.Ua.Types/State/BaseVariableState.cs: annotate four async value
  hooks (OnReadValueAsync/OnSimpleReadValueAsync/OnWriteValueAsync/
  OnSimpleWriteValueAsync) as nullable; widen the matching locals in
  ReadAttributeAsync/WriteAttributeAsync; null-forgive the simple-handler
  invocations gated by the prior null check; null-forgive the final
  `return result` since result is flow-typed nullable after assignment.
- Libraries/Opc.Ua.Client/FileSystem/FileSystemClient.cs: null-forgive
  QualifiedName.Name in CreateDirectory/CreateFile/MoveOrCopy leaf paths
  where the segment is already validated upstream.
- Libraries/Opc.Ua.Server/Fluent/EventNotifierBuilderExtensions.cs: optional
  EventPublishOptions parameter on both Publish overloads is now nullable.
- Libraries/Opc.Ua.Server/Fluent/EventPublishOptions.cs: OnError sink is
  nullable (already documented as defaulting to null).
- Libraries/Opc.Ua.Server/Fluent/EventSourceRegistry.cs: Register options
  parameter, SourceEntry.WorkerCts/WorkerTask fields and matching
  DeactivateSource locals are nullable; null-coalesce
  notifier.BrowseName.Name when projecting SourceName.
- Libraries/Opc.Ua.Server/Fluent/NodeBuilder.cs: ThrowIfSlotOccupied accepts
  Delegate? so the four async OnRead/OnWrite call sites compile.
- Libraries/Opc.Ua.Server/Fluent/NodeManagerBuilder.cs: null-forgive the
  `(QualifiedName)null` overload trampoline; EventSources backing property
  starts null until AttachEventSources runs.
- Libraries/Opc.Ua.Server/Fluent/VariableBuilder.cs: FromVariant returns
  TValue? (it already returned default on null); annotate the boxed local
  and null-forgive the setter call sites.

No runtime behavior change.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: agent

Libraries/Opc.Ua.Client/FileSystem/FileSystemClient.cs

@@ -274,7 +274,7 @@ public async ValueTask<UaDirectoryInfo> CreateDirectoryAsync(
                         $"Cannot create '{UaPath.FormatSegment(segment)}': leaf segments must not include a namespace prefix; the server picks the BrowseName namespace.",
                         nameof(path));
                 }
-                current = await CreateDirectoryInAsync(current, segment.Name, ct)
+                current = await CreateDirectoryInAsync(current, segment.Name!, ct)
                     .ConfigureAwait(false);
             }
             return current;
@@ -321,7 +321,7 @@ public async ValueTask<UaFileInfo> CreateFileAsync(
                     $"Cannot create file '{UaPath.FormatSegment(leaf)}': leaf segments must not include a namespace prefix; the server picks the BrowseName namespace.",
                     nameof(path));
             }
-            return await CreateFileInAsync(parent, leaf.Name, ct).ConfigureAwait(false);
+            return await CreateFileInAsync(parent, leaf.Name!, ct).ConfigureAwait(false);
         }
 
         /// <summary>
@@ -706,7 +706,7 @@ internal async ValueTask<UaFileSystemInfo> MoveOrCopyAsync(
                 string parentPath = UaPath.Format(segments.Take(segments.Length - 1).ToArray());
                 destDir = await GetDirectoryAsync(parentPath, ct).ConfigureAwait(false);
             }
-            return await MoveOrCopyAsync(source, destDir, segments[^1].Name, copy, ct)
+            return await MoveOrCopyAsync(source, destDir, segments[^1].Name!, copy, ct)
                 .ConfigureAwait(false);
         }
 

Libraries/Opc.Ua.Server/Fluent/EventNotifierBuilderExtensions.cs

@@ -85,7 +85,7 @@ public static class EventNotifierBuilderExtensions
         public static INodeBuilder<TNotifier> Publish<TNotifier, TEvent>(
             this INodeBuilder<TNotifier> nodeBuilder,
             Func<TNotifier, ISystemContext, CancellationToken, IAsyncEnumerable<TEvent>> factory,
-            EventPublishOptions options = null)
+            EventPublishOptions? options = null)
             where TNotifier : BaseObjectState
             where TEvent : BaseEventState
         {
@@ -130,7 +130,7 @@ public static INodeBuilder<TNotifier> Publish<TNotifier, TEvent>(
         public static INodeBuilder<TNotifier> Publish<TNotifier, TEvent>(
             this INodeBuilder<TNotifier> nodeBuilder,
             IAsyncEnumerable<TEvent> source,
-            EventPublishOptions options = null)
+            EventPublishOptions? options = null)
             where TNotifier : BaseObjectState
             where TEvent : BaseEventState
         {

Libraries/Opc.Ua.Server/Fluent/EventPublishOptions.cs

@@ -105,6 +105,6 @@ public sealed record EventPublishOptions
         /// callers that want to bubble the exception into their own
         /// telemetry. Default <c>null</c>.
         /// </summary>
-        public Action<Exception> OnError { get; init; }
+        public Action<Exception>? OnError { get; init; }
     }
 }

Libraries/Opc.Ua.Server/Fluent/EventSourceRegistry.cs

@@ -93,7 +93,7 @@ public EventSourceRegistry(
         public void Register(
             BaseObjectState notifier,
             Func<NodeState, ISystemContext, CancellationToken, System.Collections.Generic.IAsyncEnumerable<BaseEventState>> factory,
-            EventPublishOptions options)
+            EventPublishOptions? options)
         {
             if (notifier == null)
             {
@@ -373,8 +373,8 @@ private void ActivateSource(SourceEntry entry)
 
         private void DeactivateSource(SourceEntry entry, bool force)
         {
-            CancellationTokenSource cts = entry.WorkerCts;
-            Task worker = entry.WorkerTask;
+            CancellationTokenSource? cts = entry.WorkerCts;
+            Task? worker = entry.WorkerTask;
             entry.WorkerCts = null;
             entry.WorkerTask = null;
 
@@ -576,7 +576,7 @@ private static void PopulateDefaults(BaseObjectState notifier, ISystemContext co
             {
                 e.SourceName = PropertyState<string>.With<VariantBuilder>(
                     e,
-                    notifier.BrowseName.Name);
+                    notifier.BrowseName.Name ?? string.Empty);
             }
 
             if (e.Time == null || e.Time.Value.IsNull)
@@ -629,8 +629,8 @@ public SourceEntry(
             public BaseObjectState Notifier { get; }
             public Func<NodeState, ISystemContext, CancellationToken, System.Collections.Generic.IAsyncEnumerable<BaseEventState>> Factory { get; }
             public EventPublishOptions Options { get; }
-            public CancellationTokenSource WorkerCts;
-            public Task WorkerTask;
+            public CancellationTokenSource? WorkerCts;
+            public Task? WorkerTask;
             public int LeakedFaulted;
         }
 

Libraries/Opc.Ua.Server/Fluent/NodeBuilder.cs

@@ -321,7 +321,7 @@ private MethodState RequireMethod(string what)
             return m;
         }
 
-        private void ThrowIfSlotOccupied(Delegate existing, string what)
+        private void ThrowIfSlotOccupied(Delegate? existing, string what)
         {
             if (existing != null)
             {

Libraries/Opc.Ua.Server/Fluent/NodeManagerBuilder.cs

@@ -261,7 +261,7 @@ public IVariableBuilder<TValue> Variable<TValue>(NodeId nodeId)
         public IVariableBuilder<TValue> VariableFromTypeId<TValue>(NodeId typeDefinitionId)
         {
             ThrowIfSealed();
-            NodeState node = ResolveByTypeDefinition(typeDefinitionId, (QualifiedName)null);
+            NodeState node = ResolveByTypeDefinition(typeDefinitionId, (QualifiedName)null!);
             return ToVariableBuilder<TValue>(node, FormatNodeId(typeDefinitionId));
         }
 
@@ -304,7 +304,7 @@ internal VariableBuilder<TValue> ToVariableBuilder<TValue>(NodeState node, strin
         /// <c>null</c>; the <c>Publish</c> extensions surface a
         /// targeted error in that case.
         /// </remarks>
-        internal EventSourceRegistry EventSources { get; private set; }
+        internal EventSourceRegistry? EventSources { get; private set; }
 
         /// <summary>
         /// Wires the supplied registry into this builder so the

Libraries/Opc.Ua.Server/Fluent/VariableBuilder.cs

@@ -128,7 +128,7 @@ public IVariableBuilder<TValue> OnWrite(Action<ISystemContext, TValue> setter)
             ((INodeBuilder)this).OnWrite(
                 (ISystemContext context, NodeState _, ref Variant value) =>
                 {
-                    setter(context, FromVariant(value));
+                    setter(context, FromVariant(value)!);
                     return ServiceResult.Good;
                 });
             return this;
@@ -156,8 +156,8 @@ public IVariableBuilder<TValue> OnWrite(
             ((INodeBuilder)this).OnWrite(
                 async (ISystemContext context, NodeState _, Variant value, CancellationToken ct) =>
                 {
-                    TValue typed = FromVariant(value);
-                    await setter(context, typed, ct).ConfigureAwait(false);
+                    TValue? typed = FromVariant(value);
+                    await setter(context, typed!, ct).ConfigureAwait(false);
                     return new AttributeWriteResult(ServiceResult.Good);
                 });
             return this;
@@ -169,9 +169,9 @@ public IVariableBuilder<TValue> OnWrite(
         /// the variant is null so a typed lambda never has to defend
         /// against an empty variable.
         /// </summary>
-        private static TValue FromVariant(Variant value)
+        private static TValue? FromVariant(Variant value)
         {
-            object boxed = value.AsBoxedObject(Variant.BoxingBehavior.Legacy);
+            object? boxed = value.AsBoxedObject(Variant.BoxingBehavior.Legacy);
             if (boxed is null)
             {
                 return default;

Stack/Opc.Ua.Types/State/BaseVariableState.cs

@@ -575,15 +575,15 @@ public uint AccessLevelEx
         /// <see cref="ApplyIndexRangeAndDataEncoding"/> on the returned
         /// value.
         /// </summary>
-        public NodeValueEventHandlerAsync OnReadValueAsync;
+        public NodeValueEventHandlerAsync? OnReadValueAsync;
 
         /// <summary>
         /// Asynchronous sibling of <see cref="OnSimpleReadValue"/>. The
         /// returned value is post-processed by the framework
         /// (<see cref="ApplyIndexRangeAndDataEncoding"/> and copy policy)
         /// just like the synchronous path.
         /// </summary>
-        public NodeValueSimpleEventHandlerAsync OnSimpleReadValueAsync;
+        public NodeValueSimpleEventHandlerAsync? OnSimpleReadValueAsync;
 
         /// <summary>
         /// Asynchronous sibling of <see cref="OnWriteValue"/>. When set,
@@ -593,14 +593,14 @@ public uint AccessLevelEx
         /// success the framework updates the cached value, status code
         /// and timestamp.
         /// </summary>
-        public NodeValueWriteEventHandlerAsync OnWriteValueAsync;
+        public NodeValueWriteEventHandlerAsync? OnWriteValueAsync;
 
         /// <summary>
         /// Asynchronous sibling of <see cref="OnSimpleWriteValue"/>. Index
         /// range writes are not supported through this hook (just like
         /// the synchronous one).
         /// </summary>
-        public NodeValueSimpleWriteEventHandlerAsync OnSimpleWriteValueAsync;
+        public NodeValueSimpleWriteEventHandlerAsync? OnSimpleWriteValueAsync;
 
         /// <summary>
         /// Raised when the DataType attribute is read.
@@ -1795,8 +1795,8 @@ public override async ValueTask<ServiceResult> ReadAttributeAsync(
             DataValue value,
             CancellationToken cancellationToken = default)
         {
-            NodeValueEventHandlerAsync onReadValueAsync = OnReadValueAsync;
-            NodeValueSimpleEventHandlerAsync onSimpleReadValueAsync = OnSimpleReadValueAsync;
+            NodeValueEventHandlerAsync? onReadValueAsync = OnReadValueAsync;
+            NodeValueSimpleEventHandlerAsync? onSimpleReadValueAsync = OnSimpleReadValueAsync;
 
             if (attributeId != Attributes.Value ||
                 (onReadValueAsync == null && onSimpleReadValueAsync == null))
@@ -1879,7 +1879,7 @@ public override async ValueTask<ServiceResult> ReadAttributeAsync(
                     else
                     {
                         // simple async read — framework owns post-processing.
-                        AttributeSimpleReadResult simpleResult = await onSimpleReadValueAsync(
+                        AttributeSimpleReadResult simpleResult = await onSimpleReadValueAsync!(
                             context, this, cancellationToken)
                             .ConfigureAwait(false);
 
@@ -1939,7 +1939,7 @@ public override async ValueTask<ServiceResult> ReadAttributeAsync(
 
             value.WrappedValue = StatusCode.IsBad(value.StatusCode) ? Variant.Null : valueToRead;
 
-            return result;
+            return result!;
         }
 
         /// <summary>
@@ -1956,8 +1956,8 @@ public override async ValueTask<ServiceResult> WriteAttributeAsync(
             DataValue value,
             CancellationToken cancellationToken = default)
         {
-            NodeValueWriteEventHandlerAsync onWriteValueAsync = OnWriteValueAsync;
-            NodeValueSimpleWriteEventHandlerAsync onSimpleWriteValueAsync = OnSimpleWriteValueAsync;
+            NodeValueWriteEventHandlerAsync? onWriteValueAsync = OnWriteValueAsync;
+            NodeValueSimpleWriteEventHandlerAsync? onSimpleWriteValueAsync = OnSimpleWriteValueAsync;
 
             if (attributeId != Attributes.Value ||
                 (onWriteValueAsync == null && onSimpleWriteValueAsync == null))
@@ -2057,7 +2057,7 @@ public override async ValueTask<ServiceResult> WriteAttributeAsync(
                     valueToWrite = CoreUtils.Clone(valueToWrite);
                 }
 
-                AttributeWriteResult simpleResult = await onSimpleWriteValueAsync(
+                AttributeWriteResult simpleResult = await onSimpleWriteValueAsync!(
                     context, this, valueToWrite, cancellationToken)
                     .ConfigureAwait(false);