Implement MXAccess write commands in the worker

The .proto contract and MxCommandKind already defined Write, Write2,
WriteSecured, and WriteSecured2, but the worker's MxAccessCommandExecutor
had no case for any of them — every write kind fell through to
CreateInvalidRequestReply ("Unsupported MXAccess command kind Write").

Implement all four:

- VariantConverter.ConvertToComValue projects an MxValue into a
  COM-marshalable object (scalars, arrays, null) — the inverse of the
  existing COM-to-MxValue projection.
- IMxAccessServer / MxAccessComServer gain Write/Write2/WriteSecured/
  WriteSecured2, routed to ILMXProxyServer / ILMXProxyServer4.
- MxAccessSession and MxAccessCommandExecutor add the four write paths,
  following the existing ExecuteAdvise pattern; the reply is a plain OK
  reply and the outcome surfaces later as an OnWriteComplete event.

Verified live: a Write now returns PROTOCOL_STATUS_CODE_OK and produces
an OnWriteComplete event where it previously returned InvalidRequest.

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

src/MxGateway.Worker.Tests/Conversion/VariantConverterTests.cs

@@ -46,6 +46,78 @@ public sealed class VariantConverterTests
         Assert.Equal("VT_DATE", converted.VariantType);
     }
 
+    /// <summary>Verifies that scalar MxValue kinds convert to the matching boxed CLR type for a COM write.</summary>
+    [Fact]
+    public void ConvertToComValue_WithInt32_ReturnsBoxedInt()
+    {
+        object? result = _converter.ConvertToComValue(new MxValue { Int32Value = 123 });
+
+        Assert.Equal(123, Assert.IsType<int>(result));
+    }
+
+    /// <summary>Verifies that a boolean MxValue converts to a boxed bool for a COM write.</summary>
+    [Fact]
+    public void ConvertToComValue_WithBool_ReturnsBoxedBool()
+    {
+        object? result = _converter.ConvertToComValue(new MxValue { BoolValue = true });
+
+        Assert.True(Assert.IsType<bool>(result));
+    }
+
+    /// <summary>Verifies that a string MxValue converts to a string for a COM write.</summary>
+    [Fact]
+    public void ConvertToComValue_WithString_ReturnsString()
+    {
+        object? result = _converter.ConvertToComValue(new MxValue { StringValue = "abc" });
+
+        Assert.Equal("abc", Assert.IsType<string>(result));
+    }
+
+    /// <summary>Verifies that a timestamp MxValue converts to a UTC DateTime the COM marshaler renders as VT_DATE.</summary>
+    [Fact]
+    public void ConvertToComValue_WithTimestamp_ReturnsUtcDateTime()
+    {
+        DateTime dateTime = new(2026, 5, 19, 12, 0, 0, DateTimeKind.Utc);
+
+        object? result = _converter.ConvertToComValue(
+            new MxValue { TimestampValue = ProtobufTimestamp.FromDateTime(dateTime) });
+
+        Assert.Equal(dateTime, Assert.IsType<DateTime>(result));
+    }
+
+    /// <summary>Verifies that an MXAccess-null MxValue converts to a CLR null.</summary>
+    [Fact]
+    public void ConvertToComValue_WithNull_ReturnsNull()
+    {
+        object? result = _converter.ConvertToComValue(new MxValue { IsNull = true });
+
+        Assert.Null(result);
+    }
+
+    /// <summary>Verifies that an integer-array MxValue converts to an int array the COM marshaler renders as a SAFEARRAY.</summary>
+    [Fact]
+    public void ConvertToComValue_WithInt32Array_ReturnsInt32Array()
+    {
+        MxValue value = new()
+        {
+            ArrayValue = new MxArray
+            {
+                Int32Values = new Int32Array { Values = { 1, 2, 3 } },
+            },
+        };
+
+        object? result = _converter.ConvertToComValue(value);
+
+        Assert.Equal(new[] { 1, 2, 3 }, Assert.IsType<int[]>(result));
+    }
+
+    /// <summary>Verifies that an MxValue with no value kind set cannot be converted for a COM write.</summary>
+    [Fact]
+    public void ConvertToComValue_WithNoKind_Throws()
+    {
+        Assert.Throws<ArgumentException>(() => _converter.ConvertToComValue(new MxValue()));
+    }
+
     /// <summary>Verifies that file time values with expected time data type are converted to protobuf timestamps.</summary>
     [Fact]
     public void Convert_WithFileTimeAndExpectedTime_ProjectsTimestamp()

src/MxGateway.Worker.Tests/MxAccess/AlarmCommandExecutorTests.cs

@@ -379,10 +379,10 @@ public sealed class AlarmCommandExecutorTests
         public void SetBufferedUpdateInterval(int serverHandle, int updateIntervalMilliseconds) { }
         public void Suspend(int serverHandle, int itemHandle) { }
         public void Activate(int serverHandle, int itemHandle) { }
-        public void Write(int serverHandle, int itemHandle, object value, int userId) { }
+        public void Write(int serverHandle, int itemHandle, object? value, int userId) { }
-        public void Write2(int serverHandle, int itemHandle, object value, object timestampValue, int userId) { }
+        public void Write2(int serverHandle, int itemHandle, object? value, object? timestampValue, int userId) { }
-        public void WriteSecured(int serverHandle, int itemHandle, int currentUserId, int verifierUserId, object value) { }
+        public void WriteSecured(int serverHandle, int itemHandle, int currentUserId, int verifierUserId, object? value) { }
-        public void WriteSecured2(int serverHandle, int itemHandle, int currentUserId, int verifierUserId, object value, object timestampValue) { }
+        public void WriteSecured2(int serverHandle, int itemHandle, int currentUserId, int verifierUserId, object? value, object? timestampValue) { }
         public int AuthenticateUser(string userName, string password) => 0;
         public int ArchestrAUserToId(string userName) => 0;
     }

src/MxGateway.Worker.Tests/MxAccess/MxAccessComServerTests.cs

@@ -134,5 +134,26 @@ public sealed class MxAccessComServerTests
         {
             calls.Add($"AdviseSupervisory:{serverHandle}:{itemHandle}");
         }
+
+        public void Write(int serverHandle, int itemHandle, object? value, int userId)
+        {
+            calls.Add($"Write:{serverHandle}:{itemHandle}:{value}:{userId}");
+        }
+
+        public void Write2(int serverHandle, int itemHandle, object? value, object? timestamp, int userId)
+        {
+            calls.Add($"Write2:{serverHandle}:{itemHandle}:{value}:{timestamp}:{userId}");
+        }
+
+        public void WriteSecured(int serverHandle, int itemHandle, int currentUserId, int verifierUserId, object? value)
+        {
+            calls.Add($"WriteSecured:{serverHandle}:{itemHandle}:{currentUserId}:{verifierUserId}:{value}");
+        }
+
+        public void WriteSecured2(
+            int serverHandle, int itemHandle, int currentUserId, int verifierUserId, object? value, object? timestamp)
+        {
+            calls.Add($"WriteSecured2:{serverHandle}:{itemHandle}:{currentUserId}:{verifierUserId}:{value}:{timestamp}");
+        }
     }
 }

src/MxGateway.Worker.Tests/MxAccess/MxAccessCommandExecutorTests.cs

@@ -3,6 +3,7 @@ using System.Collections.Generic;
 using System.Linq;
 using System.Runtime.InteropServices;
 using System.Threading.Tasks;
+using Google.Protobuf.WellKnownTypes;
 using MxGateway.Contracts.Proto;
 using MxGateway.Worker.MxAccess;
 using MxGateway.Worker.Sta;
@@ -617,6 +618,143 @@ public sealed class MxAccessCommandExecutorTests
         Assert.Null(factory.FakeComObject.AdviseServerHandle);
     }
 
+    /// <summary>Verifies that Write dispatches the converted value to MXAccess on the STA thread.</summary>
+    [Fact]
+    public async Task DispatchAsync_Write_CallsMxAccessOnStaWithConvertedValue()
+    {
+        FakeMxAccessComObject fakeComObject = new(registerHandle: 70);
+        FakeMxAccessComObjectFactory factory = new(fakeComObject);
+        using StaRuntime runtime = CreateRuntime();
+        using MxAccessStaSession session = new(runtime, factory, new NoopEventSink());
+        await session.StartAsync(workerProcessId: 1234);
+
+        MxCommandReply reply = await session.DispatchAsync(CreateWriteCommand(
+            "write", serverHandle: 70, itemHandle: 700, value: 123, userId: 5));
+
+        Assert.Equal(ProtocolStatusCode.Ok, reply.ProtocolStatus.Code);
+        Assert.Equal(MxCommandKind.Write, reply.Kind);
+        Assert.Equal(70, fakeComObject.WriteServerHandle);
+        Assert.Equal(700, fakeComObject.WriteItemHandle);
+        Assert.Equal(123, fakeComObject.WriteValue);
+        Assert.Equal(5, fakeComObject.WriteUserId);
+        Assert.Equal(runtime.StaThreadId, fakeComObject.WriteThreadId);
+    }
+
+    /// <summary>Verifies that Write2 forwards the converted value and timestamp to MXAccess.</summary>
+    [Fact]
+    public async Task DispatchAsync_Write2_ForwardsValueAndTimestamp()
+    {
+        FakeMxAccessComObject fakeComObject = new(registerHandle: 71);
+        FakeMxAccessComObjectFactory factory = new(fakeComObject);
+        using StaRuntime runtime = CreateRuntime();
+        using MxAccessStaSession session = new(runtime, factory, new NoopEventSink());
+        await session.StartAsync(workerProcessId: 1234);
+        DateTime timestamp = new(2026, 5, 19, 12, 0, 0, DateTimeKind.Utc);
+
+        MxCommandReply reply = await session.DispatchAsync(CreateWrite2Command(
+            "write2", serverHandle: 71, itemHandle: 710, value: 456, timestamp: timestamp, userId: 6));
+
+        Assert.Equal(ProtocolStatusCode.Ok, reply.ProtocolStatus.Code);
+        Assert.Equal(MxCommandKind.Write2, reply.Kind);
+        Assert.Equal(710, fakeComObject.WriteItemHandle);
+        Assert.Equal(456, fakeComObject.WriteValue);
+        Assert.Equal(timestamp, fakeComObject.WriteTimestamp);
+        Assert.Equal(6, fakeComObject.WriteUserId);
+    }
+
+    /// <summary>Verifies that WriteSecured forwards the operator and verifier user ids to MXAccess.</summary>
+    [Fact]
+    public async Task DispatchAsync_WriteSecured_ForwardsUserIds()
+    {
+        FakeMxAccessComObject fakeComObject = new(registerHandle: 72);
+        FakeMxAccessComObjectFactory factory = new(fakeComObject);
+        using StaRuntime runtime = CreateRuntime();
+        using MxAccessStaSession session = new(runtime, factory, new NoopEventSink());
+        await session.StartAsync(workerProcessId: 1234);
+
+        MxCommandReply reply = await session.DispatchAsync(CreateWriteSecuredCommand(
+            "write-secured", serverHandle: 72, itemHandle: 720, value: 789, currentUserId: 11, verifierUserId: 22));
+
+        Assert.Equal(ProtocolStatusCode.Ok, reply.ProtocolStatus.Code);
+        Assert.Equal(MxCommandKind.WriteSecured, reply.Kind);
+        Assert.Equal(720, fakeComObject.WriteItemHandle);
+        Assert.Equal(789, fakeComObject.WriteValue);
+        Assert.Equal(11, fakeComObject.WriteCurrentUserId);
+        Assert.Equal(22, fakeComObject.WriteVerifierUserId);
+    }
+
+    /// <summary>Verifies that WriteSecured2 forwards user ids, value, and timestamp to MXAccess.</summary>
+    [Fact]
+    public async Task DispatchAsync_WriteSecured2_ForwardsUserIdsValueAndTimestamp()
+    {
+        FakeMxAccessComObject fakeComObject = new(registerHandle: 73);
+        FakeMxAccessComObjectFactory factory = new(fakeComObject);
+        using StaRuntime runtime = CreateRuntime();
+        using MxAccessStaSession session = new(runtime, factory, new NoopEventSink());
+        await session.StartAsync(workerProcessId: 1234);
+        DateTime timestamp = new(2026, 5, 19, 13, 30, 0, DateTimeKind.Utc);
+
+        MxCommandReply reply = await session.DispatchAsync(CreateWriteSecured2Command(
+            "write-secured2", serverHandle: 73, itemHandle: 730, value: 1011,
+            timestamp: timestamp, currentUserId: 33, verifierUserId: 44));
+
+        Assert.Equal(ProtocolStatusCode.Ok, reply.ProtocolStatus.Code);
+        Assert.Equal(MxCommandKind.WriteSecured2, reply.Kind);
+        Assert.Equal(1011, fakeComObject.WriteValue);
+        Assert.Equal(timestamp, fakeComObject.WriteTimestamp);
+        Assert.Equal(33, fakeComObject.WriteCurrentUserId);
+        Assert.Equal(44, fakeComObject.WriteVerifierUserId);
+    }
+
+    /// <summary>Verifies that Write without a payload returns an invalid request error.</summary>
+    [Fact]
+    public async Task DispatchAsync_WriteWithoutPayload_ReturnsInvalidRequest()
+    {
+        FakeMxAccessComObject fakeComObject = new(registerHandle: 74);
+        FakeMxAccessComObjectFactory factory = new(fakeComObject);
+        using StaRuntime runtime = CreateRuntime();
+        using MxAccessStaSession session = new(runtime, factory, new NoopEventSink());
+        await session.StartAsync(workerProcessId: 1234);
+
+        MxCommandReply reply = await session.DispatchAsync(new StaCommand(
+            "session-1",
+            "missing-write-payload",
+            new MxCommand
+            {
+                Kind = MxCommandKind.Write,
+            }));
+
+        Assert.Equal(ProtocolStatusCode.InvalidRequest, reply.ProtocolStatus.Code);
+        Assert.Null(fakeComObject.WriteServerHandle);
+    }
+
+    /// <summary>Verifies that Write without a value returns an invalid request error.</summary>
+    [Fact]
+    public async Task DispatchAsync_WriteWithoutValue_ReturnsInvalidRequest()
+    {
+        FakeMxAccessComObject fakeComObject = new(registerHandle: 75);
+        FakeMxAccessComObjectFactory factory = new(fakeComObject);
+        using StaRuntime runtime = CreateRuntime();
+        using MxAccessStaSession session = new(runtime, factory, new NoopEventSink());
+        await session.StartAsync(workerProcessId: 1234);
+
+        MxCommandReply reply = await session.DispatchAsync(new StaCommand(
+            "session-1",
+            "missing-write-value",
+            new MxCommand
+            {
+                Kind = MxCommandKind.Write,
+                Write = new WriteCommand
+                {
+                    ServerHandle = 75,
+                    ItemHandle = 750,
+                },
+            }));
+
+        Assert.Equal(ProtocolStatusCode.InvalidRequest, reply.ProtocolStatus.Code);
+        Assert.Null(fakeComObject.WriteServerHandle);
+    }
+
     private static StaCommand CreateRegisterCommand(
         string correlationId,
         string clientName)
@@ -729,6 +867,126 @@ public sealed class MxAccessCommandExecutorTests
             });
     }
 
+    private static MxValue CreateIntegerValue(int value)
+    {
+        return new MxValue
+        {
+            DataType = MxDataType.Integer,
+            VariantType = "VT_I4",
+            Int32Value = value,
+        };
+    }
+
+    private static MxValue CreateTimestampValue(DateTime timestamp)
+    {
+        return new MxValue
+        {
+            DataType = MxDataType.Time,
+            VariantType = "VT_DATE",
+            TimestampValue = Timestamp.FromDateTime(timestamp),
+        };
+    }
+
+    private static StaCommand CreateWriteCommand(
+        string correlationId,
+        int serverHandle,
+        int itemHandle,
+        int value,
+        int userId)
+    {
+        return new StaCommand(
+            "session-1",
+            correlationId,
+            new MxCommand
+            {
+                Kind = MxCommandKind.Write,
+                Write = new WriteCommand
+                {
+                    ServerHandle = serverHandle,
+                    ItemHandle = itemHandle,
+                    Value = CreateIntegerValue(value),
+                    UserId = userId,
+                },
+            });
+    }
+
+    private static StaCommand CreateWrite2Command(
+        string correlationId,
+        int serverHandle,
+        int itemHandle,
+        int value,
+        DateTime timestamp,
+        int userId)
+    {
+        return new StaCommand(
+            "session-1",
+            correlationId,
+            new MxCommand
+            {
+                Kind = MxCommandKind.Write2,
+                Write2 = new Write2Command
+                {
+                    ServerHandle = serverHandle,
+                    ItemHandle = itemHandle,
+                    Value = CreateIntegerValue(value),
+                    TimestampValue = CreateTimestampValue(timestamp),
+                    UserId = userId,
+                },
+            });
+    }
+
+    private static StaCommand CreateWriteSecuredCommand(
+        string correlationId,
+        int serverHandle,
+        int itemHandle,
+        int value,
+        int currentUserId,
+        int verifierUserId)
+    {
+        return new StaCommand(
+            "session-1",
+            correlationId,
+            new MxCommand
+            {
+                Kind = MxCommandKind.WriteSecured,
+                WriteSecured = new WriteSecuredCommand
+                {
+                    ServerHandle = serverHandle,
+                    ItemHandle = itemHandle,
+                    CurrentUserId = currentUserId,
+                    VerifierUserId = verifierUserId,
+                    Value = CreateIntegerValue(value),
+                },
+            });
+    }
+
+    private static StaCommand CreateWriteSecured2Command(
+        string correlationId,
+        int serverHandle,
+        int itemHandle,
+        int value,
+        DateTime timestamp,
+        int currentUserId,
+        int verifierUserId)
+    {
+        return new StaCommand(
+            "session-1",
+            correlationId,
+            new MxCommand
+            {
+                Kind = MxCommandKind.WriteSecured2,
+                WriteSecured2 = new WriteSecured2Command
+                {
+                    ServerHandle = serverHandle,
+                    ItemHandle = itemHandle,
+                    CurrentUserId = currentUserId,
+                    VerifierUserId = verifierUserId,
+                    Value = CreateIntegerValue(value),
+                    TimestampValue = CreateTimestampValue(timestamp),
+                },
+            });
+    }
+
     private static StaCommand CreateUnAdviseCommand(
         string correlationId,
         int serverHandle,
@@ -1080,6 +1338,118 @@ public sealed class MxAccessCommandExecutorTests
                 throw adviseSupervisoryException;
             }
         }
+
+        /// <summary>Gets the server handle passed to the most recent write, if called.</summary>
+        public int? WriteServerHandle { get; private set; }
+
+        /// <summary>Gets the item handle passed to the most recent write, if called.</summary>
+        public int? WriteItemHandle { get; private set; }
+
+        /// <summary>Gets the value passed to the most recent write, if called.</summary>
+        public object? WriteValue { get; private set; }
+
+        /// <summary>Gets the timestamp passed to the most recent timestamped write, if called.</summary>
+        public object? WriteTimestamp { get; private set; }
+
+        /// <summary>Gets the user id passed to the most recent Write/Write2, if called.</summary>
+        public int? WriteUserId { get; private set; }
+
+        /// <summary>Gets the current user id passed to the most recent secured write, if called.</summary>
+        public int? WriteCurrentUserId { get; private set; }
+
+        /// <summary>Gets the verifier user id passed to the most recent secured write, if called.</summary>
+        public int? WriteVerifierUserId { get; private set; }
+
+        /// <summary>Gets the thread ID on which the most recent write was called.</summary>
+        public int? WriteThreadId { get; private set; }
+
+        /// <summary>Writes a value to an item and tracks the operation.</summary>
+        /// <param name="serverHandle">Server handle for the write.</param>
+        /// <param name="itemHandle">Item handle to write to.</param>
+        /// <param name="value">Value to write.</param>
+        /// <param name="userId">MXAccess user id for the write.</param>
+        public void Write(
+            int serverHandle,
+            int itemHandle,
+            object? value,
+            int userId)
+        {
+            operationNames.Add($"Write:{serverHandle}:{itemHandle}");
+            WriteServerHandle = serverHandle;
+            WriteItemHandle = itemHandle;
+            WriteValue = value;
+            WriteUserId = userId;
+            WriteThreadId = Environment.CurrentManagedThreadId;
+        }
+
+        /// <summary>Writes a timestamped value to an item and tracks the operation.</summary>
+        /// <param name="serverHandle">Server handle for the write.</param>
+        /// <param name="itemHandle">Item handle to write to.</param>
+        /// <param name="value">Value to write.</param>
+        /// <param name="timestamp">Source timestamp for the write.</param>
+        /// <param name="userId">MXAccess user id for the write.</param>
+        public void Write2(
+            int serverHandle,
+            int itemHandle,
+            object? value,
+            object? timestamp,
+            int userId)
+        {
+            operationNames.Add($"Write2:{serverHandle}:{itemHandle}");
+            WriteServerHandle = serverHandle;
+            WriteItemHandle = itemHandle;
+            WriteValue = value;
+            WriteTimestamp = timestamp;
+            WriteUserId = userId;
+            WriteThreadId = Environment.CurrentManagedThreadId;
+        }
+
+        /// <summary>Performs a secured write to an item and tracks the operation.</summary>
+        /// <param name="serverHandle">Server handle for the write.</param>
+        /// <param name="itemHandle">Item handle to write to.</param>
+        /// <param name="currentUserId">Operator user id.</param>
+        /// <param name="verifierUserId">Verifier user id.</param>
+        /// <param name="value">Value to write.</param>
+        public void WriteSecured(
+            int serverHandle,
+            int itemHandle,
+            int currentUserId,
+            int verifierUserId,
+            object? value)
+        {
+            operationNames.Add($"WriteSecured:{serverHandle}:{itemHandle}");
+            WriteServerHandle = serverHandle;
+            WriteItemHandle = itemHandle;
+            WriteCurrentUserId = currentUserId;
+            WriteVerifierUserId = verifierUserId;
+            WriteValue = value;
+            WriteThreadId = Environment.CurrentManagedThreadId;
+        }
+
+        /// <summary>Performs a secured timestamped write to an item and tracks the operation.</summary>
+        /// <param name="serverHandle">Server handle for the write.</param>
+        /// <param name="itemHandle">Item handle to write to.</param>
+        /// <param name="currentUserId">Operator user id.</param>
+        /// <param name="verifierUserId">Verifier user id.</param>
+        /// <param name="value">Value to write.</param>
+        /// <param name="timestamp">Source timestamp for the write.</param>
+        public void WriteSecured2(
+            int serverHandle,
+            int itemHandle,
+            int currentUserId,
+            int verifierUserId,
+            object? value,
+            object? timestamp)
+        {
+            operationNames.Add($"WriteSecured2:{serverHandle}:{itemHandle}");
+            WriteServerHandle = serverHandle;
+            WriteItemHandle = itemHandle;
+            WriteCurrentUserId = currentUserId;
+            WriteVerifierUserId = verifierUserId;
+            WriteValue = value;
+            WriteTimestamp = timestamp;
+            WriteThreadId = Environment.CurrentManagedThreadId;
+        }
     }
 
     /// <summary>Factory for creating fake MXAccess COM objects in tests.</summary>

src/MxGateway.Worker/Conversion/VariantConverter.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Globalization;
+using System.Linq;
 using Google.Protobuf;
 using Google.Protobuf.WellKnownTypes;
 using MxGateway.Contracts.Proto;
@@ -118,6 +119,63 @@ public sealed class VariantConverter
         }
     }
 
+    /// <summary>
+    /// Converts an <see cref="MxValue"/> into a CLR object suitable for an
+    /// MXAccess COM write. The COM marshaler boxes the returned value into the
+    /// matching VARIANT, so this is the inverse of <see cref="Convert(object?)"/>.
+    /// </summary>
+    /// <param name="value">Protobuf value to convert.</param>
+    /// <returns>A COM-marshalable value, or <see langword="null"/> for an MXAccess null.</returns>
+    public object? ConvertToComValue(MxValue value)
+    {
+        if (value is null)
+        {
+            throw new ArgumentNullException(nameof(value));
+        }
+
+        if (value.IsNull)
+        {
+            return null;
+        }
+
+        return value.KindCase switch
+        {
+            MxValue.KindOneofCase.BoolValue => value.BoolValue,
+            MxValue.KindOneofCase.Int32Value => value.Int32Value,
+            MxValue.KindOneofCase.Int64Value => value.Int64Value,
+            MxValue.KindOneofCase.FloatValue => value.FloatValue,
+            MxValue.KindOneofCase.DoubleValue => value.DoubleValue,
+            MxValue.KindOneofCase.StringValue => value.StringValue,
+            // The COM marshaler renders a DateTime as VT_DATE; MXAccess accepts
+            // it as the timestamped-write time argument.
+            MxValue.KindOneofCase.TimestampValue => value.TimestampValue.ToDateTime(),
+            MxValue.KindOneofCase.ArrayValue => ConvertToComArray(value.ArrayValue),
+            MxValue.KindOneofCase.RawValue => throw new ArgumentException(
+                "MxValue raw payloads cannot be written to MXAccess.", nameof(value)),
+            _ => throw new ArgumentException(
+                "MxValue has no value kind set; nothing to write.", nameof(value)),
+        };
+    }
+
+    private static Array ConvertToComArray(MxArray array)
+    {
+        return array.ValuesCase switch
+        {
+            MxArray.ValuesOneofCase.BoolValues => array.BoolValues.Values.ToArray(),
+            MxArray.ValuesOneofCase.Int32Values => array.Int32Values.Values.ToArray(),
+            MxArray.ValuesOneofCase.Int64Values => array.Int64Values.Values.ToArray(),
+            MxArray.ValuesOneofCase.FloatValues => array.FloatValues.Values.ToArray(),
+            MxArray.ValuesOneofCase.DoubleValues => array.DoubleValues.Values.ToArray(),
+            MxArray.ValuesOneofCase.StringValues => array.StringValues.Values.ToArray(),
+            MxArray.ValuesOneofCase.TimestampValues =>
+                array.TimestampValues.Values.Select(timestamp => timestamp.ToDateTime()).ToArray(),
+            MxArray.ValuesOneofCase.RawValues => throw new ArgumentException(
+                "MxArray raw payloads cannot be written to MXAccess.", nameof(array)),
+            _ => throw new ArgumentException(
+                "MxArray has no element values set; nothing to write.", nameof(array)),
+        };
+    }
+
     private static MxValue ConvertScalar(
         object value,
         MxDataType expectedDataType)

src/MxGateway.Worker/MxAccess/IMxAccessServer.cs

@@ -56,4 +56,56 @@ public interface IMxAccessServer
     void AdviseSupervisory(
         int serverHandle,
         int itemHandle);
+
+    /// <summary>Writes a value to an item.</summary>
+    /// <param name="serverHandle">Server handle identifying the registration.</param>
+    /// <param name="itemHandle">Item handle to write to.</param>
+    /// <param name="value">COM-marshalable value to write; <see langword="null"/> writes an MXAccess null.</param>
+    /// <param name="userId">MXAccess user id (security classification) for the write.</param>
+    void Write(
+        int serverHandle,
+        int itemHandle,
+        object? value,
+        int userId);
+
+    /// <summary>Writes a value with an explicit source timestamp to an item.</summary>
+    /// <param name="serverHandle">Server handle identifying the registration.</param>
+    /// <param name="itemHandle">Item handle to write to.</param>
+    /// <param name="value">COM-marshalable value to write; <see langword="null"/> writes an MXAccess null.</param>
+    /// <param name="timestamp">COM-marshalable source timestamp for the write.</param>
+    /// <param name="userId">MXAccess user id (security classification) for the write.</param>
+    void Write2(
+        int serverHandle,
+        int itemHandle,
+        object? value,
+        object? timestamp,
+        int userId);
+
+    /// <summary>Performs a secured/verified write to an item.</summary>
+    /// <param name="serverHandle">Server handle identifying the registration.</param>
+    /// <param name="itemHandle">Item handle to write to.</param>
+    /// <param name="currentUserId">MXAccess user id of the operator performing the write.</param>
+    /// <param name="verifierUserId">MXAccess user id of the verifier authorizing the write.</param>
+    /// <param name="value">COM-marshalable value to write; <see langword="null"/> writes an MXAccess null.</param>
+    void WriteSecured(
+        int serverHandle,
+        int itemHandle,
+        int currentUserId,
+        int verifierUserId,
+        object? value);
+
+    /// <summary>Performs a secured/verified write with an explicit source timestamp.</summary>
+    /// <param name="serverHandle">Server handle identifying the registration.</param>
+    /// <param name="itemHandle">Item handle to write to.</param>
+    /// <param name="currentUserId">MXAccess user id of the operator performing the write.</param>
+    /// <param name="verifierUserId">MXAccess user id of the verifier authorizing the write.</param>
+    /// <param name="value">COM-marshalable value to write; <see langword="null"/> writes an MXAccess null.</param>
+    /// <param name="timestamp">COM-marshalable source timestamp for the write.</param>
+    void WriteSecured2(
+        int serverHandle,
+        int itemHandle,
+        int currentUserId,
+        int verifierUserId,
+        object? value,
+        object? timestamp);
 }

src/MxGateway.Worker/MxAccess/MxAccessComServer.cs

@@ -140,6 +140,74 @@ public sealed class MxAccessComServer : IMxAccessServer
         AsProxyServer4().AdviseSupervisory(serverHandle, itemHandle);
     }
 
+    /// <inheritdoc />
+    public void Write(
+        int serverHandle,
+        int itemHandle,
+        object? value,
+        int userId)
+    {
+        if (mxAccessComObject is IMxAccessServer typedFake)
+        {
+            typedFake.Write(serverHandle, itemHandle, value, userId);
+            return;
+        }
+
+        AsProxyServer().Write(serverHandle, itemHandle, value!, userId);
+    }
+
+    /// <inheritdoc />
+    public void Write2(
+        int serverHandle,
+        int itemHandle,
+        object? value,
+        object? timestamp,
+        int userId)
+    {
+        if (mxAccessComObject is IMxAccessServer typedFake)
+        {
+            typedFake.Write2(serverHandle, itemHandle, value, timestamp, userId);
+            return;
+        }
+
+        AsProxyServer4().Write2(serverHandle, itemHandle, value!, timestamp!, userId);
+    }
+
+    /// <inheritdoc />
+    public void WriteSecured(
+        int serverHandle,
+        int itemHandle,
+        int currentUserId,
+        int verifierUserId,
+        object? value)
+    {
+        if (mxAccessComObject is IMxAccessServer typedFake)
+        {
+            typedFake.WriteSecured(serverHandle, itemHandle, currentUserId, verifierUserId, value);
+            return;
+        }
+
+        AsProxyServer().WriteSecured(serverHandle, itemHandle, currentUserId, verifierUserId, value!);
+    }
+
+    /// <inheritdoc />
+    public void WriteSecured2(
+        int serverHandle,
+        int itemHandle,
+        int currentUserId,
+        int verifierUserId,
+        object? value,
+        object? timestamp)
+    {
+        if (mxAccessComObject is IMxAccessServer typedFake)
+        {
+            typedFake.WriteSecured2(serverHandle, itemHandle, currentUserId, verifierUserId, value, timestamp);
+            return;
+        }
+
+        AsProxyServer4().WriteSecured2(serverHandle, itemHandle, currentUserId, verifierUserId, value!, timestamp!);
+    }
+
     private ILMXProxyServer AsProxyServer()
     {
         return mxAccessComObject as ILMXProxyServer

src/MxGateway.Worker/MxAccess/MxAccessCommandExecutor.cs

@@ -74,6 +74,10 @@ public sealed class MxAccessCommandExecutor : IStaCommandExecutor
             MxCommandKind.Advise => ExecuteAdvise(command),
             MxCommandKind.UnAdvise => ExecuteUnAdvise(command),
             MxCommandKind.AdviseSupervisory => ExecuteAdviseSupervisory(command),
+            MxCommandKind.Write => ExecuteWrite(command),
+            MxCommandKind.Write2 => ExecuteWrite2(command),
+            MxCommandKind.WriteSecured => ExecuteWriteSecured(command),
+            MxCommandKind.WriteSecured2 => ExecuteWriteSecured2(command),
             MxCommandKind.AddItemBulk => ExecuteAddItemBulk(command),
             MxCommandKind.AdviseItemBulk => ExecuteAdviseItemBulk(command),
             MxCommandKind.RemoveItemBulk => ExecuteRemoveItemBulk(command),
@@ -223,6 +227,108 @@ public sealed class MxAccessCommandExecutor : IStaCommandExecutor
         return CreateOkReply(command);
     }
 
+    private MxCommandReply ExecuteWrite(StaCommand command)
+    {
+        if (command.Command.PayloadCase != MxCommand.PayloadOneofCase.Write)
+        {
+            return CreateInvalidRequestReply(command, "Write command payload is required.");
+        }
+
+        WriteCommand writeCommand = command.Command.Write;
+        if (writeCommand.Value is null)
+        {
+            return CreateInvalidRequestReply(command, "Write command value is required.");
+        }
+
+        session.Write(
+            writeCommand.ServerHandle,
+            writeCommand.ItemHandle,
+            variantConverter.ConvertToComValue(writeCommand.Value),
+            writeCommand.UserId);
+
+        return CreateOkReply(command);
+    }
+
+    private MxCommandReply ExecuteWrite2(StaCommand command)
+    {
+        if (command.Command.PayloadCase != MxCommand.PayloadOneofCase.Write2)
+        {
+            return CreateInvalidRequestReply(command, "Write2 command payload is required.");
+        }
+
+        Write2Command write2Command = command.Command.Write2;
+        if (write2Command.Value is null)
+        {
+            return CreateInvalidRequestReply(command, "Write2 command value is required.");
+        }
+
+        if (write2Command.TimestampValue is null)
+        {
+            return CreateInvalidRequestReply(command, "Write2 command timestamp value is required.");
+        }
+
+        session.Write2(
+            write2Command.ServerHandle,
+            write2Command.ItemHandle,
+            variantConverter.ConvertToComValue(write2Command.Value),
+            variantConverter.ConvertToComValue(write2Command.TimestampValue),
+            write2Command.UserId);
+
+        return CreateOkReply(command);
+    }
+
+    private MxCommandReply ExecuteWriteSecured(StaCommand command)
+    {
+        if (command.Command.PayloadCase != MxCommand.PayloadOneofCase.WriteSecured)
+        {
+            return CreateInvalidRequestReply(command, "WriteSecured command payload is required.");
+        }
+
+        WriteSecuredCommand writeSecuredCommand = command.Command.WriteSecured;
+        if (writeSecuredCommand.Value is null)
+        {
+            return CreateInvalidRequestReply(command, "WriteSecured command value is required.");
+        }
+
+        session.WriteSecured(
+            writeSecuredCommand.ServerHandle,
+            writeSecuredCommand.ItemHandle,
+            writeSecuredCommand.CurrentUserId,
+            writeSecuredCommand.VerifierUserId,
+            variantConverter.ConvertToComValue(writeSecuredCommand.Value));
+
+        return CreateOkReply(command);
+    }
+
+    private MxCommandReply ExecuteWriteSecured2(StaCommand command)
+    {
+        if (command.Command.PayloadCase != MxCommand.PayloadOneofCase.WriteSecured2)
+        {
+            return CreateInvalidRequestReply(command, "WriteSecured2 command payload is required.");
+        }
+
+        WriteSecured2Command writeSecured2Command = command.Command.WriteSecured2;
+        if (writeSecured2Command.Value is null)
+        {
+            return CreateInvalidRequestReply(command, "WriteSecured2 command value is required.");
+        }
+
+        if (writeSecured2Command.TimestampValue is null)
+        {
+            return CreateInvalidRequestReply(command, "WriteSecured2 command timestamp value is required.");
+        }
+
+        session.WriteSecured2(
+            writeSecured2Command.ServerHandle,
+            writeSecured2Command.ItemHandle,
+            writeSecured2Command.CurrentUserId,
+            writeSecured2Command.VerifierUserId,
+            variantConverter.ConvertToComValue(writeSecured2Command.Value),
+            variantConverter.ConvertToComValue(writeSecured2Command.TimestampValue));
+
+        return CreateOkReply(command);
+    }
+
     private MxCommandReply ExecuteAddItemBulk(StaCommand command)
     {
         if (command.Command.PayloadCase != MxCommand.PayloadOneofCase.AddItemBulk)

src/MxGateway.Worker/MxAccess/MxAccessSession.cs

@@ -227,6 +227,78 @@ public sealed class MxAccessSession : IDisposable
             MxAccessAdviceKind.Supervisory);
     }
 
+    /// <summary>Writes a value to an item.</summary>
+    /// <param name="serverHandle">Handle returned by the worker.</param>
+    /// <param name="itemHandle">Handle returned by the worker.</param>
+    /// <param name="value">COM-marshalable value to write.</param>
+    /// <param name="userId">MXAccess user id (security classification) for the write.</param>
+    public void Write(
+        int serverHandle,
+        int itemHandle,
+        object? value,
+        int userId)
+    {
+        ThrowIfDisposed();
+
+        mxAccessServer.Write(serverHandle, itemHandle, value, userId);
+    }
+
+    /// <summary>Writes a value with an explicit source timestamp to an item.</summary>
+    /// <param name="serverHandle">Handle returned by the worker.</param>
+    /// <param name="itemHandle">Handle returned by the worker.</param>
+    /// <param name="value">COM-marshalable value to write.</param>
+    /// <param name="timestamp">COM-marshalable source timestamp for the write.</param>
+    /// <param name="userId">MXAccess user id (security classification) for the write.</param>
+    public void Write2(
+        int serverHandle,
+        int itemHandle,
+        object? value,
+        object? timestamp,
+        int userId)
+    {
+        ThrowIfDisposed();
+
+        mxAccessServer.Write2(serverHandle, itemHandle, value, timestamp, userId);
+    }
+
+    /// <summary>Performs a secured/verified write to an item.</summary>
+    /// <param name="serverHandle">Handle returned by the worker.</param>
+    /// <param name="itemHandle">Handle returned by the worker.</param>
+    /// <param name="currentUserId">MXAccess user id of the operator performing the write.</param>
+    /// <param name="verifierUserId">MXAccess user id of the verifier authorizing the write.</param>
+    /// <param name="value">COM-marshalable value to write.</param>
+    public void WriteSecured(
+        int serverHandle,
+        int itemHandle,
+        int currentUserId,
+        int verifierUserId,
+        object? value)
+    {
+        ThrowIfDisposed();
+
+        mxAccessServer.WriteSecured(serverHandle, itemHandle, currentUserId, verifierUserId, value);
+    }
+
+    /// <summary>Performs a secured/verified write with an explicit source timestamp.</summary>
+    /// <param name="serverHandle">Handle returned by the worker.</param>
+    /// <param name="itemHandle">Handle returned by the worker.</param>
+    /// <param name="currentUserId">MXAccess user id of the operator performing the write.</param>
+    /// <param name="verifierUserId">MXAccess user id of the verifier authorizing the write.</param>
+    /// <param name="value">COM-marshalable value to write.</param>
+    /// <param name="timestamp">COM-marshalable source timestamp for the write.</param>
+    public void WriteSecured2(
+        int serverHandle,
+        int itemHandle,
+        int currentUserId,
+        int verifierUserId,
+        object? value,
+        object? timestamp)
+    {
+        ThrowIfDisposed();
+
+        mxAccessServer.WriteSecured2(serverHandle, itemHandle, currentUserId, verifierUserId, value, timestamp);
+    }
+
     /// <summary>Adds multiple items in bulk, returning success/failure results.</summary>
     /// <param name="serverHandle">Handle returned by the worker.</param>
     /// <param name="tagAddresses">Enumerable of item definitions to add.</param>