check-bin-obj-clash

by dotnet

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SKILL.md About

---
name: check-bin-obj-clash
description: "Detects MSBuild projects with conflicting OutputPath or IntermediateOutputPath. Only activate in MSBuild/.NET build context. 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, missing outputs in multi-project builds, multi-targeting builds where project.assets.json conflicts. Diagnoses when multiple projects or TFMs write to the same bin/obj directories due to shared OutputPath, missing AppendTargetFrameworkToOutputPath, or extra global properties like PublishReadyToRun creating redundant evaluations. DO NOT USE FOR: file access errors unrelated to MSBuild (OS-level locking), single-project single-TFM builds, non-MSBuild build systems. INVOKES: dotnet msbuild binlog replay, grep for output path analysis."
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.

## Step 2: Replay the Binary Log to Text

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

## Step 3: List All Projects

```bash
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:

```bash
# 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:

```bash
# 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:

1. **For OutputPath clashes**: Exclude restore-phase evaluations (where `MSBuildRestoreSessionId` global property is set). These don't write to output directories.

2. **For IntermediateOutputPath clashes**: Include restore-phase evaluations, as NuGet restore writes `project.assets.json` to the intermediate output path.

3. **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:

```bash
# 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:

1. **Normalize paths** - Convert to absolute paths and normalize separators
2. **Group by path** - Find evaluations that share the same OutputPath or IntermediateOutputPath
3. **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.

```bash
# 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`:

```bash
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:

```bash
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.

```xml

<OutputPath>bin\$(Configuration)\</OutputPath>
```

**Fix:** Include TargetFramework in the path:

```xml

<OutputPath>bin\$(Configuration)\$(TargetFramework)\</OutputPath>
```

Or rely on SDK defaults which handle this automatically:

```xml
<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`.

```xml

<BaseOutputPath>..\SharedOutput\</BaseOutputPath>
<BaseIntermediateOutputPath>..\SharedObj\</BaseIntermediateOutputPath>

<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:

```xml

<BaseIntermediateOutputPath>..\obj\ProjectA\</BaseIntermediateOutputPath>

<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:

```xml
<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)

## Example Workflow

```bash
# 1. Replay the binlog
dotnet msbuild build.binlog -noconlog -fl -flp:v=diag;logfile=full.log

# 2. List projects
grep 'done building project' full.log | grep -oP '"[^"]+\.csproj"' | sort -u

# 3. Check OutputPath for each evaluation
grep -i 'OutputPath\s*=' full.log | sort -u
# e.g.  OutputPath = bin\Debug\net8.0\
#       OutputPath = bin\Debug\net9.0\

# 4. Check IntermediateOutputPath
grep -i 'IntermediateOutputPath\s*=' full.log | sort -u
# e.g.  IntermediateOutputPath = obj\Debug\net8.0\
#       IntermediateOutputPath = obj\Debug\net9.0\

# 5. Compare paths → No clash (paths differ by TargetFramework)
```

## Tips

- Use `grep -i 'OutputPath\s*=' full.log | sort -u` to quickly find all OutputPath property assignments
- Check `BaseOutputPath` and `BaseIntermediateOutputPath` as they form the root of output paths
- The SDK default paths include `$(TargetFramework)` - clashes often occur when projects override these defaults
- Remember that paths may be relative - normalize to absolute paths before comparing
- **Cross-project IntermediateOutputPath clashes cannot be fixed with `AppendTargetFrameworkToOutputPath`** - files like `project.assets.json` are written directly to the intermediate path
- For multi-targeting clashes within the same project, `AppendTargetFrameworkToOutputPath=true` is the correct fix
- Common error messages indicating path clashes:
  - `Cannot create a file when that file already exists` (NuGet restore)
  - `The process cannot access the file because it is being used by another process`
  - Intermittent build failures that succeed on retry

### Global Properties to Check When Comparing Evaluations

When multiple evaluations share an output path, compare these global properties to understand why:

| Property | Affects OutputPath? | Notes |
|----------|---------------------|-------|
| `TargetFramework` | Yes | Different TFMs should have different paths |
| `RuntimeIdentifier` | Yes | Different RIDs should have different paths |
| `Configuration` | Yes | Debug vs Release |
| `Platform` | Yes | AnyCPU vs x64 etc. |
| `SolutionFileName` | No | Identifies which solution built the project — different values indicate multi-solution clash |
| `SolutionName` | No | Solution name without extension |
| `SolutionPath` | No | Full path to the solution file |
| `SolutionDir` | No | Directory containing the solution file |
| `CurrentSolutionConfigurationContents` | No | XML with project entries — count of entries reveals which solution |
| `BuildProjectReferences` | No | `false` = P2P query, not a real build - ignore these |
| `MSBuildRestoreSessionId` | No | Present = restore phase evaluation |
| `PublishReadyToRun` | No | Publish setting, doesn't change build output path but creates distinct project instances |

## Testing Fixes

After making changes to fix path clashes, clean and rebuild to verify. See the `binlog-generation` skill's "Cleaning the Repository" section on how to clean the repository while preserving binlog files.