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.

Step 2: Replay the Binary Log to Text

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

Step 3: List All Projects

grep -i 'done building project\|Building project' full.log | grep -oP '"[^"]+\.csproj"' | sort -u

This lists all project files that participated in the build.

Step 4: Check for Multiple Evaluations per Project

Multiple evaluations for the same project indicate multi-targeting or multiple build configurations:

# Count how many times each project was evaluated
grep -c 'Evaluation started' full.log
grep 'Evaluation started.*\.csproj' full.log

Step 5: Check Global Properties for Each Evaluation

For each project, query the build properties to understand the build configuration:

# Search the diagnostic log for evaluated property values
grep -i 'TargetFramework\|Configuration\|Platform\|RuntimeIdentifier' full.log | head -40

Look for properties like TargetFramework, Configuration, Platform, and RuntimeIdentifier that should differentiate output paths.

Also check solution-related properties to identify multi-solution builds:

SolutionFileName, SolutionName, SolutionPath, SolutionDir, SolutionExt — differ when a project is built from multiple solutions
CurrentSolutionConfigurationContents — the number of project entries reveals which solution an evaluation belongs to (e.g., 1 project vs ~49 projects)

Look for extra global properties that don't affect output paths but create distinct MSBuild project instances:

PublishReadyToRun — a publish setting that doesn't change OutputPath or IntermediateOutputPath, but MSBuild treats it as a distinct project instance, preventing result caching and causing redundant target execution (e.g., CopyFilesToOutputDirectory running again)
Any other global property that differs between evaluations but doesn't contribute to path differentiation

Filter Out Non-Build Evaluations

When analyzing clashes, filter evaluations based on the type of clash you're investigating:

For OutputPath clashes: Exclude restore-phase evaluations (where MSBuildRestoreSessionId global property is set). These don't write to output directories.
For IntermediateOutputPath clashes: Include restore-phase evaluations, as NuGet restore writes project.assets.json to the intermediate output path.
Always exclude BuildProjectReferences=false: These are P2P metadata queries, not actual builds that write files.

Step 6: Get Output Paths for Each Project

Query each project's output path properties:

# From the diagnostic log - search for OutputPath assignments
grep -i 'OutputPath\s*=\|IntermediateOutputPath\s*=\|BaseOutputPath\s*=\|BaseIntermediateOutputPath\s*=' full.log | head -40

# Or query a specific project directly
dotnet msbuild MyProject.csproj -getProperty:OutputPath
dotnet msbuild MyProject.csproj -getProperty:IntermediateOutputPath
dotnet msbuild MyProject.csproj -getProperty:BaseOutputPath
dotnet msbuild MyProject.csproj -getProperty:BaseIntermediateOutputPath

Step 7: Identify Clashes

Compare the OutputPath and IntermediateOutputPath values across all evaluations:

Normalize paths - Convert to absolute paths and normalize separators
Group by path - Find evaluations that share the same OutputPath or IntermediateOutputPath
Report clashes - Any group with more than one evaluation indicates a clash

Step 8: Verify Clashes via CopyFilesToOutputDirectory (Optional)

As additional evidence for OutputPath clashes, check if multiple project builds execute the CopyFilesToOutputDirectory target to the same path. Note that not all clashes manifest here - compilation outputs and other targets may also conflict.

# Search for CopyFilesToOutputDirectory target execution per project
grep 'Target "CopyFilesToOutputDirectory"' full.log

# Look for Copy task messages showing file destinations
grep 'Copying file from\|SkipUnchangedFiles' full.log | head -30

Look for evidence of clashes in the messages:

Copying file from "..." to "..." - Active file writes
Did not copy from file "..." to file "..." because the "SkipUnchangedFiles" parameter was set to "true" - Indicates a second build attempted to write to the same location

The SkipUnchangedFiles skip message often masks clashes - the build succeeds but is vulnerable to race conditions in parallel builds.

Step 9: Check CoreCompile Execution Patterns (Optional)

To understand which project instance did the actual compilation vs redundant work, check CoreCompile:

grep 'Target "CoreCompile"' full.log

Compare the durations:

The instance with a long CoreCompile duration (e.g., seconds) is the primary build that did the actual compilation
Instances where CoreCompile was skipped (duration ~0-10ms) are redundant builds — they didn't recompile but may still run other targets like CopyFilesToOutputDirectory that write to the same output directory

This helps distinguish the "real" build from redundant instances created by extra global properties or multi-solution builds.

Caveat: Multi-Solution Builds

When analyzing multi-solution builds, note that the diagnostic log interleaves output from all projects. To determine which solution a project instance belongs to, search for SolutionFileName property assignments in the diagnostic log:

grep -i "SolutionFileName\|CurrentSolutionConfigurationContents" full.log | head -20

Expected Output Structure

For each evaluation, collect:

Project file path
Evaluation ID
TargetFramework (if multi-targeting)
Configuration
OutputPath
IntermediateOutputPath

Clash Detection Logic

For each unique OutputPath:
  - If multiple evaluations share it → CLASH
  
For each unique IntermediateOutputPath:
  - If multiple evaluations share it → 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:

Consolidate solutions - Ensure each project is only built from one solution in a single build
Use different configurations - Build solutions with different Configuration values that result in different output paths
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).