AB CIP PR 7 — ISubscribable via shared PollGroupEngine. AbCipDriver now implements ISubscribable — Subscribe delegates into the PollGroupEngine extracted in PR 1, Unsubscribe releases the subscription, ShutdownAsync disposes the engine cancelling every active subscription. OnDataChange event wired through the engine's on-change callback so external subscribers see the driver as sender. The engine's reader delegate points at the driver's ReadAsync (already handles lazy runtime init + caching via EnsureTagRuntimeAsync) — each poll tick batch-reads every subscribed tag in one IReadable call. 100ms interval floor inherited from PollGroupEngine.DefaultMinInterval matches Modbus convention. Initial-data push on first poll preserved via forceRaise=true. Exception-tolerant loop preserved — individual read failures show up as DataValueSnapshot with non-Good StatusCode via the status-code mapping PR 3 established. 7 new unit tests in AbCipSubscriptionTests covering initial-poll raising per tag, unchanged value raising only once, value change between polls triggering a new event, Unsubscribe halting the loop, 100ms floor keeping a 5ms request from generating extra events against a stable value, ShutdownAsync cancelling active subscriptions, UDT member subscription routing through the synthesised Motor1.Speed full-reference (proving PR 6's fan-out composes correctly with PR 7's subscription path). Total AbCip unit tests now 137/137 passing (+7 from PR 6's 130). Validates that the shared PollGroupEngine from PR 1 works correctly for a second driver, closing the original motivation for the extraction. Full solution builds 0 errors; Modbus + other drivers untouched.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ZB.MOM.WW.OtOpcUa.Driver.AbCip/AbCipDriver.cs

@@ -20,17 +20,20 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.AbCip;
 ///     from native-heap growth that the CLR allocator can't see; it tears down every
 ///     <see cref="PlcTagHandle"/> and reconnects each device.</para>
 /// </remarks>
-public sealed class AbCipDriver : IDriver, IReadable, IWritable, ITagDiscovery, IDisposable, IAsyncDisposable
+public sealed class AbCipDriver : IDriver, IReadable, IWritable, ITagDiscovery, ISubscribable, IDisposable, IAsyncDisposable
 {
     private readonly AbCipDriverOptions _options;
     private readonly string _driverInstanceId;
     private readonly IAbCipTagFactory _tagFactory;
     private readonly IAbCipTagEnumeratorFactory _enumeratorFactory;
     private readonly AbCipTemplateCache _templateCache = new();
+    private readonly PollGroupEngine _poll;
     private readonly Dictionary<string, DeviceState> _devices = new(StringComparer.OrdinalIgnoreCase);
     private readonly Dictionary<string, AbCipTagDefinition> _tagsByName = new(StringComparer.OrdinalIgnoreCase);
     private DriverHealth _health = new(DriverState.Unknown, null, null);
 
+    public event EventHandler<DataChangeEventArgs>? OnDataChange;
+
     public AbCipDriver(AbCipDriverOptions options, string driverInstanceId,
         IAbCipTagFactory? tagFactory = null,
         IAbCipTagEnumeratorFactory? enumeratorFactory = null)
@@ -40,6 +43,10 @@ public sealed class AbCipDriver : IDriver, IReadable, IWritable, ITagDiscovery,
         _driverInstanceId = driverInstanceId;
         _tagFactory = tagFactory ?? new LibplctagTagFactory();
         _enumeratorFactory = enumeratorFactory ?? new EmptyAbCipTagEnumeratorFactory();
+        _poll = new PollGroupEngine(
+            reader: ReadAsync,
+            onChange: (handle, tagRef, snapshot) =>
+                OnDataChange?.Invoke(this, new DataChangeEventArgs(handle, tagRef, snapshot)));
     }
 
     /// <summary>Shared UDT template cache. Exposed for PR 6 (UDT reader) + diagnostics.</summary>
@@ -61,7 +68,27 @@ public sealed class AbCipDriver : IDriver, IReadable, IWritable, ITagDiscovery,
                 var profile = AbCipPlcFamilyProfile.ForFamily(device.PlcFamily);
                 _devices[device.HostAddress] = new DeviceState(addr, device, profile);
             }
-            foreach (var tag in _options.Tags) _tagsByName[tag.Name] = tag;
+            foreach (var tag in _options.Tags)
+            {
+                // UDT tags with declared Members fan out into synthetic member-tag entries addressable
+                // by composed full-reference. Parent structure tag also stored so discovery can emit a
+                // folder for it.
+                _tagsByName[tag.Name] = tag;
+                if (tag.DataType == AbCipDataType.Structure && tag.Members is { Count: > 0 })
+                {
+                    foreach (var member in tag.Members)
+                    {
+                        var memberTag = new AbCipTagDefinition(
+                            Name: $"{tag.Name}.{member.Name}",
+                            DeviceHostAddress: tag.DeviceHostAddress,
+                            TagPath: $"{tag.TagPath}.{member.Name}",
+                            DataType: member.DataType,
+                            Writable: member.Writable,
+                            WriteIdempotent: member.WriteIdempotent);
+                        _tagsByName[memberTag.Name] = memberTag;
+                    }
+                }
+            }
             _health = new DriverHealth(DriverState.Healthy, DateTime.UtcNow, null);
         }
         catch (Exception ex)
@@ -78,13 +105,25 @@ public sealed class AbCipDriver : IDriver, IReadable, IWritable, ITagDiscovery,
         await InitializeAsync(driverConfigJson, cancellationToken).ConfigureAwait(false);
     }
 
-    public Task ShutdownAsync(CancellationToken cancellationToken)
+    public async Task ShutdownAsync(CancellationToken cancellationToken)
     {
+        await _poll.DisposeAsync().ConfigureAwait(false);
         foreach (var state in _devices.Values)
             state.DisposeHandles();
         _devices.Clear();
         _tagsByName.Clear();
         _health = new DriverHealth(DriverState.Unknown, _health.LastSuccessfulRead, null);
+    }
+
+    // ---- ISubscribable (polling overlay via shared engine) ----
+
+    public Task<ISubscriptionHandle> SubscribeAsync(
+        IReadOnlyList<string> fullReferences, TimeSpan publishingInterval, CancellationToken cancellationToken) =>
+        Task.FromResult(_poll.Subscribe(fullReferences, publishingInterval));
+
+    public Task UnsubscribeAsync(ISubscriptionHandle handle, CancellationToken cancellationToken)
+    {
+        _poll.Unsubscribe(handle);
         return Task.CompletedTask;
     }
 
@@ -304,12 +343,37 @@ public sealed class AbCipDriver : IDriver, IReadable, IWritable, ITagDiscovery,
             var deviceLabel = device.DeviceName ?? device.HostAddress;
             var deviceFolder = root.Folder(device.HostAddress, deviceLabel);
 
-            // Pre-declared tags — always emitted; the primary config path.
+            // Pre-declared tags — always emitted; the primary config path. UDT tags with declared
+            // Members fan out into a sub-folder + one Variable per member instead of a single
+            // Structure Variable (Structure has no useful scalar value + member-addressable paths
+            // are what downstream consumers actually want).
             var preDeclared = _options.Tags.Where(t =>
                 string.Equals(t.DeviceHostAddress, device.HostAddress, StringComparison.OrdinalIgnoreCase));
             foreach (var tag in preDeclared)
             {
                 if (AbCipSystemTagFilter.IsSystemTag(tag.Name)) continue;
+
+                if (tag.DataType == AbCipDataType.Structure && tag.Members is { Count: > 0 })
+                {
+                    var udtFolder = deviceFolder.Folder(tag.Name, tag.Name);
+                    foreach (var member in tag.Members)
+                    {
+                        var memberFullName = $"{tag.Name}.{member.Name}";
+                        udtFolder.Variable(member.Name, member.Name, new DriverAttributeInfo(
+                            FullName: memberFullName,
+                            DriverDataType: member.DataType.ToDriverDataType(),
+                            IsArray: false,
+                            ArrayDim: null,
+                            SecurityClass: member.Writable
+                                ? SecurityClassification.Operate
+                                : SecurityClassification.ViewOnly,
+                            IsHistorized: false,
+                            IsAlarm: false,
+                            WriteIdempotent: member.WriteIdempotent));
+                    }
+                    continue;
+                }
+
                 deviceFolder.Variable(tag.Name, tag.Name, ToAttributeInfo(tag));
             }
 

src/ZB.MOM.WW.OtOpcUa.Driver.AbCip/AbCipDriverOptions.cs

@@ -54,12 +54,30 @@ public sealed record AbCipDeviceOptions(
 /// <param name="DataType">Logix atomic type, or <see cref="AbCipDataType.Structure"/> for UDT-typed tags.</param>
 /// <param name="Writable">When <c>true</c> and the tag's ExternalAccess permits writes, IWritable routes writes here.</param>
 /// <param name="WriteIdempotent">Per plan decisions #44–#45, #143 — safe to replay on write timeout. Default <c>false</c>.</param>
+/// <param name="Members">For <see cref="AbCipDataType.Structure"/>-typed tags, the declared UDT
+///     member layout. When supplied, discovery fans out the UDT into a folder + one Variable per
+///     member (member TagPath = <c>{tag.TagPath}.{member.Name}</c>). When <c>null</c> on a Structure
+///     tag, the driver treats it as a black-box and relies on downstream configuration to address
+///     members individually via dotted <see cref="AbCipTagPath"/> syntax. Ignored for atomic types.</param>
 public sealed record AbCipTagDefinition(
     string Name,
     string DeviceHostAddress,
     string TagPath,
     AbCipDataType DataType,
     bool Writable = true,
+    bool WriteIdempotent = false,
+    IReadOnlyList<AbCipStructureMember>? Members = null);
+
+/// <summary>
+///     One declared member of a UDT tag. Name is the member identifier on the PLC (e.g. <c>Speed</c>,
+///     <c>Status</c>), DataType is the atomic Logix type, Writable/WriteIdempotent mirror
+///     <see cref="AbCipTagDefinition"/>. Declaration-driven — the real CIP Template Object reader
+///     (class 0x6C) that would auto-discover member layouts lands as a follow-up PR.
+/// </summary>
+public sealed record AbCipStructureMember(
+    string Name,
+    AbCipDataType DataType,
+    bool Writable = true,
     bool WriteIdempotent = false);
 
 /// <summary>Which AB PLC family the device is — selects the profile applied to connection params.</summary>

tests/ZB.MOM.WW.OtOpcUa.Driver.AbCip.Tests/AbCipSubscriptionTests.cs

@@ -0,0 +1,184 @@
+using System.Collections.Concurrent;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Driver.AbCip;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.AbCip.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class AbCipSubscriptionTests
+{
+    private static (AbCipDriver drv, FakeAbCipTagFactory factory) NewDriver(params AbCipTagDefinition[] tags)
+    {
+        var factory = new FakeAbCipTagFactory();
+        var drv = new AbCipDriver(new AbCipDriverOptions
+        {
+            Devices = [new AbCipDeviceOptions("ab://10.0.0.5/1,0")],
+            Tags = tags,
+        }, "drv-1", factory);
+        return (drv, factory);
+    }
+
+    [Fact]
+    public async Task Initial_poll_raises_OnDataChange_for_every_tag()
+    {
+        var (drv, factory) = NewDriver(
+            new AbCipTagDefinition("Speed", "ab://10.0.0.5/1,0", "Speed", AbCipDataType.DInt),
+            new AbCipTagDefinition("Temp", "ab://10.0.0.5/1,0", "Temp", AbCipDataType.Real));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        factory.Customise = p => p.TagName switch
+        {
+            "Speed" => new FakeAbCipTag(p) { Value = 1800 },
+            "Temp" => new FakeAbCipTag(p) { Value = 72.5f },
+            _ => new FakeAbCipTag(p),
+        };
+
+        var events = new ConcurrentQueue<DataChangeEventArgs>();
+        drv.OnDataChange += (_, e) => events.Enqueue(e);
+
+        var handle = await drv.SubscribeAsync(["Speed", "Temp"], TimeSpan.FromMilliseconds(200), CancellationToken.None);
+        await WaitForAsync(() => events.Count >= 2, TimeSpan.FromSeconds(2));
+
+        events.Select(e => e.FullReference).ShouldContain("Speed");
+        events.Select(e => e.FullReference).ShouldContain("Temp");
+        await drv.UnsubscribeAsync(handle, CancellationToken.None);
+    }
+
+    [Fact]
+    public async Task Unchanged_value_raises_only_once()
+    {
+        var (drv, factory) = NewDriver(
+            new AbCipTagDefinition("Speed", "ab://10.0.0.5/1,0", "Speed", AbCipDataType.DInt));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        factory.Customise = p => new FakeAbCipTag(p) { Value = 1800 };
+
+        var events = new ConcurrentQueue<DataChangeEventArgs>();
+        drv.OnDataChange += (_, e) => events.Enqueue(e);
+
+        var handle = await drv.SubscribeAsync(["Speed"], TimeSpan.FromMilliseconds(100), CancellationToken.None);
+        await Task.Delay(500);
+        await drv.UnsubscribeAsync(handle, CancellationToken.None);
+
+        events.Count.ShouldBe(1);
+    }
+
+    [Fact]
+    public async Task Value_change_between_polls_raises_OnDataChange()
+    {
+        var (drv, factory) = NewDriver(
+            new AbCipTagDefinition("Speed", "ab://10.0.0.5/1,0", "Speed", AbCipDataType.DInt));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        var tagRef = new FakeAbCipTag(new AbCipTagCreateParams("10.0.0.5", 44818, "1,0", "controllogix", "Speed", TimeSpan.FromSeconds(2))) { Value = 100 };
+        factory.Customise = _ => tagRef;
+
+        var events = new ConcurrentQueue<DataChangeEventArgs>();
+        drv.OnDataChange += (_, e) => events.Enqueue(e);
+
+        var handle = await drv.SubscribeAsync(["Speed"], TimeSpan.FromMilliseconds(100), CancellationToken.None);
+        await WaitForAsync(() => events.Count >= 1, TimeSpan.FromSeconds(1));
+        tagRef.Value = 200; // simulate PLC change
+        await WaitForAsync(() => events.Count >= 2, TimeSpan.FromSeconds(2));
+
+        await drv.UnsubscribeAsync(handle, CancellationToken.None);
+        events.Count.ShouldBeGreaterThanOrEqualTo(2);
+        events.Last().Snapshot.Value.ShouldBe(200);
+    }
+
+    [Fact]
+    public async Task Unsubscribe_halts_polling()
+    {
+        var (drv, factory) = NewDriver(
+            new AbCipTagDefinition("Speed", "ab://10.0.0.5/1,0", "Speed", AbCipDataType.DInt));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        var tagRef = new FakeAbCipTag(new AbCipTagCreateParams("10.0.0.5", 44818, "1,0", "controllogix", "Speed", TimeSpan.FromSeconds(2))) { Value = 1 };
+        factory.Customise = _ => tagRef;
+
+        var events = new ConcurrentQueue<DataChangeEventArgs>();
+        drv.OnDataChange += (_, e) => events.Enqueue(e);
+
+        var handle = await drv.SubscribeAsync(["Speed"], TimeSpan.FromMilliseconds(100), CancellationToken.None);
+        await WaitForAsync(() => events.Count >= 1, TimeSpan.FromSeconds(1));
+        await drv.UnsubscribeAsync(handle, CancellationToken.None);
+
+        var afterUnsub = events.Count;
+        tagRef.Value = 999;
+        await Task.Delay(400);
+        events.Count.ShouldBe(afterUnsub);
+    }
+
+    [Fact]
+    public async Task Interval_below_100ms_is_floored()
+    {
+        var (drv, factory) = NewDriver(
+            new AbCipTagDefinition("Speed", "ab://10.0.0.5/1,0", "Speed", AbCipDataType.DInt));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        factory.Customise = p => new FakeAbCipTag(p) { Value = 1 };
+
+        var events = new ConcurrentQueue<DataChangeEventArgs>();
+        drv.OnDataChange += (_, e) => events.Enqueue(e);
+
+        var handle = await drv.SubscribeAsync(["Speed"], TimeSpan.FromMilliseconds(5), CancellationToken.None);
+        await Task.Delay(300);
+        await drv.UnsubscribeAsync(handle, CancellationToken.None);
+
+        // Value is stable → only the initial-data push fires; the 100 ms floor keeps polls sparse enough
+        // that no extra event is produced against a stable value.
+        events.Count.ShouldBe(1);
+    }
+
+    [Fact]
+    public async Task ShutdownAsync_cancels_active_subscriptions()
+    {
+        var (drv, factory) = NewDriver(
+            new AbCipTagDefinition("Speed", "ab://10.0.0.5/1,0", "Speed", AbCipDataType.DInt));
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        factory.Customise = p => new FakeAbCipTag(p) { Value = 1 };
+
+        var events = new ConcurrentQueue<DataChangeEventArgs>();
+        drv.OnDataChange += (_, e) => events.Enqueue(e);
+
+        _ = await drv.SubscribeAsync(["Speed"], TimeSpan.FromMilliseconds(100), CancellationToken.None);
+        await WaitForAsync(() => events.Count >= 1, TimeSpan.FromSeconds(1));
+        await drv.ShutdownAsync(CancellationToken.None);
+
+        var afterShutdown = events.Count;
+        await Task.Delay(300);
+        events.Count.ShouldBe(afterShutdown);
+    }
+
+    [Fact]
+    public async Task Subscription_on_UDT_member_uses_synthesised_full_reference()
+    {
+        var factory = new FakeAbCipTagFactory();
+        var drv = new AbCipDriver(new AbCipDriverOptions
+        {
+            Devices = [new AbCipDeviceOptions("ab://10.0.0.5/1,0")],
+            Tags =
+            [
+                new AbCipTagDefinition("Motor1", "ab://10.0.0.5/1,0", "Motor1", AbCipDataType.Structure,
+                    Members: [new AbCipStructureMember("Speed", AbCipDataType.DInt)]),
+            ],
+        }, "drv-1", factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        factory.Customise = p => p.TagName == "Motor1.Speed"
+            ? new FakeAbCipTag(p) { Value = 77 }
+            : new FakeAbCipTag(p);
+
+        var events = new ConcurrentQueue<DataChangeEventArgs>();
+        drv.OnDataChange += (_, e) => events.Enqueue(e);
+
+        var handle = await drv.SubscribeAsync(["Motor1.Speed"], TimeSpan.FromMilliseconds(100), CancellationToken.None);
+        await WaitForAsync(() => events.Count >= 1, TimeSpan.FromSeconds(2));
+
+        events.First().Snapshot.Value.ShouldBe(77);
+        await drv.UnsubscribeAsync(handle, CancellationToken.None);
+    }
+
+    private static async Task WaitForAsync(Func<bool> condition, TimeSpan timeout)
+    {
+        var deadline = DateTime.UtcNow + timeout;
+        while (!condition() && DateTime.UtcNow < deadline)
+            await Task.Delay(20);
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.AbCip.Tests/AbCipUdtMemberTests.cs

@@ -0,0 +1,217 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Driver.AbCip;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.AbCip.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class AbCipUdtMemberTests
+{
+    [Fact]
+    public async Task UDT_with_declared_members_fans_out_to_member_variables()
+    {
+        var builder = new RecordingBuilder();
+        var drv = new AbCipDriver(new AbCipDriverOptions
+        {
+            Devices = [new AbCipDeviceOptions("ab://10.0.0.5/1,0")],
+            Tags =
+            [
+                new AbCipTagDefinition(
+                    Name: "Motor1",
+                    DeviceHostAddress: "ab://10.0.0.5/1,0",
+                    TagPath: "Motor1",
+                    DataType: AbCipDataType.Structure,
+                    Members:
+                    [
+                        new AbCipStructureMember("Speed", AbCipDataType.DInt),
+                        new AbCipStructureMember("Running", AbCipDataType.Bool, Writable: false),
+                        new AbCipStructureMember("SetPoint", AbCipDataType.Real, WriteIdempotent: true),
+                    ]),
+            ],
+        }, "drv-1");
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        await drv.DiscoverAsync(builder, CancellationToken.None);
+
+        builder.Folders.ShouldContain(f => f.BrowseName == "Motor1");
+        var variables = builder.Variables.Select(v => (v.BrowseName, v.Info.FullName)).ToList();
+        variables.ShouldContain(("Speed", "Motor1.Speed"));
+        variables.ShouldContain(("Running", "Motor1.Running"));
+        variables.ShouldContain(("SetPoint", "Motor1.SetPoint"));
+
+        builder.Variables.Single(v => v.BrowseName == "Running").Info.SecurityClass
+            .ShouldBe(SecurityClassification.ViewOnly);
+        builder.Variables.Single(v => v.BrowseName == "SetPoint").Info.WriteIdempotent
+            .ShouldBeTrue();
+    }
+
+    [Fact]
+    public async Task UDT_members_resolvable_for_read_via_synthesised_full_reference()
+    {
+        var factory = new FakeAbCipTagFactory
+        {
+            Customise = p => p.TagName switch
+            {
+                "Motor1.Speed" => new FakeAbCipTag(p) { Value = 1800 },
+                "Motor1.Running" => new FakeAbCipTag(p) { Value = true },
+                _ => new FakeAbCipTag(p),
+            },
+        };
+        var drv = new AbCipDriver(new AbCipDriverOptions
+        {
+            Devices = [new AbCipDeviceOptions("ab://10.0.0.5/1,0")],
+            Tags =
+            [
+                new AbCipTagDefinition("Motor1", "ab://10.0.0.5/1,0", "Motor1", AbCipDataType.Structure,
+                    Members:
+                    [
+                        new AbCipStructureMember("Speed", AbCipDataType.DInt),
+                        new AbCipStructureMember("Running", AbCipDataType.Bool),
+                    ]),
+            ],
+        }, "drv-1", factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        var snapshots = await drv.ReadAsync(["Motor1.Speed", "Motor1.Running"], CancellationToken.None);
+
+        snapshots[0].Value.ShouldBe(1800);
+        snapshots[0].StatusCode.ShouldBe(AbCipStatusMapper.Good);
+        snapshots[1].Value.ShouldBe(true);
+        snapshots[1].StatusCode.ShouldBe(AbCipStatusMapper.Good);
+    }
+
+    [Fact]
+    public async Task UDT_member_write_routes_through_synthesised_tagpath()
+    {
+        var factory = new FakeAbCipTagFactory();
+        var drv = new AbCipDriver(new AbCipDriverOptions
+        {
+            Devices = [new AbCipDeviceOptions("ab://10.0.0.5/1,0")],
+            Tags =
+            [
+                new AbCipTagDefinition("Motor1", "ab://10.0.0.5/1,0", "Motor1", AbCipDataType.Structure,
+                    Members:
+                    [
+                        new AbCipStructureMember("SetPoint", AbCipDataType.Real),
+                    ]),
+            ],
+        }, "drv-1", factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        var results = await drv.WriteAsync(
+            [new WriteRequest("Motor1.SetPoint", 42.5f)], CancellationToken.None);
+
+        results.Single().StatusCode.ShouldBe(AbCipStatusMapper.Good);
+        factory.Tags["Motor1.SetPoint"].Value.ShouldBe(42.5f);
+    }
+
+    [Fact]
+    public async Task UDT_member_read_write_honours_member_Writable_flag()
+    {
+        var factory = new FakeAbCipTagFactory();
+        var drv = new AbCipDriver(new AbCipDriverOptions
+        {
+            Devices = [new AbCipDeviceOptions("ab://10.0.0.5/1,0")],
+            Tags =
+            [
+                new AbCipTagDefinition("Motor1", "ab://10.0.0.5/1,0", "Motor1", AbCipDataType.Structure,
+                    Members:
+                    [
+                        new AbCipStructureMember("Status", AbCipDataType.DInt, Writable: false),
+                    ]),
+            ],
+        }, "drv-1", factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        var results = await drv.WriteAsync(
+            [new WriteRequest("Motor1.Status", 1)], CancellationToken.None);
+
+        results.Single().StatusCode.ShouldBe(AbCipStatusMapper.BadNotWritable);
+    }
+
+    [Fact]
+    public async Task Structure_tag_without_members_is_emitted_as_single_variable()
+    {
+        // Fallback path: a Structure tag with no declared Members still appears as a Variable so
+        // downstream configuration can address it manually. This matches the "black box" note in
+        // AbCipTagDefinition's docstring.
+        var builder = new RecordingBuilder();
+        var drv = new AbCipDriver(new AbCipDriverOptions
+        {
+            Devices = [new AbCipDeviceOptions("ab://10.0.0.5/1,0")],
+            Tags = [new AbCipTagDefinition("OpaqueUdt", "ab://10.0.0.5/1,0", "OpaqueUdt", AbCipDataType.Structure)],
+        }, "drv-1");
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        await drv.DiscoverAsync(builder, CancellationToken.None);
+
+        builder.Variables.ShouldContain(v => v.BrowseName == "OpaqueUdt");
+        builder.Folders.ShouldNotContain(f => f.BrowseName == "OpaqueUdt");
+    }
+
+    [Fact]
+    public async Task Empty_Members_list_is_treated_like_null()
+    {
+        var builder = new RecordingBuilder();
+        var drv = new AbCipDriver(new AbCipDriverOptions
+        {
+            Devices = [new AbCipDeviceOptions("ab://10.0.0.5/1,0")],
+            Tags = [new AbCipTagDefinition("EmptyUdt", "ab://10.0.0.5/1,0", "E", AbCipDataType.Structure, Members: [])],
+        }, "drv-1");
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        await drv.DiscoverAsync(builder, CancellationToken.None);
+
+        builder.Folders.ShouldNotContain(f => f.BrowseName == "EmptyUdt");
+        builder.Variables.ShouldContain(v => v.BrowseName == "EmptyUdt");
+    }
+
+    [Fact]
+    public async Task UDT_members_mixed_with_flat_tags_coexist()
+    {
+        var builder = new RecordingBuilder();
+        var drv = new AbCipDriver(new AbCipDriverOptions
+        {
+            Devices = [new AbCipDeviceOptions("ab://10.0.0.5/1,0")],
+            Tags =
+            [
+                new AbCipTagDefinition("FlatA", "ab://10.0.0.5/1,0", "A", AbCipDataType.DInt),
+                new AbCipTagDefinition("Motor1", "ab://10.0.0.5/1,0", "Motor1", AbCipDataType.Structure,
+                    Members:
+                    [
+                        new AbCipStructureMember("Speed", AbCipDataType.DInt),
+                    ]),
+                new AbCipTagDefinition("FlatB", "ab://10.0.0.5/1,0", "B", AbCipDataType.Real),
+            ],
+        }, "drv-1");
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        await drv.DiscoverAsync(builder, CancellationToken.None);
+
+        builder.Variables.Select(v => v.BrowseName).ShouldBe(["FlatA", "Speed", "FlatB"], ignoreOrder: true);
+    }
+
+    // ---- helpers ----
+
+    private sealed class RecordingBuilder : IAddressSpaceBuilder
+    {
+        public List<(string BrowseName, string DisplayName)> Folders { get; } = new();
+        public List<(string BrowseName, DriverAttributeInfo Info)> Variables { get; } = new();
+
+        public IAddressSpaceBuilder Folder(string browseName, string displayName)
+        { Folders.Add((browseName, displayName)); return this; }
+
+        public IVariableHandle Variable(string browseName, string displayName, DriverAttributeInfo info)
+        { Variables.Add((browseName, info)); return new Handle(info.FullName); }
+
+        public void AddProperty(string _, DriverDataType __, object? ___) { }
+
+        private sealed class Handle(string fullRef) : IVariableHandle
+        {
+            public string FullReference => fullRef;
+            public IAlarmConditionSink MarkAsAlarmCondition(AlarmConditionInfo info) => new NullSink();
+        }
+        private sealed class NullSink : IAlarmConditionSink { public void OnTransition(AlarmEventArgs args) { } }
+    }
+}