fix: add fallback for nuget downloader (#9131)

This PR addresses an edge case where the nuget downloader was not
parsing the last contract DLL path on disk as expected.

fixes: #9130

Author: jorgerangel-msft

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Utilities/NugetPackageDownloader.cs

@@ -265,6 +265,16 @@ private bool TryFindPackageLibPathInCache(string packageName, NuGetVersion packa
                 }
             }
 
+            // Fallback to any of the preferred versions if none of the target frameworks matched.
+            foreach (var preferredDotNetFrameworkVersion in PreferredDotNetFrameworkVersions)
+            {
+                var dotNetFolder = Path.Combine(packageLibraryPath, preferredDotNetFrameworkVersion);
+                if (DirectoryExists(dotNetFolder))
+                {
+                    return dotNetFolder;
+                }
+            }
+
             return null;
         }
 

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/TestHelpers/TestNugetPackageDownloader.cs

@@ -19,6 +19,7 @@ internal class TestNugetPackageDownloader : NugetPackageDownloader
         private const string DefaultTargetFramework = "netstandard2.0";
         private readonly SourceRepository _mockPackageSourceRepo;
         private readonly bool _mockDirectoryExists;
+        private readonly HashSet<string>? _existingDirectories;
         private readonly bool _mockTryFindPackageInCache;
         private readonly bool _mockPackageExistsInSource;
         private readonly LocalPackageInfo? _mockLocalPackageInfo;
@@ -39,10 +40,12 @@ public TestNugetPackageDownloader(
             bool directoryExists,
             bool packageExistsInSource,
             IEnumerable<string>? targetFrameworks = null,
-            LocalPackageInfo? localPackageInfo = null) : base(packageName, version, targetFrameworks ?? [DefaultTargetFramework], settings)
+            LocalPackageInfo? localPackageInfo = null,
+            HashSet<string>? existingDirectories = null) : base(packageName, version, targetFrameworks ?? [DefaultTargetFramework], settings)
         {
             _mockPackageSourceRepo = sourceRepository;
             _mockDirectoryExists = directoryExists;
+            _existingDirectories = existingDirectories;
             _mockLocalPackageInfo = localPackageInfo;
             _mockTryFindPackageInCache = _mockLocalPackageInfo != null;
             _mockPackageExistsInSource = packageExistsInSource;
@@ -55,6 +58,10 @@ protected override SourceRepository GetCoreV3NugetSourceRepo(string source)
 
         protected override bool DirectoryExists(string path)
         {
+            if (_existingDirectories != null)
+            {
+                return _existingDirectories.Contains(path);
+            }
             return _mockDirectoryExists;
         }
 

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Utilities/NugetPackageDownloaderTests.cs

@@ -103,6 +103,73 @@ public async Task TestDownloadAndInstallPackage(bool packageExistsInSource)
             }
         }
 
+        // This test validates that when target frameworks don't match any existing directory,
+        // it falls back to the first available preferred version
+        [Test]
+        public async Task TestGetDotNetFolderPathFallback()
+        {
+            var packageName = "mockPackage";
+            var packageVersion = "1.0.0";
+            var mockDownloadResource = new Mock<DownloadResource>();
+
+            mockDownloadResource.Setup(d => d.GetDownloadResourceResultAsync(
+                It.IsAny<PackageIdentity>(),
+                It.IsAny<PackageDownloadContext>(),
+                It.IsAny<string>(),
+                It.IsAny<ILogger>(),
+                It.IsAny<CancellationToken>()))
+                .Returns(Task.FromResult(new DownloadResourceResult(new Mock<Stream>().Object, "mockSource")));
+            
+            var mockPackageSourceRepository = new Mock<SourceRepository>();
+            mockPackageSourceRepository.Setup(s => s.GetResourceAsync<DownloadResource>())
+                .Returns(Task.FromResult(mockDownloadResource.Object));
+            
+            TestNugetSettings mockSettings = new()
+            {
+                Sections =
+                [
+                    new TestNugetSettingSection(ConfigurationConstants.PackageSources, new SourceItem("mockSource", "mockSourceUri"))
+                ]
+            };
+
+            // Set up: target framework is "net9.0" but only "netstandard2.0" directory exists
+            var packagePath = "c:\\mockPath";
+            var libPath = Path.Combine(packagePath, "lib");
+            var existingDirs = new HashSet<string>
+            {
+                libPath,
+                Path.Combine(libPath, "netstandard2.0")
+            };
+
+            var mockLocalPackageInfo = new LocalPackageInfo(
+                packageName,
+                NuGetVersion.Parse(packageVersion),
+                packagePath,
+                "mockSourceUri",
+                "mockManifestPath",
+                "mockSha512Path",
+                new Lazy<NuspecReader>(),
+                new Lazy<IReadOnlyList<string>>(),
+                new Lazy<string>(),
+                new Lazy<RuntimeGraph>());
+
+            var downloader = new TestNugetPackageDownloader(
+                packageName,
+                packageVersion,
+                mockSettings,
+                mockPackageSourceRepository.Object,
+                true,
+                true,
+                ["net9.0"],
+                mockLocalPackageInfo,
+                existingDirs);
+
+            var result = await downloader.DownloadAndInstallPackage();
+            
+            Assert.NotNull(result);
+            Assert.AreEqual(Path.Combine(libPath, "netstandard2.0"), result);
+        }
+
         public static IEnumerable<TestCaseData> ParseVersionStringTestCases
         {
             get