Add clear error message (MSB4233) for .NET runtime tasks on MSBuild 17.14

eng/Versions.props

@@ -2,7 +2,7 @@
 <!-- Copyright (c) .NET Foundation and contributors. All rights reserved. Licensed under the MIT license. See License.txt in the project root for full license information. -->
 <Project>
   <PropertyGroup>
-    <VersionPrefix>17.14.32</VersionPrefix>
+    <VersionPrefix>17.14.33</VersionPrefix>
     <DotNetFinalVersionKind>release</DotNetFinalVersionKind>
     <PackageValidationBaselineVersion>17.13.9</PackageValidationBaselineVersion>
     <AssemblyVersion>15.1.0.0</AssemblyVersion>

src/Build.UnitTests/BackEnd/DotNetTaskHost_Tests.cs

@@ -0,0 +1,62 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.IO;
+using Microsoft.Build.Execution;
+using Microsoft.Build.UnitTests;
+using Shouldly;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace Microsoft.Build.Engine.UnitTests.BackEnd
+{
+    /// <summary>
+    /// End-to-end MSBuild CLI tests for .NET Runtime task error handling.
+    /// These tests verify that MSBuild.exe CLI gives a clear error when attempting to use Runtime="NET" tasks.
+    /// </summary>
+    public class DotNetTaskHost_Tests
+    {
+        private readonly ITestOutputHelper _output;
+
+        public DotNetTaskHost_Tests(ITestOutputHelper output)
+        {
+            _output = output;
+        }
+
+        /// <summary>
+        /// Test that MSBuild.exe gives clear error (MSB4233) when building a project
+        /// that uses a task with Runtime="NET".
+        /// </summary>
+        [WindowsFullFrameworkOnlyFact]
+        public void CustomTask_WithNetRuntime_ShowsClearError()
+        {
+#if NETFRAMEWORK
+            using (var env = TestEnvironment.Create(_output))
+            {
+                string taskAssemblyPath = Path.Combine(
+                    Path.GetDirectoryName(typeof(DotNetTaskHost_Tests).Assembly.Location) ?? AppContext.BaseDirectory,
+                    "Microsoft.Build.Engine.UnitTests.dll");
+
+                string projectContent = $@"
+<Project>
+    <UsingTask TaskName=""ProcessIdTask"" AssemblyFile=""{taskAssemblyPath}"" Runtime=""NET"" />
+    <Target Name='TestTask'>
+        <ProcessIdTask>
+            <Output PropertyName=""PID"" TaskParameter=""Pid"" />
+        </ProcessIdTask>
+    </Target>
+</Project>";
+                var logger = new MockLogger(_output);
+                TransientTestFile project = env.CreateFile("test.csproj", projectContent);
+                ProjectInstance projectInstance = new(project.Path);
+                projectInstance.Build(new[] { logger })
+                    .ShouldBeFalse();
+
+                logger.ErrorCount.ShouldBeGreaterThan(0);
+                logger.Errors[0].Code.ShouldBe("MSB4233");
+            }
+#endif
+        }
+    }
+}

src/Build/BackEnd/Components/Communications/NodeProviderOutOfProcTaskHost.cs

@@ -486,6 +486,20 @@ internal static string GetMSBuildLocationFromHostContext(HandshakeOptions hostCo
         /// </summary>
         internal bool AcquireAndSetUpHost(HandshakeOptions hostContext, INodePacketFactory factory, INodePacketHandler handler, TaskHostConfiguration configuration)
         {
+#if NETFRAMEWORK
+            // MSB4233: Check if .NET runtime is requested, which is not supported in .NET Framework builds of MSBuild (17.14 and earlier).
+            // All .NET Core/5+ MSBuilds and all 18.0+ builds do support .NET runtime tasks, so this check is not needed there.
+            // This provides a clear error message instead of the confusing "MSBuild.dll not found" error that would otherwise occur.
+            if ((hostContext & HandshakeOptions.NET) == HandshakeOptions.NET)
+            {
+                ProjectFileErrorUtilities.ThrowInvalidProjectFile(
+                    new BuildEventFileInfo(configuration.ProjectFileOfTask, configuration.LineNumberOfTask, configuration.ColumnNumberOfTask),
+                    "TaskRuntimeNET",
+                    configuration.TaskName,
+                    configuration.TaskLocation);
+            }
+#endif
+
             bool nodeCreationSucceeded;
             if (!_nodeContexts.ContainsKey(hostContext))
             {

src/Build/Resources/Strings.resx

@@ -1238,6 +1238,10 @@
     <value>MSB4217: Task host node exited prematurely. Diagnostic information may be found in files in the temporary files directory named MSBuild_*.failure.txt. {0}</value>
     <comment>{StrBegin="MSB4217: "}</comment>
   </data>
+  <data name="TaskRuntimeNET" xml:space="preserve">
+    <value>MSB4233: The task "{0}" from assembly "{1}" was specified to load with the .NET runtime, but this version of MSBuild does not support loading tasks with that runtime. To run .NET tasks, use "dotnet build" or Visual Studio 2026 or higher.</value>
+    <comment>{StrBegin="MSB4233: "}{Locked=".NET"}{Locked="dotnet build"}{Locked="Visual Studio 2026"}LOCALIZATION: .NET, dotnet build, and Visual Studio 2026 should not be localized.</comment>
+  </data>
   <data name="TaskParametersError" xml:space="preserve">
     <value>MSB4063: The "{0}" task could not be initialized with its input parameters. {1}</value>
     <comment>{StrBegin="MSB4063: "}</comment>