fix(build): restore multi-TFM compatibility for Release pack

adapter/MTConnect.NET-Applications-Adapter/Service.cs

@@ -3,14 +3,18 @@
 
 using MTConnect.Services;
 using NLog;
+#if NET5_0_OR_GREATER
 using System.Runtime.Versioning;
+#endif
 
 namespace MTConnect.Applications
 {
     /// <summary>
     /// Class used to implement a Windows Service for an MTConnect Agent Application
     /// </summary>
+#if NET5_0_OR_GREATER
     [SupportedOSPlatform("windows")]
+#endif
     public class Service : MTConnectAdapterService
     {
         private static readonly Logger _serviceLogger = LogManager.GetLogger("service-logger");

agent/MTConnect.NET-Applications-Agents/Service.cs

@@ -3,14 +3,18 @@
 
 using MTConnect.Services;
 using NLog;
+#if NET5_0_OR_GREATER
 using System.Runtime.Versioning;
+#endif
 
 namespace MTConnect.Applications
 {
     /// <summary>
     /// Class used to implement a Windows Service for an MTConnect Agent Application
     /// </summary>
+#if NET5_0_OR_GREATER
     [SupportedOSPlatform("windows")]
+#endif
     public class Service : MTConnectAgentService
     {
         private static readonly Logger _serviceLogger = LogManager.GetLogger("service-logger");

agent/Modules/MTConnect.NET-AgentModule-MqttRelay/Module.cs

@@ -941,11 +941,17 @@ await AsyncVoidGuard.Run(
                                     if (!conditionObservations.IsNullOrEmpty())
                                     {
                                         var multipleObservations = new List<IObservation>(conditionObservations.Count());
-                                        foreach (var observation in conditionObservations)
+                                        // Rename to avoid CS0136: the
+                                        // enclosing AgentObservationAdded
+                                        // handler takes `observation` as its
+                                        // parameter; declaring an inner
+                                        // `observation` here shadows it and
+                                        // fails to compile under net4x.
+                                        foreach (var condObservation in conditionObservations)
                                         {
-                                            multipleObservations.Add(CloneAsObservation(observation));
+                                            multipleObservations.Add(CloneAsObservation(condObservation));
                                         }
-                                    
+
                                         var result = await _entityServer.PublishObservations(_mqttClient, multipleObservations);
                                         if (result != null && result.IsSuccess)
                                         {

libraries/MTConnect.NET-HTTP/Ceen/Httpd/HttpServer.cs

@@ -2,7 +2,9 @@
 using System.Net;
 using System.Linq;
 using System.Net.Sockets;
+#if NET5_0_OR_GREATER
 using System.Runtime.Versioning;
+#endif
 using System.Threading.Tasks;
 using System.Threading;
 using System.Security.Cryptography.X509Certificates;
@@ -172,7 +174,9 @@ public void Setup(bool usessl, ServerConfig config)
 			/// <param name="socket">The socket handle.</param>
 			/// <param name="remoteEndPoint">The remote endpoint.</param>
 			/// <param name="logtaskid">The task ID to use.</param>
+#if NET5_0_OR_GREATER
 			[SupportedOSPlatform("windows")]
+#endif
 			public void HandleRequest(SocketInformation socket, EndPoint remoteEndPoint, string logtaskid)
 			{
 				RunClient(socket, remoteEndPoint, logtaskid, Controller);
@@ -865,7 +869,9 @@ public static Task ListenAsync(
 		/// <param name="remoteEndPoint">The remote endpoint.</param>
 		/// <param name="logtaskid">The log task ID.</param>
 		/// <param name="controller">The controller instance</param>
+#if NET5_0_OR_GREATER
 		[SupportedOSPlatform("windows")]
+#endif
 		private static void RunClient(SocketInformation socketinfo, EndPoint remoteEndPoint, string logtaskid, RunnerControl controller)
 		{
 			RunClient(new Socket(socketinfo), remoteEndPoint, logtaskid, controller);

libraries/MTConnect.NET-Services/MTConnectAdapterService.cs

@@ -5,15 +5,19 @@
 using System.Diagnostics;
 using System.IO;
 using System.Reflection;
+#if NET5_0_OR_GREATER
 using System.Runtime.Versioning;
+#endif
 using System.ServiceProcess;
 
 namespace MTConnect.Services
 {
     /// <summary>
     /// Class used to implement an MTConnect Adapter as a Windows Service
     /// </summary>
+#if NET5_0_OR_GREATER
     [SupportedOSPlatform("windows")]
+#endif
     public abstract class MTConnectAdapterService : ServiceBase
     {
         private const string DefaultServiceName = "MTConnect-Adapter";

libraries/MTConnect.NET-Services/MTConnectAgentService.cs

@@ -5,15 +5,19 @@
 using System.Diagnostics;
 using System.IO;
 using System.Reflection;
+#if NET5_0_OR_GREATER
 using System.Runtime.Versioning;
+#endif
 using System.ServiceProcess;
 
 namespace MTConnect.Services
 {
     /// <summary>
     /// Class used to implement an MTConnect Agent as a Windows Service
     /// </summary>
+#if NET5_0_OR_GREATER
     [SupportedOSPlatform("windows")]
+#endif
     public abstract class MTConnectAgentService : ServiceBase
     {
         private const string DefaultServiceName = "MTConnect.NET-Agent";

libraries/MTConnect.NET-Services/WindowsService.cs

@@ -3,13 +3,17 @@
 
 using System.Linq;
 using System.Runtime.InteropServices;
+#if NET5_0_OR_GREATER
 using System.Runtime.Versioning;
+#endif
 using System.Security.Principal;
 using System.ServiceProcess;
 
 namespace MTConnect.Services
 {
+#if NET5_0_OR_GREATER
     [SupportedOSPlatform("windows")]
+#endif
     internal static class WindowsService
     {
         public static bool ServiceExists(string serviceName)

libraries/MTConnect.NET-XML/MTConnect.NET-XML.csproj

@@ -17,7 +17,18 @@
 	<PropertyGroup>
 		<RootNamespace>MTConnect</RootNamespace>
 		<Configurations>Debug;Release;Package</Configurations>
-
+
+		<!--
+			XsdPreprocessor.StripXsd11Constructs uses a C# 8.0 `using var`
+			declaration (`using var reader = new StringReader(...)`). The
+			Roslyn default LangVersion on netstandard2.0 / net4x falls
+			back to 7.3, which rejects the using-declaration syntax with
+			CS8370 and breaks the multi-TFM Release pack. Pin LangVersion
+			to 8.0 — the lowest version that accepts the syntax — so the
+			Release pack survives every declared TFM.
+		-->
+		<LangVersion>8.0</LangVersion>
+
 		<Description>MTConnect.NET-XML implements the XML Document Format for use with the MTConnect.NET library. Supports MTConnect versions up to 2.7. Supports .NET Framework 4.6.1 up to .NET 9</Description>
 		<PackageReadmeFile>README-Nuget.md</PackageReadmeFile>
 

tests/MTConnect.NET-AgentModule-MqttRelay-Tests/ConditionObservationVariableScopeTests.cs

@@ -0,0 +1,119 @@
+// Copyright (c) 2026 TrakHound Inc., All Rights Reserved.
+// TrakHound Inc. licenses this file to you under the MIT license.
+
+using System;
+using System.IO;
+using System.Linq;
+using NUnit.Framework;
+
+namespace MTConnect.AgentModule.MqttRelay.Tests
+{
+    // Pins the rename PR #194 lands on Module.cs to resolve the
+    // CS0136 shadowed-local diagnostic that fires under net4x. The
+    // original code in `AgentObservationAdded` declared an inner
+    // `var observation` inside a `foreach` loop that lived in the
+    // same scope as the handler's outer `observation` parameter:
+    //
+    //     private async void AgentObservationAdded(object sender,
+    //         IObservation observation)            // <- outer name
+    //     {
+    //         ...
+    //         foreach (var observation in conditionObservations)  // <- shadow
+    //         { ... }
+    //     }
+    //
+    // The C# 5+ language spec section 7.6.2.1 forbids a local
+    // variable declaration from shadowing an enclosing
+    // parameter/local of the same name. The .NET 5+ compiler emits
+    // a warning that TreatWarningsAsErrors escalates to an error on
+    // net8.0, but the project's net4x roslyn pinned compiler emits
+    // CS0136 directly. PR #194 renames the inner declaration to
+    // `condObservation` (or similar non-shadowing name) so the
+    // multi-TFM build path stays green.
+    //
+    // This fixture reads the Module.cs source via reflection on the
+    // assembly's location and scans the relevant block, asserting
+    // that no `foreach (var observation` declaration remains inside
+    // the handler. A future contributor who re-introduces the
+    // shadow fails this test under net8.0 — the test TFM that the
+    // CI matrix exercises — instead of waiting for the next Release
+    // pack to surface the failure under net4x.
+    /// <summary>Pins the rename that resolves the net4x CS0136 shadow.</summary>
+    [TestFixture]
+    [Category("MultiTfmCompat")]
+    public class ConditionObservationVariableScopeTests
+    {
+        /// <summary>Module.cs AgentObservationAdded must not declare an inner `observation` loop variable.</summary>
+        [Test]
+        public void AgentObservationAdded_does_not_declare_inner_observation_loop_variable()
+        {
+            // Locate the Module.cs source by walking up from the test
+            // assembly to the repo root, then descending into the
+            // module's source tree. The path is stable in-tree;
+            // CI runs with the same layout.
+            var moduleSourcePath = FindModuleSourcePath();
+            Assert.That(moduleSourcePath, Is.Not.Null,
+                "Could not locate Module.cs for MTConnect.NET-AgentModule-MqttRelay.");
+
+            var source = File.ReadAllText(moduleSourcePath!);
+
+            // Extract the AgentObservationAdded handler body. The
+            // method ends at the matching close-brace of the
+            // `try { ... } finally { ... }` pair, which is the last
+            // brace at column 8 before the next `private` member.
+            var startIndex = source.IndexOf(
+                "private async void AgentObservationAdded",
+                StringComparison.Ordinal);
+            Assert.That(startIndex, Is.GreaterThanOrEqualTo(0),
+                "AgentObservationAdded handler not found in Module.cs.");
+
+            // Find the start of the next private member after the
+            // handler so we scope the scan to AgentObservationAdded's
+            // body and don't reach into AgentAssetAdded.
+            var nextMemberIndex = source.IndexOf(
+                "private async void AgentAssetAdded",
+                startIndex, StringComparison.Ordinal);
+            Assert.That(nextMemberIndex, Is.GreaterThan(startIndex),
+                "Next handler AgentAssetAdded not found after AgentObservationAdded; " +
+                "Module.cs structure has changed.");
+
+            var handlerBody = source.Substring(startIndex, nextMemberIndex - startIndex);
+
+            // Assert: the handler must NOT contain the shadowing
+            // declaration `foreach (var observation in`. The renamed
+            // form uses a different identifier such as
+            // `condObservation`.
+            var shadowingPatterns = new[]
+            {
+                "foreach (var observation in",
+                "foreach(var observation in",
+            };
+            foreach (var pattern in shadowingPatterns)
+            {
+                Assert.That(
+                    handlerBody.Contains(pattern, StringComparison.Ordinal),
+                    Is.False,
+                    $"AgentObservationAdded contains `{pattern}` which shadows " +
+                    "the outer `observation` parameter and fails CS0136 under net4x. " +
+                    "Rename the inner loop variable.");
+            }
+        }
+
+        private static string FindModuleSourcePath()
+        {
+            // The test assembly's location lives under
+            //   tests/MTConnect.NET-AgentModule-MqttRelay-Tests/bin/...
+            // walk up to find the repo root marker, then descend.
+            var dir = new DirectoryInfo(AppContext.BaseDirectory);
+            while (dir != null)
+            {
+                var candidate = Path.Combine(
+                    dir.FullName,
+                    "agent", "Modules", "MTConnect.NET-AgentModule-MqttRelay", "Module.cs");
+                if (File.Exists(candidate)) return candidate;
+                dir = dir.Parent;
+            }
+            return null;
+        }
+    }
+}

tests/MTConnect.NET-Common-Tests/MTConnect.NET-Common-Tests.csproj

@@ -17,6 +17,20 @@
 
   <ItemGroup>
     <ProjectReference Include="..\..\libraries\MTConnect.NET-Common\MTConnect.NET-Common.csproj" />
+    <!--
+      Project references added for SupportedOSPlatformAttributePresenceTests
+      (tests/MTConnect.NET-Common-Tests/Platform/). The fixture loads each
+      assembly that owns one of the six Windows-only Service / HTTP-server
+      types decorated by commit dd2eb424 (2026-05-22) and reflection-asserts
+      the [SupportedOSPlatform("windows")] attribute is still present. The
+      references are deliberately reflection-only — no test instantiates
+      WindowsService or starts an HTTP server, so the heavier transitive
+      surface is fine to drag in.
+    -->
+    <ProjectReference Include="..\..\libraries\MTConnect.NET-Services\MTConnect.NET-Services.csproj" />
+    <ProjectReference Include="..\..\libraries\MTConnect.NET-HTTP\MTConnect.NET-HTTP.csproj" />
+    <ProjectReference Include="..\..\agent\MTConnect.NET-Applications-Agents\MTConnect.NET-Applications-Agents.csproj" />
+    <ProjectReference Include="..\..\adapter\MTConnect.NET-Applications-Adapter\MTConnect.NET-Applications-Adapter.csproj" />
   </ItemGroup>
 
 </Project>