Incremental Build

How MSBuild Incremental Build Works

MSBuild's incremental build mechanism allows targets to be skipped when their outputs are already up to date, dramatically reducing build times on subsequent runs.

• Targets with Inputs and Outputs attributes: MSBuild compares the timestamps of all files listed in Inputs against all files listed in Outputs. If every output file is newer than every input file, the target is skipped entirely.
• Without Inputs/Outputs: The target runs every time the build is invoked. This is the default behavior and the most common cause of slow incremental builds.
• Incremental attribute on targets: Targets can explicitly opt in or out of incremental behavior. Setting Incremental="false" forces the target to always run, even if Inputs and Outputs are specified.
• Timestamp-based comparison: MSBuild uses file system timestamps (last write time) to determine staleness. It does not use content hashes. This means touching a file (updating its timestamp without changing content) will trigger a rebuild.

<!-- This target is incremental: skipped if Output is newer than all Inputs -->
<Target Name="Transform"
        Inputs="@(TransformFiles)"
        Outputs="@(TransformFiles->'$(OutputPath)%(Filename).out')">
  <!-- work here -->
</Target>

<!-- This target always runs because it has no Inputs/Outputs -->
<Target Name="PrintMessage">
  <Message Text="This runs every build" />
</Target>

Why Incremental Builds Break (Top Causes)

1. Missing Inputs/Outputs on custom targets — Without both attributes, the target always runs. This is the single most common cause of unnecessary rebuilds.

2. Volatile properties in Outputs path — If the output path includes something that changes between builds (e.g., a timestamp, build number, or random GUID), MSBuild will never find the previous output and will always rebuild.

3. File writes outside of tracked Outputs — If a target writes files that aren't listed in its Outputs, MSBuild doesn't know about them. The target may be skipped (because its declared outputs are up to date), but downstream targets may still be triggered.

4. Missing FileWrites registration — Files created during the build but not registered in the FileWrites item group won't be cleaned by dotnet clean. Over time, stale files can confuse incremental checks.

5. Glob changes — When you add or remove source files, the item set (e.g., @(Compile)) changes. Since these items feed into Inputs, the set of inputs changes and triggers a rebuild. This is expected behavior but can be surprising.

6. Property changes — Properties that feed into Inputs or Outputs paths (e.g., $(Configuration), $(TargetFramework)) will cause rebuilds when changed. Switching between Debug and Release is a full rebuild by design.

7. NuGet package updates — Changing a package version updates project.assets.json and potentially many resolved assembly paths. This changes the inputs to ResolveAssemblyReferences and CoreCompile, triggering a rebuild.

8. Build server VBCSCompiler cache invalidation — The Roslyn compiler server (VBCSCompiler) caches compilation state. If the server is recycled (timeout, crash, or manual kill), the next build may be slower even though MSBuild's incremental checks pass, because the compiler must repopulate its in-memory caches.

Diagnosing "Why Did This Rebuild?"

Use binary logs (binlogs) to understand exactly why targets ran instead of being skipped.

Step-by-step using binlog

1. Build twice with binlogs to capture the incremental build behavior:

   dotnet build /bl:first.binlog
   dotnet build /bl:second.binlog

   The first build establishes the baseline. The second build is the one you want to be incremental. Analyze second.binlog.

Primary: binlog MCP (preferred)

Use the binlog MCP server (Microsoft.AITools.BinlogMcp, exposed under the binlog MCP namespace) to analyze the second binlog:

1. Use the overview tool to check overall build status and duration
2. Use the search tool to find targets that executed vs were skipped — search for "Building target completely", "Building target incrementally", "Skipping target"
3. Use the search tool to find "is newer than output" messages that reveal which input file triggered a rebuild
4. Use target-related tools (target_reasons, project_targets) to inspect why specific targets ran
5. Use the expensive_targets tool to find targets that consumed the most time in the second build — these are your optimization targets

Fallback: text-log replay (when MCP is unavailable)

2. Replay the second binlog to a diagnostic text log:

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

   Then search for targets that actually executed:

   grep 'Building target\|Target.*was not skipped' second-full.log

   In a perfectly incremental build, most targets should be skipped.

3. Inspect non-skipped targets by looking for their execution messages in the diagnostic log. Check for "out of date" messages that indicate why a target ran.

4. Look for key messages in the binlog:

   • "Building target 'X' completely" — means MSBuild found no outputs or all outputs are missing; this is a full target execution.
   • "Building target 'X' incrementally" — means some (but not all) outputs are out of date.
   • "Skipping target 'X' because all output files are up-to-date" — target was correctly skipped.

5. Search for "is newer than output" messages to find the specific input file that triggered the rebuild:

   grep "is newer than output" second-full.log

   This reveals exactly which input file's timestamp caused MSBuild to consider the target out of date.

Additional diagnostic techniques

• Compare first.binlog and second.binlog side by side in the MSBuild Structured Log Viewer to see what changed.
• Use grep 'Target Performance Summary' -A 30 second-full.log to see which targets consumed the most time in the second build — these are your optimization targets.
• Check for targets with zero-duration that still ran — they may have unnecessary dependencies causing them to execute.

FileWrites and Clean Build

The FileWrites item group is MSBuild's mechanism for tracking files generated during the build. It powers dotnet clean and helps maintain correct incremental behavior.

• FileWrites item: Register any file your custom targets create so that dotnet clean knows to remove them. Without this, generated files accumulate across builds and may confuse incremental checks.
• FileWritesShareable item: Use this for files that are shared across multiple projects (e.g., shared generated code). These files are tracked but not deleted if other projects still reference them.
• If not registered: Files accumulate in the output and intermediate directories. dotnet clean won't remove them, and they may cause stale data issues or confuse up-to-date checks.

Pattern for registering generated files

Add generated files to FileWrites inside the target that creates them:

<Target Name="MyGenerator" Inputs="..." Outputs="$(IntermediateOutputPath)generated.cs">
  <!-- Generate the file -->
  <WriteLinesToFile File="$(IntermediateOutputPath)generated.cs" Lines="@(GeneratedLines)" />

  <!-- Register for clean -->
  <ItemGroup>
    <FileWrites Include="$(IntermediateOutputPath)generated.cs" />
  </ItemGroup>
</Target>