Reject Yarn Classic for TypeScript AppHosts

src/Aspire.Cli/Projects/TypeScriptAppHostToolchainResolver.cs

@@ -24,6 +24,7 @@ internal static class TypeScriptAppHostToolchainResolver
     private const string BunLockFileName = "bun.lock";
     private const string BunBinaryLockFileName = "bun.lockb";
     private const string YarnLockFileName = "yarn.lock";
+    private const string YarnClassicLockFileVersionLine = "# yarn lockfile v1";
     private const string YarnConfigFileName = ".yarnrc.yml";
     private const string PackageLockFileName = "package-lock.json";
     private const string PnpmLockFileName = "pnpm-lock.yaml";
@@ -70,8 +71,14 @@ internal static TypeScriptAppHostToolchainResolution ResolveWithReason(Directory
                 return CreateLockFileResolution(TypeScriptAppHostToolchain.Pnpm, PnpmLockFileName, candidateDirectory);
             }
 
-            if (File.Exists(Path.Combine(candidateDirectory.FullName, YarnLockFileName)))
+            var yarnLockFilePath = Path.Combine(candidateDirectory.FullName, YarnLockFileName);
+            if (File.Exists(yarnLockFilePath))
             {
+                if (IsYarnClassicLockFile(yarnLockFilePath))
+                {
+                    throw CreateYarnClassicNotSupportedException($"the Yarn lockfile at {yarnLockFilePath}");
+                }
+
                 return CreateLockFileResolution(TypeScriptAppHostToolchain.Yarn, YarnLockFileName, candidateDirectory);
             }
 
@@ -265,6 +272,11 @@ private static bool TryGetToolchainFromPackageJson(DirectoryInfo appHostDirector
             var packageManagerName = packageManager.Split('@', 2)[0];
             if (TryParseToolchain(packageManagerName, out toolchain))
             {
+                if (toolchain == TypeScriptAppHostToolchain.Yarn && IsYarnClassicPackageManager(packageManager))
+                {
+                    throw CreateYarnClassicNotSupportedException($"'{packageManager}' in {packageJsonPath}");
+                }
+
                 reason = $"packageManager '{packageManager}' found in {packageJsonPath}";
                 return true;
             }
@@ -294,6 +306,55 @@ private static bool TryParseToolchain(string packageManagerName, out TypeScriptA
         return result.HasValue;
     }
 
+    private static bool IsYarnClassicPackageManager(string packageManager)
+    {
+        const string yarnPackageManagerPrefix = "yarn@";
+
+        if (!packageManager.StartsWith(yarnPackageManagerPrefix, StringComparison.OrdinalIgnoreCase))
+        {
+            return false;
+        }
+
+        var version = packageManager[yarnPackageManagerPrefix.Length..];
+        return version.Length > 0 &&
+            version[0] == '1' &&
+            (version.Length == 1 || !char.IsAsciiDigit(version[1]));
+    }
+
+    private static InvalidOperationException CreateYarnClassicNotSupportedException(string upgradeTarget)
+    {
+        return new InvalidOperationException(
+            $"Yarn Classic is not supported for TypeScript AppHosts. Upgrade {upgradeTarget} to Yarn 4 or later, or use npm, pnpm, or Bun.");
+    }
+
+    private static bool IsYarnClassicLockFile(string yarnLockFilePath)
+    {
+        try
+        {
+            var linesRead = 0;
+            foreach (var line in File.ReadLines(yarnLockFilePath))
+            {
+                if (line.Trim().Equals(YarnClassicLockFileVersionLine, StringComparison.OrdinalIgnoreCase))
+                {
+                    return true;
+                }
+
+                linesRead++;
+                if (linesRead >= 5)
+                {
+                    return false;
+                }
+            }
+        }
+        catch (Exception ex) when (ex is IOException or UnauthorizedAccessException
+            or SecurityException or NotSupportedException)
+        {
+            return false;
+        }
+
+        return false;
+    }
+
     private static IEnumerable<DirectoryInfo> EnumerateCandidateDirectories(DirectoryInfo appHostDirectory)
     {
         yield return appHostDirectory;

tests/Aspire.Cli.EndToEnd.Tests/Helpers/TypeScriptAppHostToolchainTestHelpers.cs

@@ -67,7 +67,7 @@ internal static string GetPackageManager(string toolchain) =>
         NormalizeToolchain(toolchain) switch
         {
             "bun" => "bun@1.2.0",
-            "yarn" => "yarn@1.22.22",
+            "yarn" => "yarn@4.14.1",
             "pnpm" => "pnpm@10.0.0",
             "npm" => "npm@10.0.0",
             _ => throw new ArgumentOutOfRangeException(nameof(toolchain), toolchain, "Unsupported TypeScript AppHost toolchain.")

tests/Aspire.Cli.Tests/Projects/TypeScriptAppHostToolchainResolverTests.cs

@@ -34,6 +34,58 @@ public void Resolve_WhenPnpmLockExists_ReturnsPnpm()
         Assert.Equal(TypeScriptAppHostToolchain.Pnpm, toolchain);
     }
 
+    [Fact]
+    public void Resolve_WhenPackageManagerIsYarnClassic_Throws()
+    {
+        using var workspace = TemporaryWorkspace.Create(outputHelper);
+        var packageJsonPath = Path.Combine(workspace.WorkspaceRoot.FullName, "package.json");
+        File.WriteAllText(packageJsonPath, "{ \"packageManager\": \"yarn@1.22.22\" }");
+
+        var exception = Assert.Throws<InvalidOperationException>(() => TypeScriptAppHostToolchainResolver.Resolve(workspace.WorkspaceRoot, logger: null));
+
+        Assert.Equal($"Yarn Classic is not supported for TypeScript AppHosts. Upgrade 'yarn@1.22.22' in {packageJsonPath} to Yarn 4 or later, or use npm, pnpm, or Bun.", exception.Message);
+    }
+
+    [Fact]
+    public void Resolve_WhenPackageManagerIsModernYarn_ReturnsYarn()
+    {
+        using var workspace = TemporaryWorkspace.Create(outputHelper);
+        File.WriteAllText(Path.Combine(workspace.WorkspaceRoot.FullName, "package.json"), "{ \"packageManager\": \"yarn@4.14.1\" }");
+
+        var toolchain = TypeScriptAppHostToolchainResolver.Resolve(workspace.WorkspaceRoot, logger: null);
+
+        Assert.Equal(TypeScriptAppHostToolchain.Yarn, toolchain);
+    }
+
+    [Fact]
+    public void Resolve_WhenYarnLockIsClassic_Throws()
+    {
+        using var workspace = TemporaryWorkspace.Create(outputHelper);
+        File.WriteAllText(Path.Combine(workspace.WorkspaceRoot.FullName, "package.json"), "{ \"name\": \"apphost\" }");
+        var yarnLockPath = Path.Combine(workspace.WorkspaceRoot.FullName, "yarn.lock");
+        File.WriteAllText(yarnLockPath, "# THIS IS AN AUTOGENERATED FILE. DO NOT EDIT THIS FILE DIRECTLY.\n# yarn lockfile v1\n");
+
+        var exception = Assert.Throws<InvalidOperationException>(() => TypeScriptAppHostToolchainResolver.Resolve(workspace.WorkspaceRoot, logger: null));
+
+        Assert.Equal($"Yarn Classic is not supported for TypeScript AppHosts. Upgrade the Yarn lockfile at {yarnLockPath} to Yarn 4 or later, or use npm, pnpm, or Bun.", exception.Message);
+    }
+
+    [Fact]
+    public void Resolve_WhenParentYarnLockIsClassic_Throws()
+    {
+        using var workspace = TemporaryWorkspace.Create(outputHelper);
+        var appHostDirectory = workspace.WorkspaceRoot.CreateSubdirectory("apps").CreateSubdirectory("apphost");
+        var parentDirectory = appHostDirectory.Parent!;
+        File.WriteAllText(Path.Combine(appHostDirectory.FullName, "package.json"), "{ \"name\": \"apphost\" }");
+        File.WriteAllText(Path.Combine(parentDirectory.FullName, "package.json"), "{ \"name\": \"workspace\" }");
+        var yarnLockPath = Path.Combine(parentDirectory.FullName, "yarn.lock");
+        File.WriteAllText(yarnLockPath, "# THIS IS AN AUTOGENERATED FILE. DO NOT EDIT THIS FILE DIRECTLY.\n# yarn lockfile v1\n");
+
+        var exception = Assert.Throws<InvalidOperationException>(() => TypeScriptAppHostToolchainResolver.Resolve(appHostDirectory, logger: null));
+
+        Assert.Equal($"Yarn Classic is not supported for TypeScript AppHosts. Upgrade the Yarn lockfile at {yarnLockPath} to Yarn 4 or later, or use npm, pnpm, or Bun.", exception.Message);
+    }
+
     [Fact]
     public void Resolve_WhenPackageLockExists_ReturnsNpm()
     {

tests/Shared/Docker/Dockerfile.e2e

@@ -64,7 +64,9 @@ RUN apt-get update -qq && \
 # Install Node.js plus supported TypeScript AppHost toolchains.
 RUN curl -fsSL https://deb.nodesource.com/setup_22.x | bash - && \
     apt-get install -y --no-install-recommends nodejs && \
-    npm install -g yarn@1.22.22 pnpm@10 && \
+    corepack enable yarn && \
+    corepack prepare yarn@4.14.1 --activate && \
+    npm install -g pnpm@10 && \
     curl -fsSL https://bun.sh/install | bash && \
     ln -s /root/.bun/bin/bun /usr/local/bin/bun && \
     ln -s /root/.bun/bin/bunx /usr/local/bin/bunx && \

tests/Shared/Docker/Dockerfile.e2e-polyglot-base

@@ -66,7 +66,9 @@ RUN curl -fsSL https://cli.github.com/packages/githubcli-archive-keyring.gpg \
 # Install Node.js plus supported TypeScript AppHost toolchains.
 RUN curl -fsSL https://deb.nodesource.com/setup_22.x | bash - && \
     apt-get install -y --no-install-recommends nodejs && \
-    npm install -g yarn@1.22.22 pnpm@10 && \
+    corepack enable yarn && \
+    corepack prepare yarn@4.14.1 --activate && \
+    npm install -g pnpm@10 && \
     curl -fsSL https://bun.sh/install | bash && \
     ln -s /root/.bun/bin/bun /usr/local/bin/bun && \
     ln -s /root/.bun/bin/bunx /usr/local/bin/bunx && \