[11.0.1xx] File-based apps: improve legacy artifact support

[jjonescz]

Backport of #56160.

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🟡 Changes recommended

Critical compilation and legacy-fallback issues remain unresolved.

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Pull request overview

Backports file-based app artifact compatibility for older SDK hosts.

Changes:

• Adds an SDK capability marker.
• Implements legacy artifact-path fallback logic.
• Adds compatibility regression tests.

File summaries

File	Summary and review findings
test/dotnet.Tests/CommandTests/Run/RunFileTests_CscOnlyAndApi.cs	Adds compatibility tests. Critical (1 vote): missing System.Security import for SecurityElement.
src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.DefaultArtifactsPath.props	Adds the artifact support marker. Nit (1 vote): tests override the marker instead of verifying its assignment.
src/Cli/Microsoft.DotNet.FileBasedPrograms/VirtualProjectBuilder.cs	Adds artifact support detection and fallback. Critical (3 votes): _useLegacyArtifactsPath is undeclared. Critical (3 votes): fallback reuses cached modern project text instead of generating legacy properties.

Review details

Suppressed comments (1)

src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.DefaultArtifactsPath.props:20

• Both cases in the new test force _SupportsFileBasedAppArtifactsPath through additionalGlobalProperties; because global properties override project properties, the test never exercises this new assignment. Removing this line would still leave that test green while the current SDK is classified as legacy in real use. Add a case that evaluates without overriding the marker and verifies support is detected.

    <_SupportsFileBasedAppArtifactsPath>true</_SupportsFileBasedAppArtifactsPath>

• Files reviewed: 3/3 changed files
• Comments generated: 3
• Review effort level: Lite

[github-actions]

🔍 Build Failure Analysis

Summary — The build fails to compile across every leg (Linux/macOS/Windows, AOT and non-AOT) with CS0103: The name '_useLegacyArtifactsPath' does not exist in the current context in VirtualProjectBuilder.cs, because this backport PR added the code that uses the field _useLegacyArtifactsPath without adding the field itself (or the rest of the change that consumes it).

Root cause: incomplete backport of #56160 — missing _useLegacyArtifactsPath field and its consumer changes

This PR ([11.0.1xx] File-based apps: improve legacy artifact support) is a manual backport of #56160. The upstream PR made changes in three places of VirtualProjectBuilder.cs:

1. Field declaration — private bool? _useLegacyArtifactsPath; (class-level field).
2. Passing the field's value to WriteProjectFile — useLegacyArtifactsPath: _useLegacyArtifactsPath == true, inside the CreateProjectInstanceNoEvaluation local function's call to WriteProjectFile.
3. The hunk that sets the field (only this one made it into this PR) — the if (_useLegacyArtifactsPath is null) { ... } block that reads the new _SupportsFileBasedAppArtifactsPath MSBuild property and recurses.
4. WriteProjectFile's signature and body — adding the bool useLegacyArtifactsPath = false parameter and branching between the legacy (IncludeProjectNameInArtifactsPaths/ArtifactsPath/PublishDir/PackageOutputPath) and new (FileBasedAppArtifactsPath) property emission.

Only item 3 is present in this PR's diff (src/Cli/Microsoft.DotNet.FileBasedPrograms/VirtualProjectBuilder.cs lines 503–518) along with the companion change to Microsoft.NET.DefaultArtifactsPath.props (adding _SupportsFileBasedAppArtifactsPath) and the new test. Items 1, 2, and 4 were never carried over, so _useLegacyArtifactsPath is referenced but never declared — a plain compile error, reproduced identically in all 10 build legs (binlog_errors reports the same three CS0103 errors at lines 505/509/511 in every leg that has full error reporting, e.g. Windows x64 and Linux arm64).

Affected files / errors

• src/Cli/Microsoft.DotNet.FileBasedPrograms/VirtualProjectBuilder.cs:505 — CS0103: The name '_useLegacyArtifactsPath' does not exist in the current context
• src/Cli/Microsoft.DotNet.FileBasedPrograms/VirtualProjectBuilder.cs:509 — same root cause
• src/Cli/Microsoft.DotNet.FileBasedPrograms/VirtualProjectBuilder.cs:511 — same root cause

Proposed fix

Bring over the remaining hunks from #56160 that this backport dropped. None of the missing lines are within this PR's current diff (they live elsewhere in the same file, untouched by this PR), so they can't be expressed as single-line GitHub suggestions — apply them as additional edits to VirtualProjectBuilder.cs:

     private (ImmutableArray<CSharpDirective> Original, ImmutableArray<CSharpDirective> Evaluated)? _evaluatedDirectives;
 
+    private bool? _useLegacyArtifactsPath;
+
     internal string EntryPointFileFullPath { get; }

                 entryPointFilePath: EntryPointFileFullPath,
                 artifactsPath: ArtifactsPath,
+                useLegacyArtifactsPath: _useLegacyArtifactsPath == true,
                 includeRuntimeConfigInformation: RequestedTargets?.Any(static t => t is "Publish" or "Pack") != true);

         string? entryPointFilePath = null,
         string? artifactsPath = null,
+        bool useLegacyArtifactsPath = false,
         bool includeRuntimeConfigInformation = true,

             writer.WriteLine($"""
                 <Project>
 
                   <PropertyGroup>
-                    <FileBasedAppArtifactsPath>{EscapeValue(artifactsPath)}</FileBasedAppArtifactsPath>
+                """);
+
+            if (useLegacyArtifactsPath)
+            {
+                writer.WriteLine($"""
+                        <IncludeProjectNameInArtifactsPaths>false</IncludeProjectNameInArtifactsPaths>
+                        <ArtifactsPath>{EscapeValue(artifactsPath)}</ArtifactsPath>
+                        <PublishDir>artifacts/$(AssemblyName)</PublishDir>
+                        <PackageOutputPath>artifacts/$(AssemblyName)</PackageOutputPath>
+                    """);
+            }
+            else
+            {
+                writer.WriteLine($"""
+                        <FileBasedAppArtifactsPath>{EscapeValue(artifactsPath)}</FileBasedAppArtifactsPath>
+                    """);
+            }
+
+            writer.WriteLine($"""
                     <AssemblyName>{EscapeValue(Path.GetFileNameWithoutExtension(entryPointFilePath))}</AssemblyName>

Reference: the full, correct set of hunks for this file is visible in the merged upstream PR at dotnet/sdk#56160's diff for VirtualProjectBuilder.cs.

---

Build overview

• Failing across all 10 build legs (Windows x64/arm64, Linux x64/arm64, macOS arm64 — both AOT and non-AOT — plus the Windows FullFramework leg).
• Same 3 CS0103 errors reproduced verbatim in every leg that reports full errors (e.g. 10_0_Windows_x64_Logs, 1_0_Linux_arm64_AOT_Logs, 2_0_Linux_arm64_Logs), each hitting two projects that compile VirtualProjectBuilder.cs: Microsoft.DotNet.ProjectTools.csproj and Microsoft.DotNet.FileBasedPrograms.Package.csproj.
• Target: CoreCompile (task Csc). Exit: build failed ("Build failed.").

All MSBuild errors (3 unique, repeated per compiling project)

Code	Project	File:Line	Message
CS0103	Microsoft.DotNet.ProjectTools / Microsoft.DotNet.FileBasedPrograms.Package	VirtualProjectBuilder.cs:505	The name _useLegacyArtifactsPath does not exist in the current context
CS0103	Microsoft.DotNet.ProjectTools / Microsoft.DotNet.FileBasedPrograms.Package	VirtualProjectBuilder.cs:509	The name _useLegacyArtifactsPath does not exist in the current context
CS0103	Microsoft.DotNet.ProjectTools / Microsoft.DotNet.FileBasedPrograms.Package	VirtualProjectBuilder.cs:511	The name _useLegacyArtifactsPath does not exist in the current context

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