Guard NuGetRestoreTargets import in Microsoft.Common.CrossTargeting.targets

src/Tasks.UnitTests/CrossTargetingTargetsImportTests.cs

@@ -0,0 +1,97 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.IO;
+
+using Microsoft.Build.Evaluation;
+using Microsoft.Build.UnitTests;
+
+using Shouldly;
+
+using Xunit;
+
+namespace Microsoft.Build.Tasks.UnitTests;
+
+/// <summary>
+/// Tests for the NuGet restore targets import in <c>Microsoft.Common.CrossTargeting.targets</c>.
+/// The parallel import in <c>Microsoft.Common.CurrentVersion.targets</c> is guarded with
+/// <c>Exists('$(NuGetRestoreTargets)')</c>, but for years the cross-targeting variant was unguarded
+/// — which meant cross-targeting (multi-TFM) projects loaded in non-NuGet contexts (custom build
+/// systems, focused unit tests) failed with "Imported project not found" unless the caller
+/// manufactured a stub NuGet.targets file. These tests cover the now-guarded import.
+/// </summary>
+public sealed class CrossTargetingTargetsImportTests
+{
+    private readonly ITestOutputHelper _output;
+
+    public CrossTargetingTargetsImportTests(ITestOutputHelper output)
+    {
+        _output = output;
+    }
+
+    /// <summary>
+    /// When <c>NuGetRestoreTargets</c> points at a file that does not exist, evaluating a project
+    /// that imports <c>Microsoft.Common.CrossTargeting.targets</c> must succeed (the import is a
+    /// silent no-op), matching the long-standing behavior of <c>Microsoft.Common.CurrentVersion.targets</c>.
+    /// </summary>
+    [Fact]
+    public void ImportSucceedsWhenNuGetRestoreTargetsDoesNotExist()
+    {
+        using TestEnvironment env = TestEnvironment.Create(_output);
+        TransientTestFolder folder = env.CreateFolder(createFolder: true);
+        string nonExistentNuGetTargets = Path.Combine(folder.Path, "DoesNotExist.NuGet.targets");
+
+        string projectContents = $"""
+            <Project>
+                <PropertyGroup>
+                    <NuGetRestoreTargets>{nonExistentNuGetTargets}</NuGetRestoreTargets>
+                </PropertyGroup>
+                <Import Project="$(MSBuildToolsPath)\Microsoft.Common.CrossTargeting.targets" />
+            </Project>
+            """.Cleanup();
+
+        using ProjectFromString projectFromString = new(projectContents);
+        Project project = projectFromString.Project;
+
+        project.GetPropertyValue("NuGetRestoreTargets").ShouldBe(nonExistentNuGetTargets);
+    }
+
+    /// <summary>
+    /// When <c>NuGetRestoreTargets</c> points at a real file, the import still happens. This
+    /// guards against the new <c>Exists()</c> condition accidentally skipping a valid NuGet
+    /// targets file. The synthetic project does not set <c>IsRestoreTargetsFileLoaded</c>, and
+    /// nothing in the <c>Microsoft.Common.CrossTargeting.targets</c> import chain sets it before
+    /// the guarded import, so the canonical guard does not short-circuit here.
+    /// </summary>
+    [Fact]
+    public void ImportLoadsNuGetTargetsWhenItExists()
+    {
+        using TestEnvironment env = TestEnvironment.Create(_output);
+        TransientTestFolder folder = env.CreateFolder(createFolder: true);
+
+        const string MarkerTargetName = "_CrossTargetingNuGetStubMarker";
+        TransientTestFile nuGetStub = env.CreateFile(
+            folder,
+            "NuGet.targets",
+            $"""
+            <Project>
+                <Target Name="{MarkerTargetName}" />
+            </Project>
+            """.Cleanup());
+
+        string projectContents = $"""
+            <Project>
+                <PropertyGroup>
+                    <NuGetRestoreTargets>{nuGetStub.Path}</NuGetRestoreTargets>
+                </PropertyGroup>
+                <Import Project="$(MSBuildToolsPath)\Microsoft.Common.CrossTargeting.targets" />
+            </Project>
+            """.Cleanup();
+
+        using ProjectFromString projectFromString = new(projectContents);
+        Project project = projectFromString.Project;
+
+        project.Targets.ShouldContainKey(MarkerTargetName);
+    }
+}
+

src/Tasks/Microsoft.Common.CrossTargeting.targets

@@ -201,7 +201,7 @@ Copyright (C) Microsoft Corporation. All rights reserved.
     <NuGetRestoreTargets Condition="'$(NuGetRestoreTargets)'==''">$(MSBuildToolsPath)\NuGet.targets</NuGetRestoreTargets>
   </PropertyGroup>
 
-  <Import Project="$(NuGetRestoreTargets)" />
+  <Import Condition="'$(IsRestoreTargetsFileLoaded)' != 'true' and Exists('$(NuGetRestoreTargets)')" Project="$(NuGetRestoreTargets)" />
 
   <Import Project="$(CustomAfterMicrosoftCommonCrossTargetingTargets)" Condition="'$(CustomAfterMicrosoftCommonCrossTargetingTargets)' != '' and Exists('$(CustomAfterMicrosoftCommonCrossTargetingTargets)')"/>