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name: check-bin-obj-clash description: "Detects MSBuild projects with conflicting OutputPath or IntermediateOutputPath. USE FOR: builds failing with 'Cannot create a file when that file already exists', 'The process cannot access the file because it is being used by another process', intermittent build failures that succeed on retry, or missing/overwritten outputs in multi-project or multi-targeting builds where bin/obj (or project.assets.json) collide. Common causes: shared OutputPath, missing AppendTargetFrameworkToOutputPath, extra global properties (e.g. PublishReadyToRun), or SetTargetFramework on a ProjectReference to a single-targeting project. DO NOT USE FOR: file access errors unrelated to MSBuild (OS-level locking), single-project single-TFM builds, non-MSBuild build systems." license: MIT

Detecting OutputPath and IntermediateOutputPath Clashes

Overview

This skill helps identify when multiple MSBuild project evaluations share the same OutputPath or IntermediateOutputPath. This is a common source of build failures including:

• File access conflicts during parallel builds
• Missing or overwritten output files
• Intermittent build failures
• "File in use" errors
• NuGet restore errors like Cannot create a file when that file already exists - this strongly indicates multiple projects share the same IntermediateOutputPath where project.assets.json is written

Clashes can occur between:

• Different projects sharing the same output directory
• Multi-targeting builds (e.g., TargetFrameworks=net8.0;net9.0) where the path doesn't include the target framework
• Multiple solution builds where the same project is built from different solutions in a single build

Note: Project instances with BuildProjectReferences=false should be ignored when analyzing clashes - these are P2P reference resolution builds that only query metadata (via GetTargetPath) and do not actually write to output directories.

When to Use This Skill

Invoke this skill immediately when you see:

• Cannot create a file when that file already exists during NuGet restore
• The process cannot access the file because it is being used by another process
• Intermittent build failures that succeed on retry
• Missing output files or unexpected overwriting

Step 1: Generate a Binary Log

Use the binlog-generation skill to generate a binary log with the correct naming convention.

Primary workflow — binlog MCP

The MCP server exposes structured tools for inspecting a .binlog without parsing text logs. Call them directly instead of replaying the binlog to a text file. Call tools/list for the MCP first if you are unsure which tools are available.

Important constraints:

• The .binlog file is a binary format — do NOT try to cat, head, strings, or read it directly. Use only the MCP tools to query it.
• Synthesize findings as you go. Do not spend all available time investigating — once you have enough evidence, present your conclusions.

Step 2: Get an overview and list projects

Use the MCP overview and projects tools to understand the build and list all projects that participated.

Step 3: Check evaluations and global properties

Use the MCP evaluations and evaluation_global_properties tools to find all evaluations per project. Look for:

• Multiple evaluations for the same project (indicates multi-targeting or multiple build configurations)
• Differing global properties between evaluations (TargetFramework, Configuration, RuntimeIdentifier, SolutionFileName, PublishReadyToRun, etc.)

Step 4: Get output paths for each evaluation

Use the MCP properties tool to query OutputPath, IntermediateOutputPath, BaseOutputPath, and BaseIntermediateOutputPath for each project evaluation.

Step 5: Check for double writes

Use the MCP double_writes tool if available — it directly detects files written by multiple project instances.

Step 6: Identify clashes

Compare the OutputPath and IntermediateOutputPath values across all evaluations:

1. Normalize paths - Convert to absolute paths and normalize separators
2. Group by path - Find evaluations that share the same OutputPath or IntermediateOutputPath
3. Filter out non-build evaluations - Exclude BuildProjectReferences=false instances (P2P queries)
4. Report clashes - Any group with more than one evaluation indicates a clash

Fallback workflow — text-log replay (when MCP is unavailable)

Use this only when the MCP server cannot be started.

Replay the binlog to a diagnostic text log, then grep for the same signals the MCP tools surface:

dotnet msbuild build.binlog -noconlog -fl -flp:v=diag;logfile=full.log

Then extract the clash signals:

• Projects & evaluations — list evaluation starts and count them per project: grep 'Evaluation started' full.log | grep -oiE '"[^"]+\.[a-z]+proj"' | sort | uniq -c. Matching the full quoted path keeps same-named projects in different directories distinct (and tolerates spaces in paths); a path with a count ≥ 2 was evaluated more than once (multi-targeting or extra global properties). (grep -c alone only totals evaluations across the whole log, so it can't reveal per-project duplication.)
• Output paths — grep -iE 'OutputPath[[:space:]]*=|IntermediateOutputPath[[:space:]]*=|BaseOutputPath[[:space:]]*=|BaseIntermediateOutputPath[[:space:]]*=' full.log | sort -u, or query a project directly: dotnet msbuild MyProject.csproj -getProperty:OutputPath (and IntermediateOutputPath, BaseIntermediateOutputPath).
• Distinguishing global properties — grep -iE 'TargetFramework|Configuration|Platform|RuntimeIdentifier|SolutionFileName|PublishReadyToRun' full.log. See the Global Properties to Check table for which affect the path and which just fork a redundant instance.
• Corroborating evidence (optional) — grep 'Target "CopyFilesToOutputDirectory"' full.log plus grep 'SkipUnchangedFiles' full.log show a second instance writing (or skipping a masked write) to the same path; a long vs ~0 ms CoreCompile distinguishes the real build from a redundant instance.

Then identify clashes: normalize paths to absolute, group evaluations by OutputPath and by IntermediateOutputPath, and exclude BuildProjectReferences=false (P2P queries) — plus, for OutputPath only, MSBuildRestoreSessionId restore evaluations. Any group with more than one remaining evaluation is a clash.

Common Causes and Fixes

Multi-targeting without TargetFramework in path

Problem: Project uses TargetFrameworks but OutputPath doesn't vary by framework.

<!-- BAD: Same path for all frameworks -->
<OutputPath>bin\$(Configuration)\</OutputPath>

Fix: Include TargetFramework in the path:

<!-- GOOD: Path varies by framework -->
<OutputPath>bin\$(Configuration)\$(TargetFramework)\</OutputPath>

Or rely on SDK defaults which handle this automatically:

<AppendTargetFrameworkToOutputPath>true</AppendTargetFrameworkToOutputPath>
<AppendTargetFrameworkToIntermediateOutputPath>true</AppendTargetFrameworkToIntermediateOutputPath>

Shared output directory across projects (CANNOT be fixed with AppendTargetFramework)

Problem: Multiple projects explicitly set the same BaseOutputPath or BaseIntermediateOutputPath.

<!-- Project A - Directory.Build.props -->
<BaseOutputPath>..\SharedOutput\</BaseOutputPath>
<BaseIntermediateOutputPath>..\SharedObj\</BaseIntermediateOutputPath>

<!-- Project B - Directory.Build.props -->
<BaseOutputPath>..\SharedOutput\</BaseOutputPath>
<BaseIntermediateOutputPath>..\SharedObj\</BaseIntermediateOutputPath>

IMPORTANT: Even with AppendTargetFrameworkToOutputPath=true, this will still clash! .NET writes certain files directly to the IntermediateOutputPath without the TargetFramework suffix, including:

• project.assets.json (NuGet restore output)
• Other NuGet-related files

This causes errors like Cannot create a file when that file already exists during parallel restore.

Fix: Each project MUST have a unique BaseIntermediateOutputPath. Do not share intermediate output directories across projects:

<!-- Project A -->
<BaseIntermediateOutputPath>..\obj\ProjectA\</BaseIntermediateOutputPath>

<!-- Project B -->
<BaseIntermediateOutputPath>..\obj\ProjectB\</BaseIntermediateOutputPath>

Or simply use the SDK defaults which place obj inside each project's directory.

RuntimeIdentifier builds clashing

Problem: Building for multiple RIDs without RID in path.

Fix: Ensure RuntimeIdentifier is in the path:

<AppendRuntimeIdentifierToOutputPath>true</AppendRuntimeIdentifierToOutputPath>

Multiple solutions building the same project

Problem: A single build invokes multiple solutions (e.g., via MSBuild task or command line) that include the same project. Each solution build evaluates and builds the project independently, with different Solution* global properties that don't affect the output path.

How to detect: Compare SolutionFileName and CurrentSolutionConfigurationContents across evaluations for the same project. Different values indicate multi-solution builds. For example:

| Property | Eval from Solution A | Eval from Solution B | |---|---|---| | SolutionFileName | BuildAnalyzers.sln | Main.slnx | | CurrentSolutionConfigurationContents | 1 project entry | ~49 project entries | | OutputPath | bin\Release\netstandard2.0\ | bin\Release\netstandard2.0\ ← clash |

Example: A repo build script builds BuildAnalyzers.sln then Main.slnx, and both solutions include SharedAnalyzers.csproj. Both builds write to bin\Release\netstandard2.0\. The first build compiles; the second skips compilation but still runs CopyFilesToOutputDirectory.

Fix: Options include:

1. Consolidate solutions - Ensure each project is only built from one solution in a single build
2. Use different configurations - Build solutions with different Configuration values that result in different output paths
3. Exclude duplicate projects - Use solution filters or conditional project inclusion to avoid building the same project twice

Extra global properties creating redundant project instances

Problem: A project is built multiple times within the same solution due to extra global properties (e.g., PublishReadyToRun=false) that create distinct MSBuild project instances. These properties don't affect output paths but prevent MSBuild from caching results across instances, causing redundant target execution.

How to detect: Compare global properties across evaluations for the same project within the same solution (same SolutionFileName). Look for properties that differ but don't contribute to path differentiation:

| Property | Eval A (from Razor.slnx) | Eval B (from Razor.slnx) | |---|---|---| | PublishReadyToRun | (not set) | false | | OutputPath | bin\Release\netstandard2.0\ | bin\Release\netstandard2.0\ ← clash |

This is particularly wasteful for projects where the extra property has no effect (e.g., PublishReadyToRun on a netstandard2.0 class library that doesn't use ReadyToRun compilation).

Fix: Options include:

1. Remove the extra global property - Investigate which parent target/task is injecting the property and prevent it from being passed to projects that don't need it
2. Use RemoveGlobalProperties metadata - On ProjectReference items, use RemoveGlobalProperties="PublishReadyToRun" to strip the property before building the referenced project
3. Condition the property - Only set the property on projects that actually use it (e.g., only for executable projects, not class libraries)

Explicit <MSBuild> Build/Publish with extra global properties (self or cross-project)

Problem: A target use