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

src/Build.UnitTests/BackEnd/DotNetRuntimeTask_EndToEnd_Tests.cs

@@ -0,0 +1,77 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using Microsoft.Build.Evaluation;
+using Microsoft.Build.Execution;
+using Microsoft.Build.Framework;
+using Microsoft.Build.UnitTests;
+using Shouldly;
+using Xunit;
+using Xunit.Abstractions;
+using Xunit.NetCore.Extensions;
+
+namespace Microsoft.Build.Engine.UnitTests.BackEnd
+{
+    /// <summary>
+    /// End-to-end tests for .NET Runtime task error handling.
+    /// These tests verify that MSBuild 17.14 gives a clear error when attempting to use Runtime="NET" tasks.
+    /// </summary>
+    public class DotNetRuntimeTask_EndToEnd_Tests
+    {
+        private readonly ITestOutputHelper _output;
+
+        public DotNetRuntimeTask_EndToEnd_Tests(ITestOutputHelper output)
+        {
+            _output = output;
+        }
+
+        /// <summary>
+        /// Test that BuildManager API gives clear error (MSB4233) when attempting to build a project
+        /// that uses a task with Runtime="NET".
+        /// </summary>
+        [WindowsFullFrameworkOnlyFact]
+        public void BuildManager_WithDotNetRuntimeTask_ShowsClearError()
+        {
+            // This test uses a real task but specifies Runtime="NET" which is not supported on .NET Framework builds.
+            // We expect the build to fail with MSB4233 error that clearly explains the issue.
+#if NETFRAMEWORK
+            string projectContent = @"
+<Project>
+    <UsingTask TaskName=""ProcessIdTask"" AssemblyName=""Microsoft.Build.Engine.UnitTests"" Runtime=""NET"" />
+    <Target Name='TestTask'>
+        <ProcessIdTask>
+            <Output PropertyName=""PID"" TaskParameter=""Pid"" />
+        </ProcessIdTask>
+    </Target>
+</Project>";
+
+            using (var env = TestEnvironment.Create(_output))
+            {
+                var testProject = env.CreateTestProjectWithFiles(projectContent);
+                var logger = new MockLogger(_output);
+
+                var parameters = new BuildParameters
+                {
+                    Loggers = new[] { logger }
+                };
+
+                var result = Helpers.BuildProjectFileUsingBuildManager(testProject.ProjectFile, logger, parameters, targetsToBuild: new[] { "TestTask" });
+
+                // Build should fail
+                result.OverallResult.ShouldBe(BuildResultCode.Failure);
+
+                // Should log MSB4233 error
+                logger.ErrorCount.ShouldBeGreaterThan(0);
+                logger.Errors[0].Code.ShouldBe("MSB4233");
+
+                // Error message should contain the task name and indicate .NET runtime is not supported
+                string? errorMessage = logger.Errors[0].Message;
+                errorMessage.ShouldNotBeNull();
+                errorMessage.ShouldContain("ProcessIdTask");
+                errorMessage.ShouldContain(".NET runtime");
+                errorMessage.ShouldContain("MSBuild 18.0");
+            }
+#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).
+            // .NET Core/5+ builds of MSBuild (18.0+) 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, MSBuild 18.0 or Visual Studio 2026 or higher must be used.</value>
+    <comment>{StrBegin="MSB4233: "}{Locked=".NET"}{Locked="MSBuild"}{Locked="Visual Studio 2026"}LOCALIZATION: .NET, MSBuild, 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>